NERVA starship shuttle or even SSTO

[-Velocity-]

So how do we get more fuel for our spacecraft?

Since we're already ignoring the laws of physics, I'd say create an anti-gravity field, and use the anti-gravity field to create a perpetual motion machine. Have the perpetual motion machine drive a generator that charges up some capacitors. Once the capacitors are at full charge, you discharge them as the power source for a particle accelerator that creates antimatter. Cool the antimatter and store it in a magnetic trap. Keep doing this for a while, then hold the alien planet for ransom with your giant antimatter ball.

[Nibb31]

No, they aren't. The nuclear engines already exist and have been tested. (so what if they were only car sized prototypes that never flew, they still worked)

Ablative laser propulsion also exists, and the technology to make lasers big enough now exists as well.

None of those technologies have been capable of launching a rocket and reaching hypersonic speed on an SSTO profile. They are theoretical propulsion methods, not current or even near-future technology. And the way you describe them is just technobabble.

And again, they are convoluted answers to a simple problem. Chemical rockets can nearly do what you want. In a hypothetical future cut off from economical reality, Skylon could do it. Heck, I wouldn't be surprised if a Falcon 9 could make it as the first reusable SSTO. It would have to launch without an upper stage or payload, and you would probably want to stick a heat shield on it have it reenter nose-first, but the dV on its own probably isn't far from what's needed...

[PB666]

Since we're already ignoring the laws of physics, I'd say create an anti-gravity field, and use the anti-gravity field to create a perpetual motion machine. Have the perpetual motion machine drive a generator that charges up some capacitors. Once the capacitors are at full charge, you discharge them as the power source for a particle accelerator that creates antimatter. Cool the antimatter and store it in a magnetic trap. Keep doing this for a while, then hold the alien planet for ransom with your giant antimatter ball.

The secret is not to get more fuel but to get that fuel to be ejected at higher speeds. The problem is that uses alot of energy, so that you need an energy generator that actually coverts a sizable more mass, and a device that can do so considerably more rapidly than currently possible, and withstand much higher temperatures.

Theoretically laser containment fields like use in a fusion reactor can contain and super heat a stream of gas that is ejected at 100,000s or 1,000,000s of meter per second. All you have to due is use excitation frequencies that correspond to the absorption spectrum of the gases in a large number of states.

Bill has caught the essential problem of the line of conversation, the whole thing begins with an essentially illogical proposition, if we had a warp drive that would take only 10 days to get from Earth to point X (lets say 20 to 100 ly), or 800 to 4000 times the speed of light, would make it a material composed almost entirely of the not-known-to-exist Tachyon. Since it is almost entirely tachyon it could not be normal matter and thus it is unnecessary to speculation the best configurations for a lander.

Fundementally if warp is possible its going to be sublight and very costly which means it might take 300 years to go 100 light years, you are not going to waste your time with what is described basically a flag planting mission.

[Exoscientist]

Chemical SSTO is certainly possible if you have a throttlable engine high-thrust engine. It's chemical SSTO with re-entry landing systems and a usable payload that is pretty much impossible.

I still think that if you are stuck on using SSTO, then VTVL (rocket+tankage+payload) is more efficient than a spaceplace (rocket+tankage+payload+wings+hydraulics+landing gear). Horizontal landing requires xtra hardware that necessary eats into your payload.

My opinion, even reusable chemical SSTO with significant payload is doable now. You need to run the rocket equation to see how high can be the delta-v with both ISP and mass ratio maximized to see why. It's so high, either vertical landing or horizontal landing would work.

Bob Clark

- - - Updated - - -

You are asking a lot of chemical propulsion, even for a tripropellant engine. The russkies didn't think they could make that work, not without an air launch assist and an expendable external tank. Going nuclear with a higher isp would give you some mass fraction breathing room to stick the heatshield and ISRU equipment in.

The Russians had particularly poor mass ratios for their stages, but great ISP's for their engines. Perhaps because their stages were so heavy is what led them to develop such high efficiency engines.

On the other hand the Americans had rather poor ISP engines. Perhaps this is what led them to develop such lightweight stages.

You need both high ISP and mass ratio for a SSTO. Just marry a hign ISP engine with a high mass ratio stage and SSTO isn't even hard.

Bob Clark

[shynung]

My opinion, even reusable chemical SSTO with significant payload is doable now. You need to run the rocket equation to see how high can be the delta-v with both ISP and mass ratio maximized to see why. It's so high, either vertical landing or horizontal landing would work.

No offense, but can I see your calculations?

Edited March 9, 2015 by shynung

[Psycix]

2. For an even harder challenge, it's an SSTO that must make orbit with no missing parts except fuel

The moment you depart the water surface, you jettison the modules containing the skis and any floats.

-snip-

Once you have full tanks, you jettison the modules containing the electrolysis equipment.

-snip-

jettison the modules containing your main wings and the compressor turbines and all the equipment needed to drive them.

-snip-

You shed part of your ramjet air intake

This stings me.

I doubt that a horizontal takeoff spaceplane SSTO is the way to go.

I think it might be more effective to simply use NERVA-like nuclear thermal rockets in a vertical configuration. Going straight up makes the airframe a lot easier and lighter than having to have a hypersonic plane.

You could use the nuclear engines with multiple fuels. At first you push CO2 or N2 or something like that through them for low-isp (relatively) and very high thrust. Later in flight, once you have exited the atmosphere, you gradually switch to H2.

9K delta-V with an SSTO is a lot though, so I would suggest to top the orbit off with electrical propulsion, using the nuclear reactors to generate immense amounts of electricity.

But I've got a better idea:

Send the ship up in pieces.

Reusable first and second stages fly up and down until the entire ship is up there. This removes many limits to how big that ship can be.

Edited March 9, 2015 by Psycix

[EzinX]

But I've got a better idea:

Send the ship up in pieces.

Reusable first and second stages fly up and down until the entire ship is up there. This removes many limits to how big that ship can be.

So the ship is 2-3 stages. ISRU fills up the tanks on the ground. Bottom stage is methane, upper stages probably hydrogen. The top stage is egg or sphere shaped, and the fuel/oxidizer tank is also a heat shield. The bottom two stages both have landing legs.

Chronology :

1. Mothership launches all 3 stages as separate spacecraft. The vehicles that will become the bottom 2 stages are each wrapped in a egg shaped heat shield. None of the vehicles has much rocket fuel inside it. The extra volume inside the bottom 2 stages is packed with ISRU equipment and whatever you need to set up a base camp. (the bottom 2 stage rockets are of course cylinders to reduce air resistance on launch, so the difference between the cylinder and the egg is where all the gear is packed)

2. All 3 vehicles land close to each other, propulsively of course. You set up a base camp.

3. While the vehicles are empty of fuel, you build a launch gantry, with a crane, from materials you brought, and stack your 3-stage rocket. Maybe you launch out of a hole in the ground or something so you don't have to have structural supports.

4. Fill up all 3 stages with fuel via ISRU. Planet needs a source of carbon, hydrogen, and oxygen. Mars, Venus, Earth, the Moon technically all qualify.

5. Launch the stack. Each lower stage drops off non-destructively with enough fuel in the tank for a propulsive landing right at your crude dirt spaceport.

6. Top stage has enough fuel to rendezvous with the mother ship. It probably even has enough fuel to transfer some fuel and cargo to the mother ship, leaving enough for reentry and a propulsive landing.

7. Top stage never sheds it's heat shield - it's a gigantic egg shaped fuel tank with an engine at the bottom. Reentry, propulsive landing.

Rinse and repeat. Our machines today will age under this kind of stress and be only capable of a limited number of launches before something cracks or wears or breaks. Self repairing machinery is of course theoretically possible, however, since living organisms can do it.

Ignoring the spare parts/material aging problem, each trip only consumes fuel gathered by ISRU. Interestingly, for an earth-like planet, almost all of the rocket exhaust would be captured by the atmosphere, so you could continue to do this process forever - the only thing actually consumed is the nuclear fuel for the ISRU's nuclear reactor, or the sun's fuel if you are using solar. This is a much better idea that anything presented so far. It's practical - it's what SpaceX is trying to do, it merely requires a much more reliable version of their technology. It leaves the nuclear reactor on the ground. Nothing is consumed - the mothership actually gains net fuel and cargo doing this.

The only thing you lose is that the lower 2 stages and your ISRU equipment is pretty much a donation to this planet's gravity well.

Well. One bit of futuristic engineering. The lower 2 stages are ultimately spacecraft with efficient methane or H2 burning engines. After you launch the whole stack for the final time, you might be able to disconnect the landing gear and excess weight from the lower stages and launch each of them separately SSTO. That would give you 2 empty rocket cores that the mother ship can recover.

This reduces by a little bit the amount of stuff the mothership loses per planet explored. You could also haul your base camp, at least the part not needed for ISRU, back into space a bit at a time.

Oh. If you wanted the capability to do this forever, to have a true "shuttlecraft", then each cargo run to the mothership would need to haul back a small quantity of raw materials mined from the surface below. Do this enough times, and assuming the mothership has an onboard manufacturing plant capable of making aerospace grade parts, and the mothership could actually build replacements for all the equipment you're leaving on the planet's surface.

This also solves your material aging problem somewhat. The mothership would use the raw materials to 3d print new parts for the rockets in use, letting you replace them, although this can only be done if your part MTBF is high enough.

This would allow you to continue your "5 year mission of exploration" indefinitely.

Edited March 9, 2015 by EzinX

[Psycix]

You would probably want multiple reusable launch vehicles for redundancy. Maybe even a way to manufacture new ones. (If your starship is truly big, is likely to have a full scale manufacturing unit already.) I imagine it would allow you to recycle a stage that had many runs into a brand new one.

If the ship is big enough, and the number of rides large enough, the reusable launch vehicles will be nothing compared to the complete ship.

You'd probably want to keep as much of the big ship as possible in orbit though. No need to bring the entire warpdrive down to the planet. Orbital reconnaissance is going to be important anyway.

In all likelyhood you would also want deploy some hefty probes in various orbits to scan the planet.

[EzinX]

I really appreciate your contributions to this thread, Psycix. You didn't try to fight the OP and came up with a plausible, technically feasible solution.

[GreeningGalaxy]

I've been considering the things you might do on arrival to a planet in another star system for a hard-SF story I'm writing. My plan so far is something like this:

Phase 1:

-Begin arrival burn with a nice powerful antimatter engine. (This could either be the velocity-match burn for an Alcubierre starship, or the post-skewflip burn for a sublight torchship like my story uses.)

-At some point during arrival burn, start dropping probes, with a little bit of thrust so you can make them encounter as many planets as possible on their fast flyby trajectories. These will provide forward reconnaissance, arriving in-system long before you do and providing some preliminary analysis of various planets before they go shooting off into interstellar space again, Voyager style.

-If your crew are in cryogenic stasis, there's no need to wake them up yet. It'll be a while before you get there. That said, wake up the commanding officers right away if any of your probes receive any weird radio signals or see signs of civilization. And if any of them get shot down as they whiz past the planet, you might want to consider turning your ship away from there ASAP.

Phase 2:

-Complete arrival burn; capture into orbit of desired planet and shut engines down.

-Deploy hordes of scanning satellites into various orbits, thus pre-establishing a GPS network and obtaining a detailed surface map of the planet.

-Drop hordes of tiny UAVs into the atmosphere. Get even better map of the planet, take atmospheric data (you could probably tell whether or not this planet had an atmosphere before you left home; we can already do that with some planets today, and our tech for that kind of thing is only going to get better), find out how hard it's going to be to keep the crew alive down there. Some of these drones could also be dropships for rovers, if you like.

-Inflate some orbital habitats.

-Now wake up the crew, if they were frozen. For the moment, they can move into those nice orbital habitats you just inflated while they regroup, read over all that neat map data from the planet, and get ready for their next moves. This step and the previous one are optional if your crew weren't frozen.

Phase 3:

-Pick a nice spot to land on the surface. You'll probably want it near the ocean for refueling reasons; if you're worried about biological contamination, you might want it to be offshore. In any case, make sure it's a nice day down there.

-Get a landing party together and get in your descent/ascent vehicle. This will probably be a big planey thing with giant fusion engines. You'll probably have fusion if you have antimatter (read: at the very least, having antimatter means you'll have antimatter-initiated microfusion at the very least). You'll probably have antimatter because not much else will give you the ÃŽâ€V you need for that arrival burn.

-Descend. Ideally, this won't need much in the way of engine burning; you'll be able to just fire a short retrograde pulse to drop into the atmosphere, then glide in, pull the airbrakes, and switch over to your vertical engines once everything stalls. You'll probably want to have vertical engines, since alien planets aren't known for their abundance of runways. Your descent vehicle doesn't have to be a plane; If you like, it can just be a main-engine-down style lander, but then reentry will be more complicated and harder to control.

Most of your engines will be fusion thermal jets, which use the local atmosphere as propellant the same as a thermal rocket does. You might be able to get away with large fusion-electric ducted fans for your VTOL engines if the gravity is low enough/your reactors are powerful enough, or you could just have some high-bypass thermal turbojets capable of aiming down somewhere in there.

-Touch down. If you're coming down over the sea, inflate some kind of nifty skirt around the bottom of your plane so you don't sink or flip. If this is on land, look for someplace level. As a third alternative, you might skip the landing step entirely and instead inflate a big blimp envelope over you and hang from that for a while.

Phase 4:

-Get some science done. You're on an alien planet you've never seen before; you know the drill.

-Refuel. If you decided to come down near the ocean, pick up some of that water and stuff it in the tank; it'll do nicely as propellant. You can also electrolyze it into hydrogen if you prefer. Whatever you do, be aware of who you scoop up in that sea water. Even if you suck it through a fine mesh, there's going to be microbes in it, probably ones you don't want to haul off the planet with you. Boil the water, give it a dose of hard radiation, do whatever it takes to be sure you kill everything in it, and then test it to be absolutely sure before you leave the surface.

Of course, ISRU is optional. You'll use hardly any propellant getting down to the surface, and you won't need much more getting back up. If you don't want to haul an extra-big, extra-flexible landing craft with you, you might not want to bother with all that risky surface-refueling business. If you're trying to use a weak propulsion system like current-day fission (or worse, chemical rockets), you'll certainly need the ISRU, but if you've got a respectable fusion engine or two, you'll probably be looking at an Isp in the thousands and won't have to worry about most of the takeoff due to those nifty thermal turbojets.

Phase 5:

-Lift off. As mentioned before, the first couple km/s of velocity won't be hard to get ahold of, they'll just cost you a few kilos of deuterium and tritium. You'll need variable-geometry intakes and a number of other fun things to keep your engines from exploding at high speed, but by the time you get to thermal rocket handover, you'll be doing some serious speed. In closed-cycle mode, your engines will probably have an Isp high enough that reaching orbit won't be an issue.

-Dock with your mothership.

Phase 6:

-Do as you please. If you've come on a sublight ship, your ultimate goal might be to establish a permanent colony or research station. If you came on something FTL and can hop home whenever you please, maybe this mission is done.

[EzinX]

Fusion thermal jets? How do these work?

I'm serious. On the Talk Polywell forums, there was a discussion about this problem. One thing all the posters in the discussion - many of whom have engineering degrees - is that even if polywell does work, and gives you substantial energy : mass clean fusion, you still cannot easily make a spacecraft capable of reaching orbit.

Simply put, even with polywell (a method that might not work that if it does work, would be a lot cheaper and easier than ITER and other fusion experiments), fusion needs a nice clean vacuum, big honking magnets, and everything to be just right, or it abruptly quits working. Atmospheric air would contaminate your engines and cause them to stop almost immediately.

It would be a decent power source for a spacecraft in space, of course, because it would give a very high power : mass ratio relative to current nuclear fission reactors, which are heavy and require a heat engine. (this particular fusion reaction produces electrons directly as it's main output product which you would decelerate via charged grids to drive a circuit forward)

Nevertheless, the weight of the reactor and the electric thrusters is far too heavy to reach orbit. You can fly in the atmosphere just fine, of course, by using the electricity from fusion to turn superconducting electric motors and fly subsonic like a pure bypass jet aircraft. (you use hydrogen and boron for fuel, so you could fly at the speed of a modern-day airliner for months, probably)

But it's not getting you back into orbit, and I know of no variant that does. Antimatter, though - that would do it. Well, sorta. You are going to have a heck of a time shielding your crew from the products of that reaction.

[Exoscientist]

No offense, but can I see your calculations?

Elon Musk gave a lecture at the Royal Aeronautical Society where he gave the propellant fraction of the latest Falcon 9 as 96% or perhaps 95.5%. It's at about 30 minutes into the lecture:

These correspond to a mass ratio of 25 or 22.22. The Russian RD-170 engine has a vacuum ISP of 338 s:

RD-170.

http://en.m.wikipedia.org/wiki/RD-170

Then a Falcon 9 v1.1 first stage using a RD-170 engine instead of the Merlin's could get a delta-v of:

338*9.81ln(22.22) = 10,387 m/s.

This is well above the approx. 9,000 m/s needed for orbit.

Bob Clark

Edited March 10, 2015 by Exoscientist

[EzinX]

Taking 9000 m/s as you gave it, with the sea level ISP, it needs to burn 94.89% of it's fuel.

Most of the flight will occur in near vacuum, however. Without exact numbers, let's say that 2/3 of the flight is the upper part and circularization. So an average ISP of 328.

Then you end up with 94% of your rocket needing to be consumed as fuel. So, 1.5-2% of the rocket can be payload. Falcon 9 v1.1 lower stage is 403k kilograms, so you could get 8k kilograms barely into orbit with a single stage. Some of that orbiting mass would need to be some kind of RCS system so we don't tumble out of control, and some radios and solar panels would be nice, etc.

[Nibb31]

Remember that Falcon 9 is made of aluminium. You could improve performance by building it with modern composites, or even some sort of futuristic alloy since this thread is in the realm of warp drives. A similar incremental improvement in engine technology should make a VTVL SSTO possible with a small payload.

Personally, I'd go with something like DC-X or X-33. There was nothing stopping these projects from working on paper. Whether they made sense economically or politically is another matter, of course.

Edited March 10, 2015 by Nibb31

[EzinX]

Remember that Falcon 9 is made of aluminium. You could improve performance by building it with modern composites, or even some sort of futuristic alloy since this thread is in the realm of warp drives. A similar incremental improvement in engine technology should make a VTVL SSTO possible with a small payload.

Personally, I'd go with something like DC-X or X-33. There was nothing stopping these projects from working on paper. Whether they made sense economically or politically is another matter, of course.

Well, as mentioned earlier, this problem becomes quite easy if you have a stage or 2 underneath to get the top-most stage moving, and then you recover those stages by having them land. You can have much better mass ratios, with the top-most stage being half cargo by mass and using a vacuum optimized liquid hydrogen engine.

Interestingly, with ISRU, there's another optimizing factor at work. Not only do you not want to throw anything away - since starship mass allotment wouldn't be cheap - but you want to get the most cargo to orbit per unit of fuel burned you can (since however your ISRU plant works, it's gonna take time to produce fuel). I don't think SSTOs are optimal in that respect : they require you to burn fuel carrying a bigger engine than you need for that phase of flight, and a mostly empty fuel tank.

[Rune]

Fusion thermal jets? How do these work?

I'm serious. On the Talk Polywell forums, there was a discussion about this problem. One thing all the posters in the discussion - many of whom have engineering degrees - is that even if polywell does work, and gives you substantial energy : mass clean fusion, you still cannot easily make a spacecraft capable of reaching orbit.

Simply put, even with polywell (a method that might not work that if it does work, would be a lot cheaper and easier than ITER and other fusion experiments), fusion needs a nice clean vacuum, big honking magnets, and everything to be just right, or it abruptly quits working. Atmospheric air would contaminate your engines and cause them to stop almost immediately.

It would be a decent power source for a spacecraft in space, of course, because it would give a very high power : mass ratio relative to current nuclear fission reactors, which are heavy and require a heat engine. (this particular fusion reaction produces electrons directly as it's main output product which you would decelerate via charged grids to drive a circuit forward)

Nevertheless, the weight of the reactor and the electric thrusters is far too heavy to reach orbit. You can fly in the atmosphere just fine, of course, by using the electricity from fusion to turn superconducting electric motors and fly subsonic like a pure bypass jet aircraft. (you use hydrogen and boron for fuel, so you could fly at the speed of a modern-day airliner for months, probably)

But it's not getting you back into orbit, and I know of no variant that does. Antimatter, though - that would do it. Well, sorta. You are going to have a heck of a time shielding your crew from the products of that reaction.

Did I already tell you to google QED ARC Ploywell engines? Because they have the answer to just such a problem. The reactor, outputting high voltage current in a direct conversion scheme, is coupled to the working fluid through a few relativistic electron beams (REB), which are basically badly collimated energy weapons that get absorbed pretty well by the hydrogen propellant, and that are better driven by precisely the high voltage currents a hypothetical Polywell reactor would produce. Bussard actually worked out numbers that could lead to TWR 0.5 with ridiculously high isp (5.000s or so, IIRC), regeneratively cooled (!). Still, that ¡s very much theory right now.

You could use the nuclear engines with multiple fuels. At first you push CO2 or N2 or something like that through them for low-isp (relatively) and very high thrust. Later in flight, once you have exited the atmosphere, you gradually switch to H2.

9K delta-V with an SSTO is a lot though, so I would suggest to top the orbit off with electrical propulsion, using the nuclear reactors to generate immense amounts of electricity.

This, however, isn't theory, there is already such a design. And no, using different fuel in the same engine wouldn't work.But something along the same spirit that could is called Liquid Augmented NTR, or LANTR for short. By using liquid oxygen to run an "afterburner" for your standard hydrogen-fuelled NTR, you can build a variable isp nuclear thermal rocket, with a TWR on a low isp setting (full O2 flow on the afterburner) that approaches that of chemical engines, while getting an isp higher than them (extra energy added to your usual H20 exhaust, I think it got to around 550s in high TWR mode). Without the extra O2 in the exhaust, of course, they are your usual 900-1000s of hydrogen propelled goodness, with a clean exhaust in operation but quite the "bad breath" afterwards unless you dump the radioactive core after each firing.

But I've got a better idea:

Send the ship up in pieces.

Reusable first and second stages fly up and down until the entire ship is up there. This removes many limits to how big that ship can be.

If you have some infrastructure on the ground, that is not such a bad idea. But re-stacking a rocket after reentry is not such a trivial thing as you guys paint it... there are inherent advantages to single stage designs, besides their incredibly difficult requirements.

I also want to point out, Earth is one size of planet. Most others with similar atmospheres will have shallower gravity wells, while those with similar surface gravities should have thicker ones, in average. You know, because they probably won't have such a proportionately huge Moon to clean up most of it.

Rune. Biggest moon in relation to the parent in the whole solar system, and by a long shot, IIRC.

Edited March 10, 2015 by Rune

[GreeningGalaxy]

Fusion thermal jets? How do these work?

I'm serious. On the Talk Polywell forums, there was a discussion about this problem. One thing all the posters in the discussion - many of whom have engineering degrees - is that even if polywell does work, and gives you substantial energy : mass clean fusion, you still cannot easily make a spacecraft capable of reaching orbit.

Simply put, even with polywell (a method that might not work that if it does work, would be a lot cheaper and easier than ITER and other fusion experiments), fusion needs a nice clean vacuum, big honking magnets, and everything to be just right, or it abruptly quits working. Atmospheric air would contaminate your engines and cause them to stop almost immediately.

It would be a decent power source for a spacecraft in space, of course, because it would give a very high power : mass ratio relative to current nuclear fission reactors, which are heavy and require a heat engine. (this particular fusion reaction produces electrons directly as it's main output product which you would decelerate via charged grids to drive a circuit forward)

Nevertheless, the weight of the reactor and the electric thrusters is far too heavy to reach orbit. You can fly in the atmosphere just fine, of course, by using the electricity from fusion to turn superconducting electric motors and fly subsonic like a pure bypass jet aircraft. (you use hydrogen and boron for fuel, so you could fly at the speed of a modern-day airliner for months, probably)

But it's not getting you back into orbit, and I know of no variant that does. Antimatter, though - that would do it. Well, sorta. You are going to have a heck of a time shielding your crew from the products of that reaction.

I'm not making any assumptions that we'll be constrained to current-day designs for fusion reactors in a future that contains starships. My guess is that we'll have a pretty good handle on how to make a small, powerful deuterium/tritium fusion reactor long before we figure out how to generate antimatter in quantities measured in thousands of tons (which you would need for a proper interstellar torchship), much less find a way to bend space and make an Alcubierre bubble, which is pretty likely to be totally impossible anyway.

Theoretically speaking, heating stuff (like air or propellant) with fusion isn't that hard. The deuterium-tritium reaction results in lots of fast neutrons (which are easy enough to thermalize into the propellant), and charged alpha particles (which are even easier to thermalize). You'd then pump the propellant through channels in all that nice hot tungsten shielding surrounding the reactor, heating up to extreme temperatures and firing it out the back of the ship at high speed.

You do need a lot of heavy stuff like magnets and vacuum bottles to make fusion work, but the theoretical power density (and the power density that might one day be achieved) is certainly high enough to allow a shuttle to scream its way into orbit without burning much propellant at all. In fact, it's likely that you could just skip the thermal jet stage entirely and do the entire ascent on closed-cycle engines only, but that would make your required mass ratio a little bigger than you're probably willing to carry aboard a starship.

All this will involve building some kind of fusion reactor, obviously, but it's probably safe to say that that's less than a century away. Starships are almost certainly further off than that.

[EzinX]

Theoretically speaking, heating stuff (like air or propellant) with fusion isn't that hard. The deuterium-tritium reaction results in lots of fast neutrons (which are easy enough to thermalize into the propellant), and charged alpha particles (which are even easier to thermalize).

The difficulty is that you have to keep the fusion reaction isolated from the the air stream with a thick wall capable of keeping the fusion side in a vacuum. Also, the fusion side requires a specific geometry, or it will not work. (so no convenient way to run tubing back and forth in a complex way)

It's not that you cannot get some heat doing this. It's that getting the kind of energy you need for a rocket to lift it's own weight in Earth's gravity well and then some doesn't pass the pencil test.

All the high-thrust fusion proposals involve running the fusion reactor using outer space as the vacuum, with big honking magnets and electrostatic grids and things located out there. The reactors are deliberately made slightly faulty in order to leak fusion products a specific direction which is where thrust comes from.

None of these proposals remotely come close to even 1 G of acceleration, and this is making extremely generous assumptions about performance, and it still requires a vacuum.

[Psycix]

I really appreciate your contributions to this thread, Psycix. You didn't try to fight the OP and came up with a plausible, technically feasible solution.

Thank you for your kind words!

My inspiration draws from the Muskinator, he tends to go with what works, and then improve on that instead of relying on novel technologies that may or may not work well. Of course the reusable launch vehicle thing is exactly what SpaceX is working towards right now.

This, however, isn't theory, there is already such a design. And no, using different fuel in the same engine wouldn't work. But something along the same spirit that could is called Liquid Augmented NTR, or LANTR for short. By using liquid oxygen to run an "afterburner" for your standard hydrogen-fuelled NTR, you can build a variable isp nuclear thermal rocket, with a TWR on a low isp setting (full O2 flow on the afterburner) that approaches that of chemical engines, while getting an isp higher than them (extra energy added to your usual H20 exhaust, I think it got to around 550s in high TWR mode). Without the extra O2 in the exhaust, of course, they are your usual 900-1000s of hydrogen propelled goodness, with a clean exhaust in operation but quite the "bad breath" afterwards unless you dump the radioactive core after each firing.

If you have some infrastructure on the ground, that is not such a bad idea. But re-stacking a rocket after reentry is not such a trivial thing as you guys paint it... there are inherent advantages to single stage designs, besides their incredibly difficult requirements.

I also want to point out, Earth is one size of planet. Most others with similar atmospheres will have shallower gravity wells, while those with similar surface gravities should have thicker ones, in average. You know, because they probably won't have such a proportionately huge Moon to clean up most of it.

LANTR is a good suggestion. Though I wonder why you think it wouldn't it be possible to make a simple solid core nuclear thermal rocket that runs on any gas you push through it? Mind you this thread assumes we're in a post-warp era. LANTR is probably just as good if not better though.

The whole nuclear thing is not even needed though, this whole system could be executed with chemical rockets. It just makes the payload fraction a lot larger, and we're in post-warp anyway so NTR's should be easy.

I'm not too fussed about radiation. In space there is tons of radiation anyway, and I'm sure that mankind has found ways to deal with that by then, be it shielding, healthcare, genetic manipulation or evolution by artificial selection.

Yes, you would need infrastructure on the ground. This could potentially be created from in situ resources (Might always be a good idea to start a colony, or at least leave some hardware for one!) The ease of reusing a rocket is something I hope SpaceX will demonstrate in the next few years. Exciting!

Regarding planet sizes, it is likely to land on a larger one, simply because those are a lot easier to find. (Kepler I'm looking at you!)

This makes SSTO's less feasible.

Edited March 10, 2015 by Psycix

[GreeningGalaxy]

The difficulty is that you have to keep the fusion reaction isolated from the the air stream with a thick wall capable of keeping the fusion side in a vacuum. Also, the fusion side requires a specific geometry, or it will not work. (so no convenient way to run tubing back and forth in a complex way)

It's not that you cannot get some heat doing this. It's that getting the kind of energy you need for a rocket to lift it's own weight in Earth's gravity well and then some doesn't pass the pencil test.

All the high-thrust fusion proposals involve running the fusion reactor using outer space as the vacuum, with big honking magnets and electrostatic grids and things located out there. The reactors are deliberately made slightly faulty in order to leak fusion products a specific direction which is where thrust comes from.

None of these proposals remotely come close to even 1 G of acceleration, and this is making extremely generous assumptions about performance, and it still requires a vacuum.

You're describing an open-cycle magnetic-confinement fusion engine, which would indeed require a vacuum to operate and would have far too little thrust to take off from an earth-like planet. However, things like inertial-confinement laser fusion could potentially be done one a smaller scale and at a very high power density. And yes, I know the drive shown at that link would also need a vacuum to operate - for an atmospheric vehicle, you would make that into an internal reactor assembly which would be a vacuum chamber, and surround that chamber with shielding, through which you would run your working fluid and capture the heat from the thermalized neutrons and alpha particles. For many feasible designs, the power density would be far higher than anything you could get with fission.

Yes, there are lots of engineering hurdles - how to shield the lasers, how to make the lasers lighter, the best ways to capture the charged particles to make electricity, and so on. However, my point still stands that these are likely to be overcome on the way to finding methods of interstellar travel, so by the time we do reach other solar systems, there won't be any point in building crazy multi-stage landing craft with chemical and simple fission thermal propulsion.

[EzinX]

- for an atmospheric vehicle, you would make that into an internal reactor assembly which would be a vacuum chamber, and surround that chamber with shielding, through which you would run your working fluid and capture the heat from the thermalized neutrons and alpha particles. For many feasible designs, the power density would be far higher than anything you could get with fission.

This is completely and utterly wrong and uninformed. I need to know nothing at all about fusion's theoretical power density to make this statement.

Here's the simple reason why : ultimately, when you are producing heat through any kind of reaction - and that reaction has to be isolated from the working fluid - then the limiting factor is how hot can you make the walls of your heat exchanger before they melt.

Well, even if you make them out of tungsten-ceramic-unobtanium alloy, it's still just a few thousand kelvin, and you are rate-limited by the rate of conduction so you need a lot of surface area. All this stuff is heavy, weighing your rocket down, and preventing you from getting enough thrust to leave the ground.

But that isn't the problem. If you use pure U-235 as your reactor core, you need a very small quantity. Just a few kilograms. What you have created here is basically a nuke just a few grams of uranium from detonating. Controlling it would be...tricky. But it's technically possible to precisely remove mass or adjust neutron reflectors/absorbers enough to keep the power core from blowing up, and it would only consist of a few kilograms of fuel.

This is a higher thermal power density than fusion is every likely to have. Your limiting factor is, in this case, still the weight of your heat exchanger and whether the walls can take the heat without melting.

However, with fission, you aren't limited to an internal vacuum chamber. You can expose the fuel to air if you want to, and the reaction won't stop. This is partly why it's so dangerous...but the power you need is there.

Before you argue with me, consider my points :

1. You can produce more nuclear heat than you can possibly contain, for a prolonged period of time, with just a few kilograms of fission fuel. Fusion or antimatter is no better.

2. Your limiting factor on power:weight ratios for nuclear heat is your ability to contain the reaction.

3. Fusion absolutely requires a vacuum. Fission doesn't.

The use of fusion/antimatter/other exotics is in pure vacuum, long duration flights. Getting off the ground is a totally different technical problem, where power:weight matters far more than efficiency.

[GreeningGalaxy]

This is completely and utterly wrong and uninformed. I need to know nothing at all about fusion's theoretical power density to make this statement.

Here's the simple reason why : ultimately, when you are producing heat through any kind of reaction - and that reaction has to be isolated from the working fluid - then the limiting factor is how hot can you make the walls of your heat exchanger before they melt.

Well, even if you make them out of tungsten-ceramic-unobtanium alloy, it's still just a few thousand kelvin, and you are rate-limited by the rate of conduction so you need a lot of surface area. All this stuff is heavy, weighing your rocket down, and preventing you from getting enough thrust to leave the ground.

But that isn't the problem. If you use pure U-235 as your reactor core, you need a very small quantity. Just a few kilograms. What you have created here is basically a nuke just a few grams of uranium from detonating. Controlling it would be...tricky. But it's technically possible to precisely remove mass or adjust neutron reflectors/absorbers enough to keep the power core from blowing up, and it would only consist of a few kilograms of fuel.

This is a higher thermal power density than fusion is every likely to have. Your limiting factor is, in this case, still the weight of your heat exchanger and whether the walls can take the heat without melting.

However, with fission, you aren't limited to an internal vacuum chamber. You can expose the fuel to air if you want to, and the reaction won't stop. This is partly why it's so dangerous...but the power you need is there.

Before you argue with me, consider my points :

1. You can produce more nuclear heat than you can possibly contain, for a prolonged period of time, with just a few kilograms of fission fuel. Fusion or antimatter is no better.

2. Your limiting factor on power:weight ratios for nuclear heat is your ability to contain the reaction.

3. Fusion absolutely requires a vacuum. Fission doesn't.

The use of fusion/antimatter/other exotics is in pure vacuum, long duration flights. Getting off the ground is a totally different technical problem, where power:weight matters far more than efficiency.

Fine. Those aren't really the reasons why fusion is hard to use for aircraft, but that doesn't really matter anyway - it doesn't matter how the engine power is generated, be it gas-core fission, inertial fusion, nuclear pulse propulsion (both fission and fusion), or antimatter; all I'm saying is that a starship-age shuttle is likely to be a "simple" engine-with-wings type of thing, rather than a complex single-use multi-stage vehicle. More likely than not, we're going to have something with the power density to hop up and down between the ground and space with relative ease long before we have any ability to fly between stars.

Edited March 10, 2015 by GreeningGalaxy

[Technical Ben]

Remember that Falcon 9 is made of aluminium. You could improve performance by building it with modern composites, or even some sort of futuristic alloy since this thread is in the realm of warp drives. A similar incremental improvement in engine technology should make a VTVL SSTO possible with a small payload.

Personally, I'd go with something like DC-X or X-33. There was nothing stopping these projects from working on paper. Whether they made sense economically or politically is another matter, of course.

Fuel density means for anything Earth sized and above with an atmosphere you'll need to hope fusion gives better results. As the rocket equation does not like SSTO. Most probably any future space flight before fusion etc, would need disposable small 2-3 stage launch craft. For Mars etc, you'll have small rockets similar to what we have today, and cheap fuel tanks to dump, with possibly recoverable stages/engines etc.

Until something overcomes the fuel limit (if ever), we are stuck with payloads of 5% and lower. With most scifi making ships consist of 95% ship and 5% fuel, we are hardly going to hit the scifi designs.

[Rune]

The difficulty is that you have to keep the fusion reaction isolated from the the air stream with a thick wall capable of keeping the fusion side in a vacuum. Also, the fusion side requires a specific geometry, or it will not work. (so no convenient way to run tubing back and forth in a complex way)

It's not that you cannot get some heat doing this. It's that getting the kind of energy you need for a rocket to lift it's own weight in Earth's gravity well and then some doesn't pass the pencil test.

All the high-thrust fusion proposals involve running the fusion reactor using outer space as the vacuum, with big honking magnets and electrostatic grids and things located out there. The reactors are deliberately made slightly faulty in order to leak fusion products a specific direction which is where thrust comes from.

None of these proposals remotely come close to even 1 G of acceleration, and this is making extremely generous assumptions about performance, and it still requires a vacuum.

Re-read my comment to the OP regarding fusion engines, you could always couple the core's energy production with the propellant in a different way by, for example, having a p-b11 reaction producing high voltage currents through direct conversion that you use to drive relativistic electron beams to heat your propellant. There you go, ~80% efficient energy transfer from the reactor to the propellant, without them coming into contact. TWR is of course anybody's guess.

LANTR is a good suggestion. Though I wonder why you think it wouldn't it be possible to make a simple solid core nuclear thermal rocket that runs on any gas you push through it? Mind you this thread assumes we're in a post-warp era. LANTR is probably just as good if not better though.

The whole nuclear thing is not even needed though, this whole system could be executed with chemical rockets. It just makes the payload fraction a lot larger, and we're in post-warp anyway so NTR's should be easy.

I'm not too fussed about radiation. In space there is tons of radiation anyway, and I'm sure that mankind has found ways to deal with that by then, be it shielding, healthcare, genetic manipulation or evolution by artificial selection.

Yes, you would need infrastructure on the ground. This could potentially be created from in situ resources (Might always be a good idea to start a colony, or at least leave some hardware for one!) The ease of reusing a rocket is something I hope SpaceX will demonstrate in the next few years. Exciting!

Regarding planet sizes, it is likely to land on a larger one, simply because those are a lot easier to find. (Kepler I'm looking at you!)

This makes SSTO's less feasible.

Regarding why an NTR would be horribly difficult to design to accept more than one fuel, probably downright impossible, you have to think about how simple the design is in the first place. The propellant in heated by circulating it through channels in the core, and those channels have to be very carefully designed so that they carry away just enough heat form the core... taking into account the thermodynamic characteristics of the working fluid. Change the density of the propellant, or its heat transfer capacity, and you have to change the geometry of the propellant channels. That means a multi-propellant NTR would have to have variable geometry propellant channels. As I said, probably impossible to build, or at the very least very difficult, especially when the alternative is to add a light and mechanically simple O2 afterburning system and you get isps form 1000s to 550s, with the corresponding thrust variation and the same jet power (or slightly higher, actually, the O2 injection adds some chemical energy).

And I will still maintain my infrastructure comment, it does have some relevance even if you have magical ISRU capabilities. If you rely on a complicated launch gantry to rebuild your multistage rocket, you are talking about much longer turnaround times, without even getting into how you build and maintain said infrastructure. A single stage vehicle, on the other hand, only has to fill up its tanks and it's ready to go, potentially performing much more missions in the same amount of time.

And as to radiation, if you want to walk around your landed rocket, it's recommendable that its tail isn't irradiating the whole landing site with a partially spent core cooling down. Those things, once turned on, become very hot, and you can't dump the spent fuel anywhere like you can on orbit, because if you do the very radioactive core remains there on the surface under you. Hence my proposal of auxiliary landing engines, the delta-v for a propulsive landing of an empty stage is pretty much trivial.

Rune. Seriously guys, you should all check this paper. Fusion-electric SSTO engine with variable specific impulse and up to 0.5 TWR!