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Comment by jhgb | Hacker News Reader

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.

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I'm skeptical too, but DARPA is saying the DRACO program is for getting to/from Mars quickly:

> The DRACO program intends to develop novel nuclear thermal propulsion (NTP) technology to enable time-critical missions over vast distances in cislunar space. Unlike propulsion technologies in use today, NTP can achieve high thrust-to-weights similar to chemical propulsion but with two to five times the efficiency. This enables NTP systems to be both faster and smaller than electric and chemical systems, respectively. The propulsive capabilities afforded by NTP will enable the United States to maintain its interests in space, and to expand possibilities for the National Aeronautics and Space Administration (NASA)’s long-duration human spaceflight missions (i.e., to Mars). Because of the ability to transit space faster than other propulsion systems, the NTR engine can return astronauts to Earth much faster in case of an emergency and similarly ensure reduction of overall trip time and exposure to deleterious impacts to astronaut health which come with long-term spaceflight.

> but with two to five times the efficiency

I suspect from the number that they're talking purely about Isp. Once one performs a whole system analysis, it's much less rosy for (non-LA)NTR.

I agree. Furthermore, besides the mention of Mars they're also talking about cislunar space in that same paragraph, but chemical propulsion seems sufficient in cislunar space. It's only takes a days to return from the moon with chemical propulsion, which proved sufficient in the past.

LANTR would improve performance of lunar landers/cislunar shuttles, especially for variable specific impulse which is what LANTR could plausibly do without much trouble -- start with high oxygen flow for high thrust and high propellant mixture density, decrease oxygen flow later in flight for higher terminal Isp. This brings you the performance of a multi-stage vehicle without staging, and LANTR can even with high oxygen flow deliver Isp significantly higher than what hydrolox has, with propellant density several times higher than what pure-hydrogen NTR gives you.

I've thought about trying to optimize the performance of such a variable Isp vehicle, but it requires calculus of variations skills that I'm lacking at the moment. I guess I need to take a look at that. But there's a decent chance that with a such a vehicle, you could move from the "we need to mine ice on the Moon" to the "we just need to extract oxygen from lunar soil; we can bring hydrogen from LEO" territory, which would be a win for lunar flights (for example you wouldn't be limited to polar region bases where you'd need to mine water to get back home).

It's not for getting to/from Mars quickly. It's for giving money to congressional districts quickly.

This is more the sort of engine you develop if you're going for an Apollo style mission where there's a mother craft that goes into orbit and a separate lander goes down to the surface. A NTR's poor TWR compared to conventional combustion rockets means it would be a bad ascent stage.

I wouldn't assume the plan relies on ISRU at all but if it do having to carry the resulting hydrogen up to orbit on the ascent stage will be a big limiting factor so not keeping the oxygen isn't so large a flaw. And if you're carrying the fuel to orbit on another rocket you want to get as high an ISP as you can manage with what you bring up.

All of which isn't to say this would be a good plan. I've drunk the SpaceX koolaid on the topic. But if it's a bad plan at least it isn't a stupid one and there are reasons behind things.

Yeah, I did notice that the original NTR plans arose from the wish to upgrade Saturn V with its limited "throw weight" at third stage separation (http://www.astronautix.com/s/saturnc-5n.html). It doesn't seem to make a lot of sense to design a propulsion unit for a sixty year old mission architecture today, though.

Yeah, the SpaceX Kool-aid, at least as regards Mars colonization, is about as lethal as the Jonestown variety.

There will be no Mars colonization on Starships, whatever Elon says. Starship is just wholly inadequate to the task. Neither will there be any sub-orbital passenger or freight service on it.

Starship should be adequate for lofting lots of Starlinks, for getting to the moon, and for boosting just amazingly well-equipped 100t outer solar system probes and telescopes. It might suffice for a quick visit to Mars with a half-dozen crew. (BTW, 9, not 6 months, each way.)

Probably the only way to make even that work would be to send two ships strung on a cable, nose to nose, spun for centrifugal gee force, so they could still walk when they got there. Maybe the second ship carries hydrogen (as ammonia?) to bond to ISR carbon to come home on. And solar panels, to crack the carbon.

But the first thing any attempted colonist would transmit back is "Can I please come home?"

A Mars colony will involve living in small metal boxes, surrounded by danger, with ruinous prices on things, if they are available at all.

We can get the same experience today, without going to Mars, in places called "prisons".

I think, in the medium term, all Musk is actually aiming at, is a crewed research station on Mars, with a few dozen people living in it. Rather similar to what we already have in Antarctica.

People will sign up to go. First person to step foot on Mars gets their name in the history books, next to Neil Armstrong. The rest get to join a very elite club. I suppose prison is kind of a club too, but nothing elite about it.

And Musk will call it a "colony"–aspirational naming. And maybe, one day, in centuries to come, it will actually evolve into one. I don't think Musk has really thought a lot about how to get from the "colony-in-name-only crewed research station" to a genuine colony – that's too many steps ahead. He just trusts he'll work it out when he gets there, or if he doesn't live that long, somebody else will.

"Aspirational naming" is quite a curious synonym for lying.

And to say that Musk has not thought much about X, for any X, is quite an understatement. The closer you look at anything he says, the less evidence of thought you can find. Today is a golden age for glib grifters.

Well, look at SpaceX: he founded it, he remains its CEO&CTO, and in 10 years it has gone from 0% market share to over 50% global market share–which was achieved, not through anticompetitive subterfuge, but simply by building a substantially better product at a substantially lower cost (and whatever government subsidies were involved, were made available in even greater amounts to competing companies which failed to leverage them into the same market success). Obviously he must have some capacity for intelligent thought to be able to pull that off. Of course, he employs many brilliant engineers, without whom none of that would have been possible–but, as founder/CEO, he created and sustained the corporate environment which made it possible for them to achieve that.

Yet, he does literally none of the work, and every unscripted public statement shows he understands nothing of the technical details beyond what he has learned to parrot.

He did not found PayPal, Tesla, or Neuralink, although he has often claimed to. Hyperloop is 100% grift.

> Yet, he does literally none of the work

This sounds like labor-theory-of-value BS. Management and leadership is work, and it's essential.

Compare SpaceX to Blue Origin - while SpaceX has succeeded in conquering over 50% of the orbital launch market, Blue Origin still hasn’t made it to orbit - nor has ULA’s new rocket using Blue’s engines. Is that the fault of Blue’s engineers? I don’t think that’s fair - some of them are just as brilliant as SpaceX’s. The real blame, I think, is at the executive level. Bezos has never invested anywhere near as much of his time and energy and personal wealth into Blue as Musk has invested into SpaceX. And Musk has made much better choices of executive leadership (Gwynne Shotwell vs Bob Smith). That’s just one example of the massive difference the ability and commitment of a founder can make to the success of a business. (You’d think on a forum owned by a Silicon Valley VC firm, that point would be uncontested and accepted as obviously true.)

Shotwell anyway has to know that the whole Mars colonization shtick is total BS.

He has been lucky in some of his hires.

I have perhaps naive expectations about somebody styling himself Chief Technical Officer. I guess Chief Grift Officer would be too revealing.

CTOs aren’t always super-technical-and even those who are, while the CTO of a small startup might realistically have an expert-level understanding of all the business’s core technologies, that is no longer a realistic standard when talking about a multi-billion dollar firm with a highly complex or diverse tech stack. Arguably, one of the most important tasks for a CTO, is to be able to tell the difference between good engineering executives and bad ones. And, judged by that standard, Musk actually has done a very good job as SpaceX CTO, much better than many of its major competitors. Doing that requires understanding the technology well-enough to distinguish engineers and engineering leaders who really understand it from those who are just pretending to do so-and I think it is obvious Musk does understand the technologies at SpaceX (and Tesla too) well-enough to successfully make that distinction. People seem to be holding him to an unrealistic standard, which I doubt they’d actually apply to a CTO who wasn’t named Elon Musk.

I would say he gets a free pass nobody else does. If the CTO of Intel spouted things as idiotic as he does routinely, they would have to resign.

What about Larry Ellison, CTO of Oracle? Frankly I think Larry Ellison could say any crazy thing he liked, and no one would really care, and he'd stay CTO and chair of Oracle's board. Because C-suite executives get a "free pass" all the time–especially when they combine their C-suite role with a substantial ownership interest in the company (true of both Ellison and Musk). But most C-suite execs, the average person has never heard of them, and so they don't care what they say. Whereas, Musk is a controversial celebrity, so people judge him by rather different standards than the thousands of other near-anonymous CEOs, CTOs and billionaires in the world.

> you have to throw away 88.9% of the water that you mine on site

Nobody cares about that. People care only about the end results, not about the efficiency (or inefficiency) of the intermediate steps. As long as you can travel to Mars and back in half the time it takes with a chemical rocket, exactly zero people will shed tears for all the oxygen wasted after splitting water on Mars.

Besides that, chances are you will be able to find uses for the oxygen you produce. I don't need to remind you that people breathe oxygen, astronauts included.

> People care only about the end results, not about the efficiency (or inefficiency) of the intermediate steps.

Look up for the guy claiming hydrogen is best and most efficient, even though performance per dollar is poor.

Speaking of performance per dollar, what does the nuclear fuel for an NTR cost?

The NERVA engine [1], [2] used 60kg of 92.5% enriched Uranium.

The online nuclear fuel cost calculator [3] shows that at current Uranium market prices, the cost for 1kg of such highly enriched Uraniums is about $50k, so the whole engine core would come at about $3 MM.

This engine had a weight of about 2.5 metric tons, a thrust of 75 kN and a specific impulse of 860s.

For comparison, the weight of a SpaceX Raptor engine is about 1.5 tons, it has a thrust of 1.8 MN (24 times higher than Nerva) and a specific impulse of 360 s (2.4 times lower than Nerva's).

[1] https://en.wikipedia.org/wiki/NERVA

[2] http://www.astronautix.com/n/nervaalphaengine.html

[3] https://www.uxc.com/p/tools/FuelCalculator.aspx

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.

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.

Ion drives don't scale up in thrust fast enough for it to be worthwhile for manned missions.

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.

I think that's basically what the VASIMIR engine is.

[1] https://en.wikipedia.org/wiki/Variable_Specific_Impulse_Magn...

I don’t know that voluminous tanks and heavy engines are necessarily a problem for something that’s designed to permanently live in space - the tanks can essentially just be onion-layered gasbags, and could be km3 in volume if you wanted. As to fuel - don’t get it from heavy bodies. Mine asteroids, minor moons, whatever.

> I don’t know that voluminous tanks and heavy engines are necessarily a problem for something that’s designed to permanently live in space - the tanks can essentially just be onion-layered gasbags, and could be km3 in volume if you wanted.

It's the opportunity cost. At low and moderate speeds (we're talking delta-Vs of 10 km/s and less), the same tankage simply gives you higher performance with chemical propulsion, so for no size of tankage may it actually be advantageous to use an NTR instead of a chemical engine. Only at extreme delta V levels do NTRs actually get better performance, but that's not a mission-to-Mars territory. LANTRs could possibly lower the crossover point, especially with variable Isp, but properly estimating how much requires calculus of variations, as I already said elsewhere.

> As to fuel - don’t get it from heavy bodies. Mine asteroids, minor moons, whatever.

Same issue. Your supply may be limited and/or require effort to extract. NTRs throw oxygen away; hydrolox and methalox engines use it for propulsion. For every tonne of water extracted, you'll go MUCH further if you go chemical, or at least with LANTR instead of NTR.

>Only at extreme delta V levels do NTRs actually get better performance, but that's not a mission-to-Mars territory

Why is that not mission-to-Mars territory? You can shave months off the transit time with >15km/s delta-v.

1) The travel time benefits are degressive owing to increasingly eccentric heliocentric trajectories - the changes in trajectory length get smaller and smaller as your velocity vector stops being colinear with the planet's orbit upon intercept, so you don't really save a lot of additional time. (But you get the most benefits with even small increases above Hohmann transfer speed.)

2) Intercept velocities, on the other hand, are progressive -- pretty much for the same reason, combined with Pythagoras' theorem. At one point you stop being able to aerocapture, even with exerting downward lift in Martian atmosphere to prolong the braking phase.

Owing to these two things, I'm not quite sure that propelling yourself from LEO to Mars at 15 km/s would be a good idea, unless you intend to crash into the planet.

I am assuming that you can spend the delta-v at both launch and intercept, it sounds like you are assuming it is all used at launch?

This starts becoming disadvantageous even faster than just trying to speed up. At that point a much better improvement for you is the development of a magnetoshell decelerator. Mass-wise, there's no situation where propulsive intercept is better since a magnetoshell decelerator will be much more lightweight than even an NTR stage.

Why? Propellant is relatively light in an NTR system as far as I'm aware. I am also assuming that making a nuclear ship aerodynamic would be unfeasible (it will likely be long and only structurally reinforced longitudinally).

That's not quite right.

Using a Hohmann transfer orbit [1], you get from Earth to Mars in about 9 months.

Using an Aldrin Mars cycler [2], you can get in as little as 75 days. Of course, the Aldrin Mars cycler requires more delta-v, but that's the point, if you have more delta-v you get there sooner.

[1] https://en.wikipedia.org/wiki/Hohmann_transfer_orbit

[2] https://en.wikipedia.org/wiki/Mars_cycler

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