Will SpaceX ever go nuclear propulsion?

[MP99]

Jason: Actually, you're wrong. Thin film PV, in combo with either regen fuel cells or (more relevant nowadays with recent advances) state of the art Lithium Ion or Lithium sulfur (both of which are better than older regen fuel cells) beats nuclear power pound for pound and volume stowage wise for surface power on Mars. See this paper: http://systemarchitect.mit.edu/docs/cooper10.pdf

And in-space, PV trounces nuclear (ie how much power for a given mass) until you get past the asteroid belt. It's not even fair, solar is like 5-10x more powerful (if you compare existing or historical in-space nuclear to existing solar, OR credible new developments for nuclear compared with credible new developments for solar). That's why no one has nuclear powered satellites anymore.

The problem with PV on Mars is that your PV panels build up an electric charge from both use as well as dust storms, after a while, simply brushing off the panels doensn't work so well because the dust is now electrostatically stuck on the PV cell faces.  Plus, the fine dust would start to cloud the surface of the PV cells just from simple abrasion.  This is a small part of why the Mars rovers using PV cells are slowly but surely becoming unable to generate power.

    MIND YOU, this has not happened NEARLY as fast as anyone at NASA expected, thus the decade plus mission on a rover that was supposed to only last 90 days.  But there has, over the years, been a noticable and steady drop off of power that the cells can generate.

Maybe to some degree, but IIRC a cleaning event earlier this year got Opportunity up to 94% of its original capacity. So most of the loss does seem to be easily removable dust.

(And Opportunity apparently tends to get cleaning events on hillsides, so static panels placed on hills would probably do better than Opportunity has, since they would be on the hill all the time.)

Do you have a source about the 94% result? I'd be very interested in it.

"in May Opportunity's solar array dust factor went from 0.832 to 0.962, which is close to as good as it can get and a record for a rover more than 10 years into its mission."

http://www.planetary.org/explore/space-topics/space-missions/mer-updates/2014/05-mer-update-opportunity-hunts-ancient-clays.html (it's next to the picture of a hypothetical early Mars with oceans)

So it's actually 96%, even better.

That only says the dust layer has low impact. Doesn't seem to say how the underlying panels are degrading.

Cheers, Martin

[RanulfC]

actually both chemical and nuclear thermal rocket engines are significantly constrained by material properties and temperature limits. NTR needs to conduct heat through a solid heat exchanger to the gas, but chemical rockets can have the gas combust without contacting any solid surface directly, so for the same materials you can tolerate HIGHER temperatures with a chemical rocket. The big thing ISN'T the energy per kilogram of fuel but instead the molecular weight of the fuel in combination with energy. A typical hydrolox chemical rocket already tends to be constrained by temperature, so adding nuclear power would ONLY add more mass if operating at the same mixture ratio.

Point? Nuclear tends to greatly exceed the possible thermal limits of chemical, usually by a large margin. While LOX/LH2 can reach combustion temperatures of 2,985K/2,712c/4,913f NTRs can reach (and actually slighly exceed given the right design) near melting point of tungsten temperatures of 3,695K/3,422c/6191f using solid core designs which is where the very high ISP comes from. There's been no forseen issues with operations at those temperatures for NTR chambers and nozzles found. You simply can't get the higher temperatures with "just" chemical combustion.

Randy

[Robotbeat]

So you're operating at temperatures higher than the melting point of your fuel? Explain.

[R7]

While LOX/LH2 can reach combustion temperatures of 2,985K/2,712c/4,913f ... You simply can't get the higher temperatures with "just" chemical combustion.

With all due respect, nonsense. The first thing NASA tells Joe 6pack about SSME is how it's hotter than 6000F inside it and iron would boil there. RD-170 family runs even hotter. Tc is connected to Pc but not linearly. Attaching couple examples how Tc changes with O/F.

[Nindalf]

I'm going to be a bit stupid here and say this without doing the math, but I'm pretty sure that solid-fuel nuclear thermal rockets have a higher Isp mostly because of the lower average molecular mass of the exhaust, and its exclusively diatomic composition, not because of higher temperatures.

For instance, when you switch from hydrogen propellant to ammonia (the second-best choice for Isp), you tend to get specific impulse similar to a good hydrogen/oxygen chemical rocket.

Chemical hydrolox propellant is mostly oxygen by mass.  Even so, there's a large excess of hydrogen which does not react, but generates thrust efficiently.  You're primarily carrying the oxygen to heat the hydrogen (similarly, in a hydrocarbon/oxygen rocket, you're mostly carrying the carbon for that purpose as well, though it also makes the hydrogen easier to handle).  It's this excess mass of oxygen which holds the Isp down.

[R7]

I'm going to be a bit stupid here and say this without doing the math, but I'm pretty sure that solid-fuel nuclear thermal rockets have a higher Isp mostly because of the lower average molecular mass of the exhaust, and its exclusively diatomic composition, not because of higher temperatures.

You are exactly right. Among other things (ratio of specific heats, chamber temperature, pressure ratio) the exhaust velocity from nozzle is inversely proportional to the square root of mean molecular mass (M) of the gas. Quadruple M and exhaust velocity halves etc.

For instance in SSME the M is about 14 (notice how it isn't 18, the molecular mass of water, because the engines runs fuel rich). In hydrogen NTR the M is that of hydrogen gas, 2. If other factors are kept the same the exhaust from NTR is (1/sqrt(2))/(1/sqrt(14)) = ~2.6 times faster than from SSME, resulting the equivalent increase to Isp.

edit: forgot the root. Is there more elegant way of saying that. inverse-square-root...tish  ?

[Robotbeat]

Tank volume is a very important consideration as well as engine T/W. Both those things are terrible for NTR, so much so that even the bIg ISP advantage you started out with ends up being wasted.

[RanulfC]

So you're operating at temperatures higher than the melting point of your fuel? Explain.

Near melting point actually, but dynamic cooling was demonstrated during the DUMBO tests where elements were run at temps above the melting point of the core/fuel. It's a balancing game but it was the main reason the NERVA nozzle would not work with the DUMBO design.

Intersting charts I hadn't seen those before, thanks.

Tank volume is a very important consideration as well as engine T/W. Both those things are terrible for NTR, so much so that even the bIg ISP advantage you started out with ends up being wasted.

Tank volume hasn't been shown to be as big an issue as you make it out to be. Specifically tank mass for orbit-to-orbit ops can be very light. T/W is important during only part of the mission. That's where the idea of LANTR comes in. Higher ISP overall means more efficent so its not wasted.

Randy

[Nindalf]

T/W is important during only part of the mission.

Not true.  You get your delta-v from the Isp and the proportional difference in mass after you've used all your propellant.  More empty mass means less delta-v.

There's also the Oberth effect.  Low thrust maneuvers are inefficient, so you need more delta-v to get to the same destination.

T/W (or rather, thrust-to-mass ratio) is always important.  For any Isp advantage, in any application, there's some degree of T/W terribleness that would make it worthless.

[PreferToLurk]

I would like to echo other commenters, in that "ever" is a very long time.  Also, does the nuclear propulsion need to be thermal, or does nuclear/electrical/ion satisfy the question?  Lockheed Martin (I know, not the biggest fans of SpaceX) just announced plans to have a container car sized fusion plant operational in less than 10 years and a prototype in less than 5. See link here (scientificamerican.com) 

If SpaceX ever gets its monster raptor powered BFR built, they will have payload capacity to spare for exotic solutions.

[Robotbeat]

The issue is that people automatically assume the answer will be a certain magic solution. Maybe NTR just, you know, isn't that great?

[Patchouli]

T/W is important during only part of the mission.

Not true.  You get your delta-v from the Isp and the proportional difference in mass after you've used all your propellant.  More empty mass means less delta-v.

There's also the Oberth effect.  Low thrust maneuvers are inefficient, so you need more delta-v to get to the same destination.

T/W (or rather, thrust-to-mass ratio) is always important.  For any Isp advantage, in any application, there's some degree of T/W terribleness that would make it worthless.

NTR would still be able to take advantage of the Oberth effect but solar electric would not.

[Joey S-IVB]

Well the Skunk Works may have a production FUSION reactor available for SpaceX to purchase for the MCT by 2024. Just announced today in an Aviation Week article. http://aviationweek.com/technology/skunk-works-reveals-compact-fusion-reactor-details

[Robotbeat]

We shall see. Doubt it'd be useful for MCT, though. At best, it'd be useful for Mars surface power for ISRU. (And for the record, I take the LM news with a huge grain of salt. Alt-fusion projections are always optimistic.)

Chemical rockets are really awesome. They take stored chemical energy and release it very quickly and efficiently with very little dry mass. Once you get reusability down, then formerly-totally-good-ideas like SEP and whatnot look like a lot of work just to save a few pounds when you could do 20 chemical rocket flights (with the same rocket) while the SEP or NEP tug is still spiralling out of Earth's gravity well. From a capital-return standpoint, a fully and rapidly reusable chemical rocket vehicle is incredibly difficult to beat, plus both handling and development are WAY more straight-forward than they would be for an NTR... and fuel costs may be actually lower (!) since you can use methane and ultra-cheap oxygen instead of a WHOLE bunch of difficult-to-handle hydrogen.

Just use the nuclear power to crack water and/or carbon dioxide into propellant for the chemical rocket.

[Vultur]

Once you get reusability down, then formerly-totally-good-ideas like SEP and whatnot look like a lot of work just to save a few pounds when you could do 20 chemical rocket flights (with the same rocket) while the SEP or NEP tug is still spiralling out of Earth's gravity well.

For operations around Earth orbit/Moon/Lagrange points and at least initial trips to Mars, sure.

But for high delta V missions (really fast trips to Mars, manned missions to outer planet moons, etc.) and for pushing asteroids around (or anything that involves moving really huge masses, otherwise the fuel mass would become ridiculous quickly) eventually you'll need something with a higher specific impulse.

[Robotbeat]

I didn't say SEP has no uses. But abundant chemical gives you a lot of delta-v for those high-delta-v missions.

[clongton]

I didn't say SEP has no uses. But abundant chemical gives you a lot of delta-v for those high-delta-v missions.

For an extremely limited amount of time, and then you're, well, all done.
That's the Achilles heel of chemical.

[Robotbeat]

I didn't say SEP has no uses. But abundant chemical gives you a lot of delta-v for those high-delta-v missions.

For an extremely limited amount of time, and then you're, well, all done.
That's the Achilles heel of chemical.

You have to fuel up, my friend! SEP is awesome, but.... A full mission to Mars could mean 1000 launches if you built an RLV instead of an SEP stage ... It's really hard to reuse an SEP Mars Transfer Vehicle more than 10-15 times because now it's 3 decades old.

[Nilof]

I didn't say SEP has no uses. But abundant chemical gives you a lot of delta-v for those high-delta-v missions.

That depends on what you consider a high delta-v mission. For example, going to Mercury and back(or any mission with a similar delta-v) with chemical is going to be really impractical even with abundant fuel in Earth orbit, while a similar mission with SEP is very feasible.

Sure, you can go to the Moon and Mars with chemical. You can cut down travel time to Mars a bit with abundant chemical. But there isn't that many other interesting things you can do with it. If you want routine impulsive transfers to these targets, you're better off looking at tether transport systems than you are looking at ultra-large scale propellant depots. Reusable reaction mass is the way to go.

[RanulfC]

I didn't say SEP has no uses. But abundant chemical gives you a lot of delta-v for those high-delta-v missions.

For an extremely limited amount of time, and then you're, well, all done.
That's the Achilles heel of chemical.

You have to fuel up, my friend! SEP is awesome, but.... A full mission to Mars could mean 1000 launches if you built an RLV instead of an SEP stage ... It's really hard to reuse an SEP Mars Transfer Vehicle more than 10-15 times because now it's 3 decades old.

And? So what really, most if not all the parts are modular and replacable unless you're imagining some sort of off-the-wall (or NASA ) once-off system. Lest we forget that SEP also allows that you have those nice convienent solar panels now in Mars orbit where you can beam the power to any point on the surface you need it.

I'm going to point out again however that SEP isn't something that SpaceX seems interested in and they SEEM to be focused solely on "abundant chemical" only architecture.

Randy