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The new BBC News item New engine tech that could get us to Mars faster says:

Once Orion has been connected to the transfer vehicle, a nuclear electric rocket would be used to get the crew capsule and the transport module to Mars, where they link up with a Mars orbiter and lander, which are waiting in Mars' orbit.

In a nuclear thermal electric rocket, a small nuclear reactor heats up liquid hydrogen. The gaseous form of the element expands and shoots out of the thruster.

"If we can cut transit time [to Mars] down by 30-60 days, it will improve the exposure to radiation facing the crew," says Mr Cassidy. "We're looking at nuclear thermal as a key technology because it can enable faster transit times."

Dale Thomas, together with UAH, has a study contract with Nasa to design a space rocket featuring a nuclear thermal engine. He thinks nuclear thermal electric is the closest new engine technology to being ready for use.

"Some of the trajectories we run in my lab, we can get the transit time down to three months, which is still a very long journey, but it's about a third of the time that chemical propulsion requires to get us there," he says.

To my understanding the description "...a small nuclear reactor heats up liquid hydrogen. The gaseous form of the element expands and shoots out of the thruster." is that of a "normal" nuclear propulsion design and doesn't really use electricity.

I think that in nuclear thermal electric propulsion, a nuclear source of thermal energy would be used to produce electricity which would then be used to ionize and electrostatically accelerate the reaction mass, rather than rely on expansion within a nozzle to produce thrust. For the same amount of electricity, it this would generate much more bang for the buck, though the power supplies and coils used to maintain the plasma could be a huge weight penalty.

Question: Is the BBC wrong (again?) or perhaps am I wrong, and the nuclear heat source is used to make electricity which is then used to heat the hydrogen so that it expands through a nozzle?


1, 2, first one still has problems, 2nd one the BBC promptly corrected and I revised accordingly.

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  • $\begingroup$ The fact that they use H instead of Xe should be a hint that it is not a pure ion engine... $\endgroup$ – Antzi Dec 25 '19 at 5:21
  • $\begingroup$ @Antzi since it's a theoretical design there may be other things going on. From an impulse argument you want the lowest mass/charge ratio ion you can get, and protons beat Xe$^{+1}$ by a factor of $\sqrt{131}$ in terms of mass-specific impulse at a given acceleration voltage. I worked that out in some post somewhere but can't find it right now. $\endgroup$ – uhoh Dec 25 '19 at 5:29
  • $\begingroup$ @Antzi I forgot to finish that thought: "But from an ionization potential argument Xenon is preferred because 1) Hydrogen as H2 needs 1/2 of 4.5 eV for each naked neutral H then separately13.6 eV to ionize it whereas Xe is monatomic and needs only 12.1 eV to ionize. By comparison monatomic He needs 24.6 eV to ionize and will want to steal an electron from almost anything it touches quickly, before it can be extracted" $\endgroup$ – uhoh Dec 25 '19 at 6:36
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    $\begingroup$ @Antzi If they have nuclear instead of solar power, they may have a surplus of electrical power available, and so a lighter, less efficient, but more powerful RF system may be able to ionize hydrogen which is always the most desirable reaction mass for singly ionized atoms at a given high voltage, from an impulse point of view. Dunno about reactivity of the ions with the grid material... $\endgroup$ – uhoh Dec 25 '19 at 6:38
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The description given matches that of the NERVA/Timberwind style nuclear thermal rocket, but it also applies to a more complex hybrid nuclear-thermal-electric engine described partway down this page (the section titled 'The True Hybrid'). In this scheme, hydrogen makes two passes through the reactor: in the first pass it's heated in order to drive a turbine for electrical power, and in the second pass it's heated and exhausted through a nozzle as in the conventional NTR, but there's an electromagnetic accelerator ring to further accelerate the exhaust, more or less as you guessed. According to this writeup, the NTR portion of the engine would reach a specific impulse in the 800s, and the electric accelerator would boost it to over 1000s. Thrust-to-weight would be in the NTR realm rather than the ion thruster realm.

I'm not sure if this is the tech that UAH is working on, however. This UAH press release seems to be talking about fiddling with the propellant composition in a conventional NTR.

Is the BBC wrong (again?) or perhaps am I wrong, and the nuclear heat source is used to make electricity which is then used to heat the hydrogen so that it expands through a nozzle?

The BBC's description is over-edited, in that it no longer distinguishes the NTER from the NTR, but it's not wrong per se.

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