3 ms·

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

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.
  - silencedogood3 4y ago
unchocked 4y ago
Long term hydrogen storage isn't that bad with the proper architecture. You need a cryocooler which can be powered by the nuke, and thermal shielding for the tank which in vacuum is thin film and of minuscule weight. Hydrogen leakage and structural embrittlement are overblown, i.e. the Space Shuttle tank is one of the most mass efficient architectures in history and it was full of liquid hydrogen. Terrestrially, you can buy a Toyota hydrogen car today. Materials matter, but people act like the thing needs to be made of 4" plate and will fall apart if you look at it. Scaling helps here too, as volume increases to the third power while wall area increases to the second. The thing will, if there is any sense in the architecture, be assembled in orbit so gossamer heat shields and the like won't be a problem, nor will an extended assembly program that makes with a separately launched nuclear reactor. For ISRU Mars return, water is incredibly abundant and there's no concern with "wasting" residual oxygen. For lunar applications, water may be scarce but oxygen is abundant in regolith. You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input.
- Filligree 4y ago
  > You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input. This is probably right, but the way you said it made me wonder. Would it be possible to strip electrons from atoms, then use just the electrons as propellant? Or would the ensuing static charge of the spaceship render this infeasible? I imagine it'd pull in electrons from all around itself, but I don't know how the numbers come out.
  - toopok4k3 4y ago
    Funny that you mention this, Ion thrusters do exist. They are a thing but with very limited uses cases. They still need a kind of propellant gas like Xenon or Krypton that gets used. https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster
  - ben_w 4y ago
    If you strip the electrons off some atoms and use just the electrons[0] as reaction mass, you will eventually get a large enough electric charge you can no longer throw the electrons away from you. Electric forces behave similarity to gravity, so while it wouldn’t normally be phrased like this, you could say your engine exhaust will eventually no longer have escape velocity from your ship. For this reason, ion drives do things to neutralise the net charge. (If you meant using them as a power source rather than reaction mass, it’s technically possible but that’s called a capacitor and they have very low energy density). [0] or, by symmetry, just the nucleus.

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.
- silencedogood3 4y ago

unchocked 4y ago

Long term hydrogen storage isn't that bad with the proper architecture. You need a cryocooler which can be powered by the nuke, and thermal shielding for the tank which in vacuum is thin film and of minuscule weight. Hydrogen leakage and structural embrittlement are overblown, i.e. the Space Shuttle tank is one of the most mass efficient architectures in history and it was full of liquid hydrogen. Terrestrially, you can buy a Toyota hydrogen car today. Materials matter, but people act like the thing needs to be made of 4" plate and will fall apart if you look at it. Scaling helps here too, as volume increases to the third power while wall area increases to the second. The thing will, if there is any sense in the architecture, be assembled in orbit so gossamer heat shields and the like won't be a problem, nor will an extended assembly program that makes with a separately launched nuclear reactor. For ISRU Mars return, water is incredibly abundant and there's no concern with "wasting" residual oxygen. For lunar applications, water may be scarce but oxygen is abundant in regolith. You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input.

Filligree 4y ago
> You can't beat hydrogen as a fuel. As the lightest molecule, you get the highest exhaust velocity for the least energy input. This is probably right, but the way you said it made me wonder. Would it be possible to strip electrons from atoms, then use just the electrons as propellant? Or would the ensuing static charge of the spaceship render this infeasible? I imagine it'd pull in electrons from all around itself, but I don't know how the numbers come out.
- toopok4k3 4y ago
  Funny that you mention this, Ion thrusters do exist. They are a thing but with very limited uses cases. They still need a kind of propellant gas like Xenon or Krypton that gets used. https://en.wikipedia.org/wiki/Ion_thruster https://en.wikipedia.org/wiki/Ion_thruster
- ben_w 4y ago
  If you strip the electrons off some atoms and use just the electrons[0] as reaction mass, you will eventually get a large enough electric charge you can no longer throw the electrons away from you. Electric forces behave similarity to gravity, so while it wouldn’t normally be phrased like this, you could say your engine exhaust will eventually no longer have escape velocity from your ship. For this reason, ion drives do things to neutralise the net charge. (If you meant using them as a power source rather than reaction mass, it’s technically possible but that’s called a capacitor and they have very low energy density). [0] or, by symmetry, just the nucleus.