8 ms·

The PDFs here have a lot more information: https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view

The PDFs here have a lot more information: https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view https://sam.gov/opp/af490b568d2a438498afa1e80bce63e5/view A few takeaways; they intend for such an engine to eventually support long duration human spaceflight (going to Mars.) The propellant for the NTR engine to be liquid hydrogen. One of the problems DARPA anticipates with using such an engine for such a mission is needing to store liquid hydrogen longer than the present state of the art. The PDF doesn't seem to mention it, but I think the Advanced Cryogenic Evolved Stage (ACES) is probably relevant to this project. Does anybody know what kind of duration they expect to get from ACES? I'm not sure but I think it's weeks, not months.

ceejayoz 4y ago
Could you make the hydrogen en-route? Solar power to crack water, use the oxygen for breathing? https://www.nasa.gov/content/space-applications-of-hydrogen-and-fuel-cells https://www.nasa.gov/content/space-applications-of-hydrogen-... says this is done on the ISS currently.
- nicoburns 4y ago
  Where are they going to get the water from?
  - ceejayoz 4y ago
    You'd be bringing oxygen and hydrogen along anyways. Why not bring it in water form?
  - evgen 4y ago
    Much easier to store water for long duration than to store hydrogen. It even serves a useful purpose as radiation shielding for some solar events.
- robonerd 4y ago
  Maybe? I believe LH2 has about 35 mols of hydrogen per liter, while water is 55 mols per liter. Storing hydrogen as water seems practical from that perspective, but what of the power needed to split that water? I think you'd need quite a lot of power to split that much water fast (starting a few days before running the engine.) Splitting it slowly over time using solar energy would seem to still leave you with a storage problem, but perhaps a more tractable one. Maybe instead of electrolysis, they could use heat from the reactor? Thermolysis needs 2500 C though.
  - jandrese 4y ago
    Wouldn't you just size the engines small enough that they instantly burn off the H2 as you crack it? The solar power should be even and constant so you can size the system to match. It is going to require a very large solar array, especially since your spacecraft is going to be really heavy with all of that water.
    - jhgb 4y ago
      You could design a pulse detonation engine for this. Electrolyze water continuously; detonate it in a pipe every now and then. It's a very simple design that gives you quite a bit of performance for hopping in the asteroid belt. Specific impulse similar to a hydrolox engine, or slightly worse than regular hydrolox engines if operating stoichiometrically, although the detonation mode could compensate for that (detonation rocket engines can potentially get ~10% better Isp performance than "classical" rocket engines). However, you get triple the propellant density (water has ~1000 kg/m³; hydrolox is at around 340 kg/m³). This makes it much better compared to a classical hydrolox vehicle wherever gravity is near zero so that you don't need lift-off thrust.
  - chipsa 4y ago
    NTRs are basically open-cycle gas cooled reactors. The thermal limit on the reactor temp is when does stuff start to melt. Project Rho[0] suggests that's the reactor temp anyways. But you need to be able to separate out the oxygen from the thermolysis stream, rather than just feeding the entire thing into your engine, both because your Isp would go to crap if you tossed the oxygen out too, and you'd have oxidizing your reactor problems. Though, you could just store it all as ammonia, and you get more hydrogen for your buck, and can probably just feed that all through the reactor. [0]:http://www.projectrho.com/public_html/rocket/enginelist2.php http://www.projectrho.com/public_html/rocket/enginelist2.php
- jandrese 4y ago
  If you are lugging all of that water mass along you could store it in a jacket around the crew compartment, providing additional radiation protection for most of the trip. You're going to need humongous solar panels to support this, but since you are in space this isn't an intractable problem. A small but constant acceleration would probably make life better in the spacecraft as well.
- colechristensen 4y ago
  The mass of the oxygen in water is 8x that of the hydrogen, and you just don't need all the much for humans, and what you do have after respiration (CO2) gets recycled through the Sabatier process (H2O -> O2, H2; CO2 + H2 -> CH4 + H2O) I.e. water is a quite inefficient storage medium for hydrogen and you're probably better of making heavier containment vessels for liquid hydrogen (of course a calculation could be shown to demonstrate the balance, but a tank weighing 8x the contents is a very long way from the extremely light tanks used in spaceflight)
dotnet00 4y ago
ACES is effectively dead, although a lot of its ideas ended up getting into Vulcan's upper stage. Long duration in its context means days. It was intended to do something similar to what SpaceX is doing with Starship, using the boiloff gas to pressurize the tanks. The problem with longer duration storage of hydrogen is that there really isn't any option besides going with a denser or thicker material, while modern rocket wall thicknesses are measured in millimeters of lightweight metals or composites. However, the convenient thing about NTR is it should be a lot easier to switch to something less prone to seeping through everything. It would be a matter of weighing the losses from needing a heavier tank against the losses from using heavier propellant.
- robonerd 4y ago
  Apparently Bruno has been talking about two orders of magnitude improvement to Centaur V's duration. Seems far fetched to me, but I think months of duration would be necessary to make this engine worthwhile (the PDF is talking about the value of this engine for getting astronauts home from Mars quickly in emergencies; that would only be possible with months of duration at least I think.) DARPA says they're expecting designs using liquid hydrogen, and as far as I understand liquid hydrogen would be the most efficient propellant for an NTR. What might the best storable alternative be?
  - dotnet00 4y ago
    From what I understand, Bruno didn't say that Centaur V has those two orders of magnitude improvements, rather that they're aiming to push improvements of that level over the next few years. That said, I don't think it's too far fetched, assuming that the long duration version is separate from the regular version (ie it can be heavier to support denser tanks). Liquid Hydrogen would be most efficient in a pure physics sense, but due to the mass tradeoffs with storage tech, there may be other propellants that are comparable in a practical sense. I'm not informed enough on the matter to say exactly which would be better, but for a somewhat comparable point of reference, Hydrogen+Oxygen is the most efficient propellant for chemical rockets but when accounting for the special tanks needed for storing hydrogen, methane can achieve pretty comparable performance due to being perfectly fine in a thin-walled stainless steel tank.
jhgb 4y ago
I don't see how an NTR helps you in any way to get to Mars or back. Heavy engine, voluminous tanks (~70 kg/m³), criminally wasted ISRU material (you have to throw away 88.9% of the water that you mine on site, whereas a hydrolox or methalox system uses almost all of it and the methalox system can even mix it with considerable amount of CO₂ for better system performance). The performance figures for such a system will be terrible. At best a LANTR (not just an NTR) might be somewhat useful for cislunar uses. For Mars flights not even LANTR may be useful.
- snek_case 4y ago
  Seems like you could probably get more efficient by using a nuclear reactor to power an ion drive? Also wouldn't need to cool fuel down to cryogenic temperatures.
  - jhgb 4y ago
    At 1 AU from the Sun, and possibly all the way to Mars, advanced photovoltaics may very well be better than a nuclear reactor for powering ion engines: It has very high system-level power/weight ratio (in lab around 300 W/kg, currently in operation around 150-200 W/kg), possibly could even power an ion engine without heavy power conditioning equipment ("direct drive electric thruster"), and also scales down for smaller probes. So for a trip to Pluto, a reactor would be useful, for a trip to Mars, it's hardly necessary.
  - chipsa 4y ago
    Ion drives don't scale up in thrust fast enough for it to be worthwhile for manned missions.
  - Symmetry 4y ago
    For a trip to Mars the time it takes an ion drive rocket to reach cruising speed isn't negligable compared to the overall flight time. And missing out on the Oberth effect is fairly significant. If this were a flight to, say, Jupiter though electric drives all the way.
  - credit_guy 4y ago

ceejayoz 4y ago

Could you make the hydrogen en-route? Solar power to crack water, use the oxygen for breathing? https://www.nasa.gov/content/space-applications-of-hydrogen-and-fuel-cells https://www.nasa.gov/content/space-applications-of-hydrogen-... says this is done on the ISS currently.

nicoburns 4y ago
Where are they going to get the water from?
- ceejayoz 4y ago
  You'd be bringing oxygen and hydrogen along anyways. Why not bring it in water form?
- evgen 4y ago
  Much easier to store water for long duration than to store hydrogen. It even serves a useful purpose as radiation shielding for some solar events.
robonerd 4y ago
Maybe? I believe LH2 has about 35 mols of hydrogen per liter, while water is 55 mols per liter. Storing hydrogen as water seems practical from that perspective, but what of the power needed to split that water? I think you'd need quite a lot of power to split that much water fast (starting a few days before running the engine.) Splitting it slowly over time using solar energy would seem to still leave you with a storage problem, but perhaps a more tractable one. Maybe instead of electrolysis, they could use heat from the reactor? Thermolysis needs 2500 C though.
- jandrese 4y ago
  Wouldn't you just size the engines small enough that they instantly burn off the H2 as you crack it? The solar power should be even and constant so you can size the system to match. It is going to require a very large solar array, especially since your spacecraft is going to be really heavy with all of that water.
  - jhgb 4y ago
    You could design a pulse detonation engine for this. Electrolyze water continuously; detonate it in a pipe every now and then. It's a very simple design that gives you quite a bit of performance for hopping in the asteroid belt. Specific impulse similar to a hydrolox engine, or slightly worse than regular hydrolox engines if operating stoichiometrically, although the detonation mode could compensate for that (detonation rocket engines can potentially get ~10% better Isp performance than "classical" rocket engines). However, you get triple the propellant density (water has ~1000 kg/m³; hydrolox is at around 340 kg/m³). This makes it much better compared to a classical hydrolox vehicle wherever gravity is near zero so that you don't need lift-off thrust.
- chipsa 4y ago
  NTRs are basically open-cycle gas cooled reactors. The thermal limit on the reactor temp is when does stuff start to melt. Project Rho[0] suggests that's the reactor temp anyways. But you need to be able to separate out the oxygen from the thermolysis stream, rather than just feeding the entire thing into your engine, both because your Isp would go to crap if you tossed the oxygen out too, and you'd have oxidizing your reactor problems. Though, you could just store it all as ammonia, and you get more hydrogen for your buck, and can probably just feed that all through the reactor. [0]:http://www.projectrho.com/public_html/rocket/enginelist2.php http://www.projectrho.com/public_html/rocket/enginelist2.php
jandrese 4y ago
If you are lugging all of that water mass along you could store it in a jacket around the crew compartment, providing additional radiation protection for most of the trip. You're going to need humongous solar panels to support this, but since you are in space this isn't an intractable problem. A small but constant acceleration would probably make life better in the spacecraft as well.
colechristensen 4y ago
The mass of the oxygen in water is 8x that of the hydrogen, and you just don't need all the much for humans, and what you do have after respiration (CO2) gets recycled through the Sabatier process (H2O -> O2, H2; CO2 + H2 -> CH4 + H2O) I.e. water is a quite inefficient storage medium for hydrogen and you're probably better of making heavier containment vessels for liquid hydrogen (of course a calculation could be shown to demonstrate the balance, but a tank weighing 8x the contents is a very long way from the extremely light tanks used in spaceflight)

dotnet00 4y ago

ACES is effectively dead, although a lot of its ideas ended up getting into Vulcan's upper stage. Long duration in its context means days. It was intended to do something similar to what SpaceX is doing with Starship, using the boiloff gas to pressurize the tanks. The problem with longer duration storage of hydrogen is that there really isn't any option besides going with a denser or thicker material, while modern rocket wall thicknesses are measured in millimeters of lightweight metals or composites. However, the convenient thing about NTR is it should be a lot easier to switch to something less prone to seeping through everything. It would be a matter of weighing the losses from needing a heavier tank against the losses from using heavier propellant.

robonerd 4y ago
Apparently Bruno has been talking about two orders of magnitude improvement to Centaur V's duration. Seems far fetched to me, but I think months of duration would be necessary to make this engine worthwhile (the PDF is talking about the value of this engine for getting astronauts home from Mars quickly in emergencies; that would only be possible with months of duration at least I think.) DARPA says they're expecting designs using liquid hydrogen, and as far as I understand liquid hydrogen would be the most efficient propellant for an NTR. What might the best storable alternative be?
- dotnet00 4y ago
  From what I understand, Bruno didn't say that Centaur V has those two orders of magnitude improvements, rather that they're aiming to push improvements of that level over the next few years. That said, I don't think it's too far fetched, assuming that the long duration version is separate from the regular version (ie it can be heavier to support denser tanks). Liquid Hydrogen would be most efficient in a pure physics sense, but due to the mass tradeoffs with storage tech, there may be other propellants that are comparable in a practical sense. I'm not informed enough on the matter to say exactly which would be better, but for a somewhat comparable point of reference, Hydrogen+Oxygen is the most efficient propellant for chemical rockets but when accounting for the special tanks needed for storing hydrogen, methane can achieve pretty comparable performance due to being perfectly fine in a thin-walled stainless steel tank.

jhgb 4y ago

I don't see how an NTR helps you in any way to get to Mars or back. Heavy engine, voluminous tanks (~70 kg/m³), criminally wasted ISRU material (you have to throw away 88.9% of the water that you mine on site, whereas a hydrolox or methalox system uses almost all of it and the methalox system can even mix it with considerable amount of CO₂ for better system performance). The performance figures for such a system will be terrible. At best a LANTR (not just an NTR) might be somewhat useful for cislunar uses. For Mars flights not even LANTR may be useful.

snek_case 4y ago
Seems like you could probably get more efficient by using a nuclear reactor to power an ion drive? Also wouldn't need to cool fuel down to cryogenic temperatures.
- jhgb 4y ago
  At 1 AU from the Sun, and possibly all the way to Mars, advanced photovoltaics may very well be better than a nuclear reactor for powering ion engines: It has very high system-level power/weight ratio (in lab around 300 W/kg, currently in operation around 150-200 W/kg), possibly could even power an ion engine without heavy power conditioning equipment ("direct drive electric thruster"), and also scales down for smaller probes. So for a trip to Pluto, a reactor would be useful, for a trip to Mars, it's hardly necessary.
- chipsa 4y ago
  Ion drives don't scale up in thrust fast enough for it to be worthwhile for manned missions.
- Symmetry 4y ago
  For a trip to Mars the time it takes an ion drive rocket to reach cruising speed isn't negligable compared to the overall flight time. And missing out on the Oberth effect is fairly significant. If this were a flight to, say, Jupiter though electric drives all the way.
- credit_guy 4y ago