We spend a lot of time talking about how to get an interstellar probe up to speed. But what happens if we do achieve a cruise speed of 12 percent of the speed of light, as envisioned by the designers who put together Project Daedalus back in the 1970s? Daedalus called for a 3.8-year period of acceleration that would set up a 46-year cruise to its target, Barnard’s Star, some 5.9 light years away. That’s stretching mission duration out to the active career span of a researcher, but it’s a span we might accept if we could be sure we’d get good science out of it.
Maximizing the Science Return
But can we? Let’s assume we’re approaching a solar system at 12 percent of c and out there orbiting the target star is a terrestrial planet, just the sort of thing we’re hoping to find. Assume for the sake of argument that the probe crosses the path of this object at approximately ninety degrees to its orbital motion trajectory. As Kelvin Long shows in a recent post on the Project Icarus blog, the encounter time, during which serious observations could be made, is less than one second. A Jupiter-class world, much larger and observable from a greater distance, itself offers up something less than ten seconds at best for scientific scrutiny.
That’s a paltry return on decades of construction and flight time, not to mention the probable trillion or more dollars it would take to build such a probe, and it hardly compares well to what we’ll be able to achieve with even ground-based telescopes as the next generation of optics becomes available. What to do? Long is looking into these issues as part of the Project Icarus team, which is revisiting the Daedalus concept to see how changing technologies could alter the flight profile and produce a mission whose results would be substantially more useful.
Image: The Daedalus starship arrives in the Barnard’s Star system. Credit and copyright: Adrian Mann.
One option is to do the unthinkable. Instead of ramping up flight speed to get to the destination more quickly, perhaps a better alternative is to slow the mission down. There are two ways to do this: 1) Aim for a slower cruise speed in the first place and/or 2) attempt to decelerate the vehicle. The latter choice is a genuine conundrum for reasons Long makes clear:
Another option being examined [for deceleration] is reverse engine thrust, but the problem with this is that if we assume an equal acceleration-deceleration profile then the mass ratio scales as squared compared to a flyby mission and so requires an enormous amount of propellant; definitely a turn-off for a design team seeking efficient solutions.
What this boils down to is that if you want to carry enough propellant to turn your spacecraft around and decelerate, you have to carry that additional propellant with you from the start of the mission. The rocket equation yields a stubborn result — the requirement for propellant increases not proportionally but exponentially in relation to the final velocity required. The initial fuel mass becomes vast beyond comprehension when we apply the numbers to slowing an interstellar craft, which is why the Icarus team, as it looks into deceleration, is examining ideas like magsails, where the incoming vehicle can brake against the star’s stellar wind.
A magsail or, for that matter, various other sail possibilities (Robert Forward described decelerating a manned interstellar vehicle by lightsail in his novel Rocheworld) offers the unique advantage of leaving the fuel out of the spacecraft — you’re braking against a stellar particle flux, or against starlight itself. But whether or not such ideas prove feasible, they’re more likely to at least help if the spacecraft is traveling slower to begin with, making it easier to decelerate further. A slower transit also reduces stress on the vehicle’s engines and structure during the boost phase.
The Case Against Going Faster
Long notes that Project Icarus is far from having answers on just what cruise speed will be optimal — Icarus is a work in progress. But these issues are at the heart of the interstellar quest:
…all of this analysis goes to the heart of whether a flyby probe such as Daedalus is really useful given what it took to get there. The potential science return is massively amplified by performing a deceleration of the vehicle and although it is a significant engineering challenge this is why the Icarus team decided to address this problem; and it is a problem, even if you choose to just decelerate sub-probes. Coming up with a viable solution to the deceleration problem in itself would justify Project Icarus and the five years it took to complete the design process.
Supposing you gave up on trying to stop the probe in the destination system, but simply made your goal to slow it down enough to make protracted scientific observations as it passed through? It’s clearly an option, and again we’re considering a trade-off between the shortest travel time and the ability to maximize science return. Interstellar flight is a challenge so daunting that it makes us question all our assumptions, not the least of which has always been that faster is better. Not necessarily so, the Icarus team now speculates, and perhaps a fusion/magsail hybrid vehicle will emerge, a significant upgrade from the Daedalus design. And this reminds me of something I wrote about magsails back in 2004 in my Centauri Dreams book:
At destination, a magnetic sail is our best way to slow [the] probe down, with perhaps a separate solar sail deployment at the end that can brake the vessel into Centauri orbit. If you had to bet on the thing — if the human race decided a fast probe had to be launched and was willing to commit the resources to do so within the century — this is where the near-term technology exists to make it happen.
Of course, I now look back on that passage and shudder at my use of the phrase ‘near-term’ to describe the vehicle in question, but maybe a very loose definition of ‘near-term’ to mean ‘within the next few centuries’ will suffice (hey, I’m an optimist). In any case, when we’re talking journeys of forty trillion kilometers (the distance to the nearest stellar system) and more, a century or two seems little enough to ask. And while I do believe this, I rejoice at the spirit of Project Icarus, whose team presses on to discover whether such a thing could be attempted in an even shorter time-frame.
To achieve the required velocity change to complete the mission in the required time we’re running up against the limits imposed by the specific impulse being assumed for the fusion drive, the Daedalus team seems to have assumed an efficiency of only 10% for the propulsion, which I assume means only about 10% of the propellant undergoes fusion. Getting even a modest efficiency gain here would improve the propellant mass ratios considerably, so would trapping some of the unfused propellant and recycling it be an option?
I just found this:
http://www.ibiblio.org/lunar/school/InterStellar/Explorer_Class/External_fueled_Drive.html
It feels like cheating, but makes a lot of sense, one way of going about it would be to use an EM launcher to accelerate a long line of pellets of fusion propellant up to just short of Icarus’s cruising speed, Icarus is then launched fueled, burns all its fuel catching up with the stream of propellant pellets, gathers them slowly (the pellets and Icarus are travelling at a very low relative velocity) and Icarus is fully fueled when it’s time to decelerate at its destination.
Because the pellets are able to survive accelerations far higher than a probe could, the EM launcher could be a fraction of the size of a probe launcher.
Andrew W I think you are right about the thrust efficiency assumption. Can we ask how the efficiency has been estimated ? This is an absolutely key parameter, so getting a realistic estimate is what the whole mission profile depends on.
For example, if we can assume close to 100% efficiency, there is no problem (! OK I know) in providing the desired outcome, that is, travel time to neighbouring stars within a few decades and thrust braking on arrival.
If we can assume only a few per cent efficiency, then we have major problems in providing this outcome. So you see it is very important.
Clearly acceleration/deceleration is the main hurdle.
Forgive me if this has been discussed already, but what about using the gravity of the star? If you could get the spacecraft captured in orbit, that could make deceleration a much easier process, as you wouldn’t have to worry about coming to a complete stop before leaving the system. You could gradually decelerate while orbiting and make plenty of observations in the process.
However, I don’t know what the specifications of this would be. How fast could you go and still achieve an orbit in the outer solar system? How would the star type/mass affect this? Would you have to slow down significantly, or would you use a side thruster to change direction until gravity took over, or both?
It’d be useful to know the viability of this.
kzb,
I like the Arthur C Clarke quote – please tell me the reference?
Kelvin
Several theories, including String theory, predict that gravity and electromagnetism unify in higher dimensions. There is a theory, supported by the fact that gravity moves at about the speed of light and yet can escape black holes, that gravity is the weakest force because it can escape normal spacetime. If a electromagnetic field can be made to leave normal spacetime and form a sort of carpet outside it, that would trap gravity into normal spacetime. That would solve Alcubierres problem of unrealistic amounts of negative energy. A vacuum energy deficiency can suck an electromagnetic field out of normal spacetime. That vacuum energy deficiency can be created by the Casimir effect. Manipulating gravity can also be used for much cheaper launches than with chemical rockets. Test if graphene can generate a Casimir effect!
Thinking about the idea of aerobraking I wondered if it might be possible (after the main probe had safely passed) to crash the empty fuel tanks into a convenient KBO, creating a huge nebula that could be used to aerobrake some trailing subprobes, a hundred tonnes moving at 0.1c striking a KBO would release about 10,000 megatons of energy.
But it looks like it could only work if the explosion of the KBO was shaped ie. the mass striking it did so in a way that shaped the distribution of the debris so as to create a relatively dense jet that the subprobes could aerobrake down for millions of kilometres, while decelerating at thousands of g’s, even if such a thing could in theory be engineered, the heat produced through aerobraking would still be a huge problem.
None of the braking solutions discussed here have any potential that I can see to do more than a minuscule fraction of the necessary braking to stop at the target. Certainly not gravity, as the escape velocity argument shows. Certainly not any form of aerobraking, as the heat argument shows. Not any sort of sail, either, because there is not enough wind until it is much to late. Absent any workable and not yet considered out-of-the-box solutions, turning the engine around appears to be the only realistic way to break down to a stop.
While the square-of-mass-ratio argument seems forbidding, a better way to think of it is double-the-travel time, as has been said. That is right, by doubling the trip time we can extend the observation time from a few seconds to many decades, at the same mass ratio. Put this way, braking to a stop really seems like a no-brainer.
Braking to anything less than a stop seems futile, as the gain in observation time will be minutes rather than decades for anything short of a complete stop.
The resolution of the gravitational lens is determined by three things: 1) the diffraction limit, where the extremely large aperture leads to resolutions down well below a meter (cm, even, IIRC). 2) The aperture of the detector elements (think rods and cones of the FOCAL eye), which in order to resolve the Einstein ring needs to be ~10cm, translating to a resolution of ~100 m at nearby stars. 3) The light gathering power, for which the 10^8 amplification factor is relevant. A 100*100 patch on a distance exoplanet would provide about as many photons to a detector element as the same patch on Neptune would without the solar focus. I don’t know what that would do to the resolution. It might be devastating. It is quite possible that FOCAL will be restricted to extremely bright objects, such as stars and AGNs.
I agree Eniac, using some medium in the target system to significantly slow or stop the probe is a dead duck, there’s just not enough there to work with. I think the idea of mailing fuel to decelerate to the probe once it had reached its cruise speed has merit, but if you have a system to launch fuel pellets to the probe at say 0.1c, why even use on-board fusion to accelerate the probe in the first place? Might as well just use the pellets as a form of beam propulsion from the outset, which is outside the Icarus rules.
The rocket equation is only relevant if the spaceship carries its fuel on board.
But you can use a particle accelerator, situated in your home star system, to accelerate the fuel, creating a ‘bread crumbs’ trail.
The ship, equipped with a VERY small Bussard ramjet, will follow this trail, gathering the fuel it needs en route.
Thus, you need not worry about the rocket equation – and you can send more than enough fuel for both acceleration and deceleration, at only a fraction of the cost (the fuel quantity needed will NOT grow exponentially, but linearly).
Andrew W
“if you have a system to launch fuel pellets to the probe at say 0.1c, why even use on-board fusion to accelerate the probe in the first place”
Because only on-board fusion allows you to both accelerate and decelerate; and because on-board fusion converts a little sent fuel into a LOT of energy.
ProtoAvatar: I am afraid this one is a no-go. Particle beams are MUCH too divergent to create a usable trail as envisioned.
All the various deliver-the-fuel-separately ideas require “intelligent” maneuverable fuel packages that can line themselves up, actively. That in itself would not be so much of a problem, the real showstopper is that we lack any remotely plausible way to accelerate macroscopic objects of any kind to relativistic velocity. Such a relativistic gun would have to be obscenely long, and no magnets or charges could be switched fast enough to keep up with the speed. We have trouble enough reaching orbital velocity with guns.
Kare’s microsail concept has merit, but there is a huge gap between the flimsiness of the sail and the enormous acceleration required that seems a very tough nut to crack, both mechanically and thermally.
Consider, however, that at 0.1c, the kinetic energy of a pellet is about the same as could possibly be liberated by fusion. If I recall correctly, Kare has his microsails propel the ship by impact alone. It really improves your design parameter space if you do not have to worry about maintaining fusion. Which, let us not forget, we cannot do yet.
“on-board fusion converts a little sent fuel into a LOT of energy.”
The kinetic energy of a stream of fuel pellets moving at 0.1c relative to the probe is greater than the energy released by fusing that fuel – at least with the fusion efficiencies that have been hypothesized by the Project Daedalus team. If the pellets were zapped to form a plasma that could then be bounced by the probe you would get twice the thrust you would by emitting that mass as propellant at the same velocity by the probe, you would also avoid, at least for that phase of the journey, a lot of wear on your engines.
“Because only on-board fusion allows you to both accelerate and decelerate”
That’s the theory, but there are a couple of practical considerations if you’re talking about using a Bussard ramjet to decelerate at the destination using a stream of pellets that have traveled independently to that destination.
Firstly the pellet stream needs to hold together until it’s captured by the interstellar probe, I think this would be challenging even for the first fraction of a LY during the probes initial acceleration phase, the pellet stream would need to be aimed with incredible accuracy or somehow marshaled as it traveled, for the stream to travel for decades across light years to the vicinity of another star and still be intact…?
Secondly anything traveling at the velocities we’re talking about experiences a lot of heating through collisions with the interstellar medium, this was mentioned by someone a while ago on another thread, from memory, temperatures in the thousands of degrees would be experienced on leading surfaces. I figure even for a stream of fuel pellets, stand-off shields within the stream could keep the H/He pellets frozen by protecting them from being exposed to this leading surface heating, at least for the acceleration phase, but again, given this bombardment by interstellar particles, how could it all be held together over interstellar distances well enough to be utilizable for deceleration?
Another problem with using the fuel to decelerate is that the fuel needs to travel at high velocity to get to the destination at the same time as the probe (unless the pellets are launched centuries before the probe) so to use that fuel you’re back to the rocket equation – the fuel must be slowed as the probe slows – unless you figure on your probe further accelerating a stream of propellant that’s moving faster and faster relative to the probe as the probe slows.
Thanks to Andrew for pointing out what I missed: Decelerating with sent fuel at 0.1c is difficult, as it would require performing the fusion while the fuel is passing through, and expelling the exhaust at even greater velocity towards the front.
Perhaps a suitable pellet sent through a magnetic nozzle at 0.1c could be made to fuse passively and expand out the other end, like in a scramjet, but I doubt that the ~10^-8 seconds or so that fuel and ship can spend together is enough time to perform much fusion regardless of the mechanism. Fusion is not instantaneous, it requires time for the reactions to occur.
Eniac, Andrew W
“ProtoAvatar: I am afraid this one is a no-go. Particle beams are MUCH too divergent to create a usable trail as envisioned. ”
Only if the atoms in the beam are electrically charged. If not, the divergence will be minimal. A small Bussard ramscoop (radius of 2-4 km) could pick the atoms up effectively throughout the journey.
“Decelerating with sent fuel at 0.1c is difficult, as it would require performing the fusion while the fuel is passing through, and expelling the exhaust at even greater velocity towards the front.”
Only if the difference in speed between the fuel and the ship is 0.1c. If the difference in speed is minimal (the fuel being just a little slower than the ship), from the ships’ perspective, the fuel atoms will float in front of the ship, making it easy to perform fusion (if you can fusion atoms in the first place, that is).
And, of course, you can easily send the fuel so as to have approximately the same speed as the ship in every segment of the journey. This means, of course, that you won’t accelerate fuel at an uniform velocity.
If you choose to propel the ship by hitting it with particles whose speed by comparison to the ships’ is high:
The ship will be battered by the particles, resulting in structural damage; you will be unable to decelerate by using this method; and, in order to accelerate particles to 0.1c, you will still need to consume energy (presumably, fusion generated energy) – it’s just that you’ll do it in your home system.
“for the stream to travel for decades across light years to the vicinity of another star and still be intact”
The stream will need to travel between two stars – two gravity wells, not in the vicinity of a star. I’m confident it can be targeted precisely enough.
“anything traveling at the velocities we’re talking about experiences a lot of heating through collisions with the interstellar medium”
All the dust betweeen here and Alpha Centauri can be compressed in a gas at room pressure only mm thick. I doubt friction will generate enough heat so as to make this mission architecture impractical.
“Consider, however, that at 0.1c, the kinetic energy of a pellet is about the same as could possibly be liberated by fusion.”
Yes – you’ll need to consume energy to accelerate the fuel.
Let’s see how much energy you’ll consume by comparison to carrying the fuel on-board (rocket equation):
Let’s say the ship (minus the fuel) needs a certain amount of fuel to accelerate/decelerate.
By using a particle accelerator in your home system, you send enough fuel to only accelerate the ship. And the particle accelerator consumes the energy required to accelerate the fuel.
Now you’re sending enough fuel to both accelerate/decelerate.
You must double the amount of fuel sent. And the particle accelerator will consume twice as much energy.
The consumption grows linearly with increase in delta V. Definitely NOT “back to rocket equation”.
By carrying the fuel on-board, you need to carry the fuel needed to accelerate the fuel which is there to accelerate the fuel etc. The consumption grows exponentially with increase in delta V.
Here’s the thread in which heating through interaction with the interstellar medium was discussed.
https://centauri-dreams.org/?p=14918
ProtoAvatar, when the probe and the fuel stream are approaching the target star both are traveling at about the same speed, no problem in itself for the probe to use the fuel for decelerating, but while the probe slows, the fuel stream does not, if both are traveling at 0.1c at the start of braking by the time the probe has slowed to 0.05c its fuel is passing it at a relative speed of 0.05, to use that fuel the probe has to accelerate that fuel further relative to itself, if it achieves this, by the time the probe has slowed to 0.01c, its fuel is wiping through at a relative speed of 0.09c. Eniac’s comparison with a scramjet is appropriate, there’s a major issue over fusing the fuel fast enough during the nanoseconds of its passage through the engine, easier just to accept the rocket equation.
“you can easily send the fuel so as to have approximately the same speed as the ship in every segment of the journey.”
So for the final phase of deceleration the fuel will be traveling at 0.02c? so it’ll take that part of the pellet stream over 200 years to get to Alpha Centauri, while the probe takes only ~45 years? the fuel gets posted ~150 years before the probe leaves this solar system?
Not a problem, we send the pellets back in time. Recently there was a lot of research showing that time travel is possible.
Apart from the fact that neutral relativistic beams are not in our accelerator repertoire (and won’t be for a very long time), the kind of collimation you are looking for is far out in fantasy-land. Even fully coherent laser beams from a kilometer sized aperture (which we do not have) are more divergent than that. Forget particle beams.
The idea, which isn’t mine, is to vaporize, ionize and then reflect the incoming pellets in a magnetic field, avoiding damage to the ship. This should permit the transmission of up to twice the momentum of the particles to the ship, and the energy in these kinetic explosions would be every bit as powerful as a fusion explosion, rendering the fusion part unnecessary. See http://en.wikipedia.org/wiki/Jordin_Kare#Sailbeam for the whole idea, the most realistic beam-based propulsion concept I have seen.
The rest of what you say in the above quote is true, except that you have to accelerate your particles to 0.1c anyway, whether you use fusion or not. Unless, of course, you have that time machine to send them a few thousand years ahead of time, at a lower velocity. For the same reason, you cannot use fusion fuel for deceleration, either, at least not the way you describe.
Andrew W
About heating through friction:
As said, due to the extremely low density of the interstellar particles, I don’t think heating will pose a significant problem.
About acceleration/deceleration:
Accelerating by using a particle accelerator to seed the ship’s trajectory with fuel is clearly cheaper (less fuel+energy expended) than carrying the fuel on-board.
Decelerating by using the same method, is, indeed, a bit trickier.
One solution – wait a few decades until the slower-travelling fuel needed for deceleration reaches an appropiate position before launching the ship.
Another solution – equip the ship with fuel tanks which are empty during the acceleration phase; these fuel tanks are to be filled when the ship reaches its top velocity, with fuel you sent before the ship, travelling at a litle less than the ship’s top velocity. The ship will then decelerate by using this fuel. There is an disadvantage – for portion of the journey (but only for this deceleration), the sent fuel’s quantity will have to obey the rocket equation, and your particle accelerator will have to consume the appropriate energy to send it on its way.
Of course, the energy the particle accelerator consumes grows linearly with the amount of fuel you accelerate (and its speed), meaning you still expend far less energy in the accelerator’s fusion reactors than what would be required by an exponential curve (such as the one required by the rocket equation in order to send the same amount of fuel on-board the ship).
Of course, the preferred solution will be in between the ones I presented – wait 5-10 years before launching the ship, so that you will have to fill the ship’s tanks later during the deceleration phase; thus, you can send less fuel for the deceleration phase controlled by the rocket equation/you can make the ships fuel tanks smaller.
Eniac
“the fact that neutral relativistic beams are not in our accelerator repertoire (and won’t be for a very long time)”
They are far closer to our technological horizon than interstellar ships, let alone interstellar ships that can accelerate/decelerate with on-board fuel or other propositions discussed here.
“looking for is far out in fantasy-land”
What I just said.
“coherent laser beams from a kilometer sized aperture (which we do not have) are more divergent than that.”
Who said anything about kilometer sized aperture? You don’t have to send the fuell all at once. 1 meter sized aperture and the ship spending larger sections of the journey collecting the sent fuel will be adequate.
Alpha Centauri is 4.3 light years distance from Earth. How much does a laser beam 1 meter wide diverge until it reaches there?
And, of course, the ship can be equipped with a Bussard ramscoop 2-4 km wide – also far closer to our technological horizon than several propositions discussed here.
Going back through the thread I linked to above, it looks like the heating would only be of the order of a few Watts/sq cm at 0.1c, not such a major problem.
I ran some numbers and, apparently, a few of my predictions regarding time intervals were too timid.
Here’s my corrected post:
About acceleration/deceleration:
Accelerating by using a particle accelerator to seed the ship’s trajectory with fuel is clearly cheaper (less fuel+energy expended) than carrying the fuel on-board.
Decelerating by using the same method, is, indeed, a bit trickier.
One solution – wait 100-200 years until the slower-travelling fuel needed for deceleration reaches an appropiate position before launching the ship.
Another solution – equip the ship with fuel tanks which are empty during the acceleration phase; these fuel tanks are to be filled when the ship reaches its top velocity, with fuel you sent before the ship, travelling at a litle less than the ship’s top velocity. The ship will then decelerate by using this fuel. There is an disadvantage – for this portion of the journey (but only for this deceleration), the sent fuel’s quantity will have to obey the rocket equation, and your particle accelerator will have to consume the appropriate energy to send the fuel on its way.
Of course, the energy the particle accelerator consumes grows linearly with the amount of fuel you accelerate (and its speed), meaning you still expend far less energy in the accelerator’s fusion reactors than what would be required by an exponential curve (such as the one needed by the rocket equation in order to send the same amount of fuel on-board the ship).
Of course, the preferred solution will be in between the ones I presented – wait ~20-40 years before launching the ship, so that the first phase of the deceleration can be accomplished by the ‘slower’ fuel which, in the meantime, reached the appropriate position; thus, you will have to fill the ship’s tanks only later during the deceleration.
The rocket equation shows that a small decrease of the delta v correponds to a large decrease in the needed fuel. Which means, of course, that you can send less fuel for the deceleration phase controlled by the rocket equation/you can make the ship’s fuel tanks smaller.”
Surely you mean “centuries” or “millenia”? How slow do we want to decelerate to, specifically?
No, they aren’t. There is not even a conceptual device that would produce neutral beams collimated to the degree you need. Unless you can propose one? Much less even for macroscopic pellets. For ships, on the other hand, we have some pretty good and detailed concepts, one of them is called Daedalus.
Now, let’s see, by diffraction limit, ah, http://www.aleph.se/Trans/Tech/Space/laser.txt, Dspot = 2.44 d (lambda)/Dlens, d = 5 ly = 5*10^16 m, lambda = 5*10^-7 m, Dlens = 1 m, Dspot = 6.1 * 10^10 m.
The answer is: 61 million kilometers.
Of course, particle beams cannot be coherent and diverge MUCH more than laser beams.
Oh, but it is, unless your pellets are very large and/or heat resistant or have cooling systems. Realize that “a few Watts” is like a small light bulb and generates a fair amount of heat. Certainly not something that would permit frozen deuterium to remain frozen for many decades.
To the heat, add erosion, and you see that this is just one of many show-stoppers.
Who said anything about kilometer sized aperture?That was me, trying to stretch things in your favor.
As surely you know, spot size decreases with increasing aperture of the laser. To accomodate your 2-4 km ramscoop (or sail, rather), we would need an aperture of 15-30 thousand kilometers.
Well, not such a major problem compared to the several kw/sq cm at 0.9c mentioned in that thread.
Keep in mind that I’m not advocating a ship collecting fuel pellets at its destination anyway.
My opinion now is that the best method of interstellar probe propulsion would be to use an EM launcher – possibly a coil gun firing metal pellets – to small fly-by probes, the pellets bounced (vapourised and bounced?) with the distance between launcher and probe during the acceleration phase kept down by using higher accelerations. Perhaps 10 tonne probes accelerated at 20g to 0.5c over a distance of about 400 AU.
This gives us a system that can launch additional probes at minimal extra cost, because most of the capital invested in the reusable launcher and the systems that power it.
The high velocity of the probes means that about 1400 star systems with 2000 stars would be in range if we allow a flight time of 100 years.
The same launcher could then be used to accelerate larger fuel carrying deceleration capable probes with lower cruise velocities to the more interesting star systems. I see little advantage in launching the fuel for deceleration of these probes separately, may as well launch the probes fully fueled and save the hassle of catching fuel pellets in flight.
Eniac
“”They [impossible beam collimation] are far closer to our technological horizon than interstellar ships, let alone interstellar ships that can accelerate/decelerate with on-board fuel or other propositions discussed here.”
No, they aren’t. There is not even a conceptual device that would produce neutral beams collimated to the degree you need.””
In my post ‘they’ denoted particle accelerators that can accelerate ions to 0.1c and then turn these atoms neutral.
Considering that at 0.1c, relativistic effects are minimal, even calling these accelerators ‘relativistic’ is not really accurate.
And they are closer to our technological level than many of the propositions discussed here.
About diffraction:
“http://www.aleph.se/Trans/Tech/Space/laser.txt”
To accomodate your 2-4 km ramscoop (or sail, rather), we would need an aperture of 15-30 thousand kilometers.”
Point taken.
There are ways to deal with this problem
For example – you could accelerate fuel pellets, each containing a ferromagnetic substance, permanently magnetised. The fuel pellets will tend to attract each other, opposing diffraction. Additionally, you could increase the size of the aperture (of course, not to thousands of km).
You yourself advocated the solvability of this problem:
“The idea, which isn’t mine, is to vaporize, ionize and then reflect the incoming pellets in a magnetic field, avoiding damage to the ship. This should permit the transmission of up to twice the momentum of the particles to the ship, and the energy in these kinetic explosions would be every bit as powerful as a fusion explosion, rendering the fusion part unnecessary. See http://en.wikipedia.org/wiki/Jordin_Kare#Sailbeam for the whole idea, the most realistic beam-based propulsion concept I have seen.”
Calling a proposition concept based upon pellets accelerated to o.1c “the most realistic beam-based propulsion concept I have seen” and then “There is not even a conceptual device that would produce neutral beams collimated to the degree you need.”?
“Going back through the thread I linked to above, it looks like the heating would only be of the order of a few Watts/sq cm at 0.1c, not such a major problem.”
Oh, but it is, unless your pellets are very large and/or heat resistant or have cooling systems. Realize that “a few Watts” is like a small light bulb and generates a fair amount of heat. Certainly not something that would permit frozen deuterium to remain frozen for many decades.”
The most these ‘few watts’ could do is melt the deuterium pellets located at the front of your fuel trail. It will have little effect on the rest of the sent fuel.
So yes, it’s not much of a problem.
About diffraction:
“For example – you could accelerate fuel pellets, each containing a ferromagnetic substance, permanently magnetised. The fuel pellets will tend to attract each other, opposing diffraction. Additionally, you could increase the size of the aperture (of course, not to thousands of km).”
The problem can be solved even easier, with no need for a ferromagnetic substance or for increasing the size of the accelerator’s aperture.
All you have to do is send fuel pellets with a size of a few centimeters. At this scale, diffraction is no longer an issue/problem.
If diffraction is no longer an issue, some of the parameters for the accelerator/ship can be changed for the better:
First – the particle accelerator:
It can have an aperture of as little as 10m, it has to be able to accelerate H or He to speeds ranging from low to only (by particle accelerator standards) 0.1c, and it must release the fuel as pellets a few centimeters large, made of neutral atoms.
As said, such an accelerator is close to our technological level. And, compared to most support systems for interstellar travel discussed here, it’s dirt cheap.
Second – the Bussard ramscoop (and it is a ramscoop, Eniac, not a sail; it gathers fuel, it does NOT deflect anything in order to propel the ship):
Now it can be as small as 1km; because the fuel found near the ship will always have almost the same velocity as the ship, a magnetic field of only moderate strength can be used to gather the nearby fuel pellets (after these fuel pellets have been ionised).
Such a ramscoop is very close to our technological level; it may even be doable today. And it is, too, dirt cheap by comparison to most other propositions discussed here regarding interstellar travel.
A 0.5c EM launcher? I don’t think so. We’ll perfect black hole propulsion before that (kidding…). First consider how long this EM launcher would have to be, then calculate the transition time of the projectile through a coil and the rate of change of field strength required. I bet both will be way, way, far out of technological reach. While much more practical, we have not yet figured out a good way to reach orbital velocity with an EM launcher, which is ~ 0.00003c.
ProtoAvatar: Diffraction is not a problem with particle beams. I brought up lasers because they are the most collimated beams available. Particle beams can not be coherent, and their collimation is MUCH worse than lasers.
Really? Please elaborate. Can you name any research or engineering projects that come close? How long would this accelerator be? How would it exert force on the projectiles? What acceleration do you have in mind? See my above answer to Andrew’s suggestion.
What can be done, and I have not previously realized, is relativistic neutral particle beams (note: particles, as in atoms, not pellets). It is done by accelerating negative ions and stripping them of their extra electron before sending them off. However, this does nothing to address the collimation problem, which remains insurmountable for particles or dumb projectiles.
As I have said, the only way to keep a beam collimated over interstellar distances is by making its constituents actively maneuver to stay on aim, call them “smart projectiles”. That requires, of course, the projectiles to be macroscopic, with propulsion and navigation systems. As noted above, accelarating macroscopic objects to near light speed is mostly the stuff of fantasy. With the possible exception of those laser accelerated microsails, which you should take a look at. While it is still far from projects such as Daedalus in terms of feasibility, at least it is a start to deal realistically with the issues.
Of course, then there is the ISM heating and erosion problem, which affects any and all such concepts:
Oh, are you now suggesting the cm sized pellets will line up with such precision that they would shield each other? Good luck with that. At least then your ramscoop could be the size of a tea cup, which is an advantage.
More likely, the high density cloud generated by the disintegration of prior projectiles would contribute to the demise of following ones.
This is exactly what I meant. You missed the distinction between smart projectiles and neutral particles, and you did not look at the reference enough to realize those are not pellets, but laser driven microsails.
Read this paper, it will vastly improve your perspective on this matter. It’s main flaw, if I recall correctly, is that it does not deal with the ISM erosion issue, which would be severe for such VERY flimsy sails….
http://www.niac.usra.edu/files/studies/final_report/597Kare.pdf
How many pellets do you want to send? For them to act on each other magnetically, they’d have to be no more than centimeters apart. That means there would have to be, let’s see, ah, 0.1*3*10^8*100 = 3 billion per second.
As said before, with incoherent beams of particles or projectiles, diffraction is irrelevant. I brought it up only as the limit on lasers, which are the best collimated beams we have. The divergence of particle or pellet beams is the consequence of non-zero transverse velocity. To keep beam spread at 1 km over 5 ly at 0.1c, you would need to control transverse velocity of projectiles to within 1 km / 50 years = 0.000,000,7 m/s, while accelerating them in the longitudinal direction to 30,000,000 m/s. Furthermore, you have to maintain this precision during the long flight, in the face of gravitational, magnetic, collisional and all sorts of other unpredictable small, but relevant forces. This is completely infeasible for too many reasons to list. It is approximately equivalent to shoot a gun from the Earth to the moon, hitting a penny there every time.
Eniac
About diffraction:
Diffraction is a direct consequence of Heisenberg uncertainty.
The position of photons/particles becomes too well defined causing their momentum to become too fuzzy on the axis perpendicular to the direction of travel.
If you use fuel pellets a few centimeters large, their position will not become too well defined, meaning their momentum will not become fuzzy.
In other words, you don’t have to worry about diffraction.
About heat:
A few watts per cm2 per journey will barely manage to melt a few fuel pellets, regardless of whether they shield each other or not. Meaning heat it’s a non-problem at 0.1c.
About technology:
A particle accelerator and a Bussard ramscoop with the properties I mentioned are FAR closer to our technological horizon than most propositions heard here for interstellar travel.
We have them today? No. But we’re close.
Eniac
About the sailbeam:
It has major disadvantages: its sail will have to support impacts from projectiles at 0.1c for the acceleration phase, meaning the protecting magnetic field will have to be strong meaning it will need a lot of power meaning it will have to carry heavy reactor/fuel for it.
But most of all, it can not decelerate from 0.1c to 0 by using the scarce interstellar particles unless its sail is gigantic (aka imprectically large) meaning if it wantss to decelerate it can not afford to accelerate to 0.1c meaning the journey will take much much longer.
Eniac, thanks for your last couple of links, I’ll have to bow to your greater knowledge.
I’ll take solace in that the sail beam paper you linked to works on the principle of accelerating pellet sized particles (microsails) that are bounced by the probe in theory achieving a similar outcome in terms of probe performance to what I was looking for.
I was surprised by the high initial rate of deceleration that was suggested achievable using the magsail.
ProtoAvatar said: “A few watts per cm2 per journey will barely manage to melt a few fuel pellets” Watts is a measure of power; Joules/second, that’s more intense radiation than Earth receives at noon on the equator from the sun (0.13w/cm^2).
Andrew W
“ProtoAvatar said: “A few watts per cm2 per journey will barely manage to melt a few fuel pellets” Watts is a measure of power; Joules/second, that’s more intense radiation than Earth receives at noon on the equator from the sun (0.13w/cm^2).”
As said – this much heat per journey is a non-problem.
Close? Please name examples.
From what little I know, our best pellet accelerators (aka guns, a well researched field) do 2 km/s maybe 5 or 6 under exceptional circumstances. That leaves us four orders of magnitude short. In terms of energy, 8 orders of magnitude. Not close. Not close at all.
This amount of heat will vaporize every single one of the pellets in a matter of seconds, if they are made of frozen deuterium. If Andrew is correct with his numbers, the equilibrium temperature should be around 600 K.
I don’t know what you mean by “per journey”, maybe you do not fully understand the physical concepts involved, see Andrew’s explanation.
You have provided very little reasoning with your latest assertions, you have repeated my own statement that diffraction is irrelevant as if it was disputed, and you have not attempted to explain how you plan to reach the 0.000,000,7 m/s transverse accuracy, the penny-on-the-moon shot. Or what you envision that fantastic accelerator to look like (hint: It would be very, very, VERY long). If you cannot come up with more persuasive reasoning, I will cease my futile attempts at getting anywhere with this discussion.
Of course it has disadvantages. But it is an honest and ingenious attempt to deal with all the difficult issues in some reasonable depth (except ISM erosion, IIRC), to the extent a NIAC study can. The best I have seen, although Forward’s giant lens and lightsails and his starwisp are close contenders. In comparison, your idea is full of unbridgeable gaps, some of which I have pointed out.
Eniac
“Close? Please name examples.”
And you name examples of any sails we’ve built that are close to magnetic sails large enough/strong enough to stop a ship travelling at 0.1c by friction with the incredibly rarefied interstellar medium.
By comparison, ‘4 orders of magnitude’ is far closer to our technology level.
The LHC/a current rail-gun is closer to the particle accelerator needed than today’s sails are to such a magsail.
As for the ‘8 orders of magnitude’ more energy, this would come from fusion reactors.
Not to mention, the sailbeam needs similarly large amounts of energy – for its laser – another piece of technology we don’t have at anything close to the ‘level of magnitude’ required (I’m reminded of the ‘star wars’ program).
“how you plan to reach the 0.000,000,7 m/s transverse accuracy, the penny-on-the-moon shot.”
First – it’s not 1km/5ly.
It’s 1km/~2.3/ly (unless you want to spend 100-200 years waiting before launching the ship); ~0.0000014m/s error.
Plus, not all fuel pellets will need to be accelerated to almost 0.1c.
How do I plan on attaining this accuracy?
There’s no need to make the particle accelerator ‘very, very, VERY long’, Eniac.
You make the particle accelerator relatively short – in it, the fuel pellets are accelerated up to 0.1c.
A few light minutes away, around the trajectory of the fuel pellets, you put devices whose purpose is to measure the direction of the fuel pellets exiting the particle accelerator and, if necessary, correct it.
You launch shuch devices a few light minutes before the ship, too – their function will be to correct the direction of the fuel pellets that are about to be gathered by the ship.
“This amount of heat will vaporize every single one of the pellets in a matter of seconds, if they are made of frozen deuterium”
Eniac, fuel pellets can be easily made to withstand a few watts/cm2 – j/s/cm2 – ~600k – if you prefere:
By NOT making the fuel pellets only out of deuterium;
By coating the fuel pellets (shielding).
The heat is not a serious problem.
“In comparison, your idea is full of unbridgeable gaps, some of which I have pointed out”
The ‘gaps’ in the sailbeam ideea make the ‘gaps’ in my ideea seem manageable.
“From what little I know, our best pellet accelerators (aka guns, a well researched field) do 2 km/s maybe 5 or 6 under exceptional circumstances. That leaves us four orders of magnitude short. In terms of energy, 8 orders of magnitude. Not close. Not close at all.”
The speed of light is ~300km/s.
0.1c is ~30km/s.
Eniac, 5km/s (muzzle velocity for a rail gun) is NOT 4 orders of magnitude lower than 30km/s (an order of magnitude = times 10).
5km/s is not even 1 order of magnitude lower than 30km/s.
In my previous post I started from the assumption that these numbers of yours were correct. They are not.
Meaning we are far closer to building a particle accelerator along my specifications that you think.
Eniac
Well, this is embarassing.
I mistakenly took the speed of light as 300000m/s instead of 300000km/s. That’s what happens when you write posts when you’re half asleep.
I would be obliged if you would ignore my previous post. Obviously, it’s not accurate.
The relationship between length, acceleration, and final velocity is L=1/2 v^2/a, if I remember correctly. Let us make it “relatively short”, say 1 million km (short relatively to the distance we are going). The acceleration you would need in such a “short” accelerator is 0.5*3*10^7*3*10^7/10^9 = 4.5*10^5 m/s^2 = 45,000 g.
There is no electromagnetic device that could possibly exert such large acceleration on a macroscopic object, continuously. Even if you could rig it, 1 million km of it would be structurally problematic (I am trying for the understatement of the year, here…) and rather expensive (enter this, too).
You will say “make it circular”, ignoring the fact that the acceleration required to go around in a circle of a “relatively short” radius is governed by the same above equation, give or take a small factor.
You will say “what about particle accelerators?”, not realizing that only particles have the very large charge/mass ratio by which they can be accelerated at such incredible rates. They also have no structure that could be disintegrated by large acceleration.
For an exercise, look up or calculate the pressure needed to contain a certain amount of deuterium in a certain volume at 600K, design a structure that does so and calculate its fuel/structure mass ratio.
Shielding does nothing to avoid heating. Also, consider that every impacting particle is likely to knock off a little piece of your shielding, and calculate how much shielding you would need to keep going for a decade or two. Warning: there is little data on high-energy particle erosion, most of it in connection with engineering accelerator targets, which have to be changed frequently because of erosion.
I won’t, because I consider that impossible. Just like your fuel pellets. My baseline for an interstellar mission is Daedalus, modified to support deceleration, by taking more fuel or simply going half as fast. I think the Icarus team is generally on the right track with that.
Kare’s sail-beam, as I have said, is plausible *for a beam-based proposal*, but not very plausible at all in comparison with Daedalus/Icarus.
Now you are inching a little closer to actively guided projectiles, which is indeed a way to make progress here. You need to explore by what means you will launch those “devices”, and eventually you will realize that the pellets themselves will have to be them. Now, if you would then realize that laser-pushing is a more realistic form of acceleration than EM accelerators (because it replaces millions of km of magnets with millions of km of laser beam), you would be homing in on something much like the Kare proposal.
“The speed of light is ~300km/s.
0.1c is ~30km/s.”
No, c is 300,000 km/s
oops! Shouldn’t have commented until I’d read your last comment.
wiki:”The Yugoslavian MTI (MTI – Military – technology institute) developed, within a project named EDO-0, a rail gun with 7 kJ kinetic energy, in 1985. In 1987 a successor was created, project EDO-1, that used projectile with a mass of 0.7 g and achieved speeds of 3,000 m/s, and with a mass of 1.1 g reached speeds of 2,400 m/s. It used a track length of 0.7 m.”
That comes out at 6,428,571 m/s^2 acceleration for the 0.7g projectile, and I’ve come across similar accelerations for other rail guns. 45,000g I think is a very conservative figure for the achievable (initial at least) acceleration.
Eniac
“Kare’s sail-beam, as I have said, is plausible *for a beam-based proposal*, but not very plausible at all in comparison with Daedalus/Icarus.”
When it comes to acceleration/deceleration, Daedalus/Icarus is unfeasible – unless you’re willing to spend a LOT of time on the journey.
“”Eniac, fuel pellets can be easily made to withstand a few watts/cm2 – j/s/cm2 – ~600k – if you prefere:
By NOT making the fuel pellets only out of deuterium;
By coating the fuel pellets (shielding).”
[…]pressure needed to contain a certain amount of deuterium in a certain volume at 600K[…]Shielding does nothing to avoid heating.”
Yes, shielding for the pellets will be challenging – but not impossible.
But I had another solution – make the pellets out of a substance rich in deuterium or helium, substance that remains solid at 600k.
“You will say “make it circular”
You will say “what about particle accelerators?””
I say ‘what about a high power laser’?
Make the fuel pellets in the form of lenses (as in the sailbeam proposal), out of a substance rich in deuterium or helium, that can withstand its acceleration by laser and 600k once accelerated.
It won’t be accelerated by the laser energy as effectively as the microsails in the sailbeam proposal (due to the necessity of including fusionalbe material in its composition); but it will allow for both acceleration and deceleration of the starship.
And – somewhat more exotic – I say ‘make it circular, around a moon/planet’.
An satelite stays in orbit around a moon/planet because it follows the curvature of the space around the planet, not because it’s accelerated in any way. From its perspective, the satellite flies in a straight path; from an exterior perspectice, it follows a circular path.
Thus, you can accelerate the pellets as if you had a straight accelerator of arbitrary length.
The moon/planet’s motions will make targeting the pellets (with the accelerator/devices light-minutes distant) more difficult, but far from impossible.
“”A few light minutes away, around the trajectory of the fuel pellets, you put devices whose purpose is to measure the direction of the fuel pellets exiting the particle accelerator and, if necessary, correct it.
You launch shuch devices a few light minutes before the ship, too – their function will be to correct the direction of the fuel pellets that are about to be gathered by the ship.”
You need to explore by what means you will launch those “devices”, and eventually you will realize that the pellets themselves will have to be them.”
Not necessarily.
The devices near the accelerator/laser won’t have to be launched at all. they’ll keep their position by using fusion-based propulsion (much like the propulsion of the starship, only on a far smaller scale)
The devices in front of the ship will be launched on the starship and will be equipped with their own fusion-bsed propulsion. When the starship nears a portion of its trajectory seeded with fuel pellets, these deveices will be sent and will accelerate to a few light minutes in front of the starship.”
About the circular accelerator around a moon/planet:
The obvios problem with it is that, once the pellets reach escape velocity, they’ll tend to leave the orbit of the moon/planet, at which point you’ll have to start delfecting them electromagnetically, ‘helping’ gravity.
Fortunately, Andrew W put things in perspective.
6,428,571 m/s^2 acceleration was acieved decades ago – and potentially much larger acceleration values can be reached.
As it turns out, a circular accelerator is feasible after all – without bothering to build it around a planet.
I understand that 45,000 g is not so unusual as an acceleration, but we have to sustain that contact free and continuously for a long time, at extreme velocities.
There is a lot of interesting stuff on coil guns and the like here:
http://www.coilgun.info/theorymath/electroguns.htm
The current state of the art being a few thousand km/s, using projectiles whose mass is entirely devoted to the electromagnetic interaction. With lots of serious hurdles to getting better. With muzzle energy as the relevant success parameter, of which there are 8 orders of magnitude to go.
Good to speculate about in terms of getting to orbit, but interstellar? Please…
“only particles have the very large charge/mass ratio by which they can be accelerated at such incredible rates. They also have no structure that could be disintegrated by large acceleration.”
As proven by experiment, rail gun bullets – with their charge/mass ratio – can be accelerated at far more than 45,ooog.
Also – as per the sailbeam proposal, microsails should be able to withstand the brutal acceleration imparted by the laser.
Fuel pellets (made out of a substance containing deuterium or helim) should be able to withstand the much smoother acceleration curve of an accelerator.
ProtoAvatar: “But I had another solution – make the pellets out of a substance rich in deuterium or helium, substance that remains solid at 600k.”
It really is time to drop this idea of mailing the pellets over interstellar distances.
There are not molecules that have He as a component that remain solid at 600k because He is noble.
Any high melting point molecule that incorporates hydrogen is only going to have hydrogen as a minor component.
Are you still arguing this all to avoid the rocket equation for the deceleration? Because you’ve now removed any possibility of fusing the H/He as it whips through your buzzard ramjet at some high relative velocity, you’ll first have to reprocess the pellets to remove all these impurities., if you some how manage to fuse the H/He without removing the shielding etc you’re going to end up with a far lower fraction of the total pellet weight undergoing fusion which will lower the Isp.