With Ultra Safe Nuclear engines and hydrogen propellant, how far to Mars could you get and still be able to return to Earth in an emergency?

Question

The NPR news item (audio + transcript) Could Nuclear Power Aid In Travel To Mars? contains the following:

VISHAL PATEL: If you want to go to Mars, nuclear is a smart choice.

BRUMFIEL: Vishal Patel is a nuclear rocket scientist with a company called Ultra Safe Nuclear Corporation. They're working on a version of a rocket that would use a nuclear reactor. The reactor would heat hydrogen gas and shoot it out a nozzle. It's way more efficient than a chemical engine. A nuclear rocket could make a round-trip mission possible in as little as half the time needed using conventional rockets. It would also allow astronauts to turn back towards home if they encountered an emergency at the start of their trip. Patel recognizes that launching a nuclear reactor from Earth might make people nervous. But, he says, Ultra Safe Nuclear Corporation is working hard to make it ultrasafe.

Question: With what is known or suggested about Ultra Safe Nuclear Corporation's engine's performance using hydrogen as reaction mass and a reasonable estimate of what a one-way transport¹ of a small crew to Mars might look like, how far from Earth could one get and still be able to return to Earth?

To keep it simple it will be okay if you want to assume that Earth-intercept is sufficient and the spacecraft can safely reenter even at interplanetary velocity (like sample return capsules do).

I don't think we have enough information to answer the question. The nuclear element only specifies the amount of power that's on board. To turn on an ion engine, one needs power but also fuel, which will help determine the thrust and isp. Without that info, I don't think we can adequately guess. — ChrisR, Feb 25 '21 at 23:54
@ChrisR I always wonder who "we" is when people say "we don't know". I think each person can speak to their own knowledge (e.g. "I don't know") but should not a priori decide that nobody can know enough to compose answer. I believe that this problem is a simple delta-v calculation to re-intercept the Earth; one only needs a dry to total mass fraction and an Isp, right? As far as I know, ion engines would not factor in to an answer here. — uhoh, Feb 25 '21 at 23:57
@ChrisR NTP systems achieve expanded payload mass capabilities due to their two-fold increase in specific impulse compared with chemical propulsion systems. — uhoh, Feb 26 '21 at 00:16
A delta-v calculation only involves orbital mechanics, so the thruster selection and its isp is irrelevant. Hence, if you want an answer specific to the thruster in question, you need to account for the mass of the vehicle, the thrust output, and the isp. (Source: my thesis on round-trips to Mars using low thrust propulsion) — ChrisR, Feb 26 '21 at 00:16
@ChrisR thanks for the semantics tutorial; one first determines how much delta-v is needed at each point from orbital mechanics, then determines how much delta-v is available by assuming a dry/total mass fraction at each point and Isp in order to decide if/when it's possible. (Source: knowledge) And again, low thrust or ion engines are irrelevant here; this is nuclear thermal propulsion. We can guess this from "The reactor would heat hydrogen gas and shoot it out a nozzle." — uhoh, Feb 26 '21 at 00:19
The talk about heating up hydrogen clearly indicates that it's a solid core nuclear thermal rocket, And we'll get the same performance as every other solid core nuclear thermal rocket. — ikrase, Feb 26 '21 at 05:44
@ikrase wait, don't answer that, I'll ask it as a new question! — uhoh, Feb 26 '21 at 05:49
@ikrase I've just asked Are nuclear thermal engine designs limited to about twice the Isp of existing chemical rocket engines? If so, why; what's the limiting factor? — uhoh, Feb 26 '21 at 05:58

Mark · Answer 1 · 2021-02-25T23:56:57.883

2

All the way?

This isn't really a meaningful question. If you've got sufficient delta-V to get to Mars and back, you've got sufficient delta-V to get to Mars and back. Unless you're on a continuous-thrust trajectory, abort modes aren't limited by distance or time, they're limited by remaining delta-V.

"Ultra Safe Nuclear" sounds like it's a variant of a nuclear thermal rocket. These act pretty much like conventional chemical rockets, only somewhat more efficiently. Once you've finished the trans-Mars injection burn, there's no "turn around and go back" option. You might be able to do some sort of gravity assist at Mars to get a faster return, but more likely, you'd abort by establishing an orbit with a period of two years.

Pulling some numbers from a Solar System delta-V map, trans-Mars injection from low Earth orbit requires about 3.6 km/s of delta-V. Orbital mechanics is symmetric, so that's how much delta-V you'd need to turn around and go back. If you're only fueled for a one-way trip to Mars, you've only got enough fuel left for propulsive capture: 2.1 km/s, not enough for a "turn around" abort.

edited Feb 25 '21 at 23:56

answered Feb 25 '21 at 23:47

Mark

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Thanks for the edit! Btw I've specified that propellant be sufficient for only one-way in the question. Nuclear is used for crewed missions because humans are perishable and go crazy sometimes, but supplies and fuel for a return mission would presumably be landed on Mars ahead of time via uncrewed robotic delivery missions. – uhoh Feb 26 '21 at 00:02
If you decide to abort soon after TMI, and if you have sufficient delta-v, you can effectively undo the TMI and return in much less than two years. The longer the “soon” timeframe, the more delta-v you need for “sufficient”, so I don’t think there’s a sensible way to answer the question. – Russell Borogove Feb 26 '21 at 04:18
Time to sing the I've Got The Need For Delta V song. – ikrase Feb 26 '21 at 05:45

With Ultra Safe Nuclear engines and hydrogen propellant, how far to Mars could you get and still be able to return to Earth in an emergency?

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