Nuclear Propulsion and Space Exploration

What would have been the effect of allowing nuclear propulsion in space exploration?

I'm asking specifically for effects on exploration, not cultural effects.

What's the likely cost of nuclear propulsion compared to OTL craft?
Are there materials restrictions that needed to be overcome?
What projects/targets does this technology make viable?

It depends on exactly what you want, because there might need to be socio-political PODs that need to be tweaked first, especially if you're thinking of an ATL space program where something like Orion is possible.

Mike Stearns said:

It depends on exactly what you want, because there might need to be socio-political PODs that need to be tweaked first, especially if you're thinking of an ATL space program where something like Orion is possible.

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I'm really just asking about the science and the exploration, not necessarily working out a POD where something is possible politically.
That said, if something is really prohibitively expensive where we're basically pauperizing the Earth to make it happen, that's...um...salient information.

Based on what we know of nuclear propulsion, where could it get us, what can it do for us, where *couldn't* it get us/what *couldn't* it do for us?

What do you mean by "nuclear propulsion"?

An RTG powered ion engine is different from a reactor powered ion engine is different from nuclear thermal (e.g. NERVA) is different from an Orion style spaceship powered by throwing bombs out the back, is different from a vented fusion reaction.

A nuclear powered ion engine would help a LOT in the exploration of the outer planets (as it's too far for decent solar power). Inside the orbit of Mars, you can use solar power for the engine, and it's a lot easier to do politically.

A NERVA style engine is interesting, but not all that great. Yes, its ISP is almost twice that of LH2/Lox chemical engines, but they are massive (which makes the dry mass higher, which undermines some of that advantage), AND they can't be used as tugs because there's a strong limit on how long the engine can run (if I remember what EofPi said).

VASIMIR engines powered by a high powered fission reactor are interesting.

Anything that involves setting off dozens of nukes for propulsion is highly unlikely to be funded.

Anything that requires controlled fusion requires that we DEVELOP that first before we can use it.

Dathi THorfinnsson said:

What do you mean by "nuclear propulsion"?

A NERVA style engine is interesting, but not all that great. Yes, its ISP is almost twice that of LH2/Lox chemical engines, but they are massive (which makes the dry mass higher, which undermines some of that advantage), AND they can't be used as tugs because there's a strong limit on how long the engine can run (if I remember what EofPi said).

VASIMIR engines powered by a high powered fission reactor are interesting.
.

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I agree. Nuclear propulsion only really get's interesting once we have the ability to build some type of spacecraft that stays in orbit that is then refurbished between missions. Reminds me what was used in the book "The Martian".

Dathi THorfinnsson said:

What do you mean by "nuclear propulsion"?

An RTG powered ion engine is different from a reactor powered ion engine is different from nuclear thermal (e.g. NERVA) is different from an Orion style spaceship powered by throwing bombs out the back, is different from a vented fusion reaction.

A nuclear powered ion engine would help a LOT in the exploration of the outer planets (as it's too far for decent solar power). Inside the orbit of Mars, you can use solar power for the engine, and it's a lot easier to do politically.

A NERVA style engine is interesting, but not all that great. Yes, its ISP is almost twice that of LH2/Lox chemical engines, but they are massive (which makes the dry mass higher, which undermines some of that advantage), AND they can't be used as tugs because there's a strong limit on how long the engine can run (if I remember what EofPi said).

VASIMIR engines powered by a high powered fission reactor are interesting.

Anything that involves setting off dozens of nukes for propulsion is highly unlikely to be funded.

Anything that requires controlled fusion requires that we DEVELOP that first before we can use it.

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A very helpful rundown! I'm definitely asking as a non-expert. I was certainly thinking of technology that could have been reasonably developed in deployed over the last 60 years, not something more theoretical like fusion.

I guess what prompted the thread was an actual TL on the board- Shift in Priorities- but I didn't want to get bogged down in the politics of that thread. Suffice it to say, there are no international nuclear controls as space exploration gets kicked off. A shuttle is on the way, which means (barring political realities) stations and tugs* should be the next logical steps.

*If by tugs we're talking about craft built or assembled in space intended to remain in space.

A couple other basic questions, and I appreciate you taking the time:

1) would these methods be cost-effective for unmanned craft? The benefit in my mind would be not having to rely on rare and complex alignments to study the solar system, but if it's not cost-effective then scratch that.

2) Were all of these methods available 60 years ago, or did they require subsequent advances to go from purely theoretical to merely politically impossible? What's a general "release date" for each of them, given favorable political circumstances?

Expat said:

2) Were all of these methods available 60 years ago, or did they require subsequent advances to go from purely theoretical to merely politically impossible? What's a general "release date" for each of them, given favorable political circumstances?

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Orion was probably doable with 1970s tech.

It wasn't the bombs, as much as the pusher plate

marathag said:

Orion was probably doable with 1970s tech.

It wasn't the bombs, as much as the pusher plate

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But an Orion style craft's biggest advantage is getting gigantic spacecraft off the ground. You NEED to have tonnes of mass to absorb the impulse of atomic bombs going off regularly. The other thing is an earth launch Orion would be an Apollo frequency thing - you'd likely only launch one a year or so.

If you tried using one as an Earth orbit tug, on a regular basis, you'd be be fouling up cis-lunar space something fierce with energetic particles, EMP, etc. What they'd do to the Van Allen Belts, I don't want to know. Or unshielded electronics on earth.

Again, with Orion, if it's only once a year or so, you can have everyone in the Western Hemisphere shut down all electronics on launch day. If you need to.

A regular tug? Not so much.


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AFAIK, the best space nuclear reactor so far has been the TOPAZ reactor the Soviets built. That was ground tested in 1971, and was rated for 5kWe for 3-5 years, although the only two flown apparently only operated for 6 months and a year.

That reactor apparently uses 12kg of U235 fuel, with a total reactor mass of 320kg.

This reactor, and the BES 5 reactor (2kWe, much heavier) used in the Soviet RORSAT (radar sat) program, e.g. Kosmos 954 that crashed in Canada, used thermionic conversion to turn heat into electricity. This has the advantage of no moving parts - but is relatively low efficiency.

Thermionic converters (not nuclear) only had their first prototype in 1957, and one clearly needs time to develop them, increasing efficiency, finding what might work with your reactor, etc.

So. A reactor powered ion engined mission to the Outer Planets is probably only possible iOTL in the mid 70s by the Soviets, and that if they wanted to throw a bucket of money at it.

How early could you push that forward? With a PoD of, say the end of WWII?

Hmmm... With luck you could get the thermionic converter a few years earlier. Ion engines and building the nuclear powered satellite would take a bit. ... Plus you need satellite reliability to be good enough for a multi-year voyage.

Late '60s?

Expat said:

1) would these methods be cost-effective for unmanned craft? The benefit in my mind would be not having to rely on rare and complex alignments to study the solar system, but if it's not cost-effective then scratch that.

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Not particularly. You're talking easily in the hundreds of millions per unit for a NERVA, beyond the billions in development costs--it is after all a reactor about 4x the power of a Nimitz class carrier, and for BEO that reactor isn't coming home. Even for a LEO-to-LLO or LEO-to-LLO, that's expensive when you'll only get a few flights out of it. Even for a Flagship mission like MSL, that'd be a 10% increase in cost, and for very little benefit--launch windows are plenty frequent for science missions that are being planned years in advance anyway (they come around about every two years for Mars and Venus, and every year or so for most other places).

2) Were all of these methods available 60 years ago, or did they require subsequent advances to go from purely theoretical to merely politically impossible? What's a general "release date" for each of them, given favorable political circumstances?

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NERVA was test-fired pretty extensively in the 60s and 70s, but the question of storing hydrogen in space for extended use of NERVA wasn't really addressed--solving that ironically helps conventional LH2/LOX chemical fuels a lot more, since the propellant for that is mostly more-easily-stored LOX, rather than entirely hydrogen as the propellant for nuclear thermal rockets is.

Orion was in a much less technically advanced state, but was perhaps achievable by the 80s...if the world had a sudden attack of the crazy.
Dathi answered nuclear-electric pretty well, so I'll leave him to that one.

Thanks, folks! I feel better informed.

An aqueous homogeneous reactor could be an interesting power source for a spacecraft. It would he able to provide significantly more power than a nuclear battery at a lower weight, so much that it would need a sizable radiator array for cooling due to the difficulty of radiating heat into space. Apart from powering an ion engine, I'm not sure what else the power could be used for. Perhaps there is something interesting that spacecraft engineers haven't thought of or included given the anemic power budgets provided by solar panels and nuclear batteries.

There three ways for nuclear propulsion that really work

ORION
you take a bucket put a ignited fire-crackers under it and watch it fly
now replace the Bucket with large Steel plate with schock absorber and the fire-crackers with allot small nuclear bombs.
theoretical a 4000 ton Orion space ship launch from earth, can carry 1500 tons payload into solar system.
yep, it's radioactive fallout is hellish...

Nuclear Thermal Rocket aka NERVA
intrinsic a immersion heater, using nuclear reactor to heat liquid hydrogen
nearly double payload compare to LoxHydro chemical engine
But next the radioactivity of just used Nerva engine (keep more that 14 kilometer away form it in Orbit)
It lose piece of the reactor during function, also can a Nerva go supercritical and melt (what happen with Chernobyl reactor)

Nuclear Electric Propulsion
that Ion or Plasma Engine power by nuclear reactor
it's heavy and slow in begin, do low thrust, but once gain momentum, it can get really fast.
for unmanned cargo mission Earth-moon with travel time of Year, it can bring allot payload.
on Mars trips it same like Nerva engine performance.
but it's real game changer for trips into outer solar system here on long distance it get really fast
were chemical engine need 5 year to get Jupiter and back and 12 years for Saturn
can NEP reach Jupiter faster in 2 years and Saturn under 5 years

but there is catch Money and politic
let face it, No one is sufficed crazy to build a ORION space ship.
A NERVA engine cost billions and it's environmentalist ultimative nightmare.
NEP has good chance to be build one day, either by Russian or Chinese
and they don't care about environmentalist...

Orion is one of those things we keep around for emergancies, normally its madness but in a bad situation (say an asteroid inbound) it could be used to launch an emergancy mission. Beyond that its unlikely ever to fly although its successors like Deadelus might be possible one day.

Dathi THorfinnsson said:

AFAIK, the best space nuclear reactor so far has been the TOPAZ reactor the Soviets built. That was ground tested in 1971, and was rated for 5kWe for 3-5 years, although the only two flown apparently only operated for 6 months and a year.

That reactor apparently uses 12kg of U235 fuel, with a total reactor mass of 320kg.

This reactor, and the BES 5 reactor (2kWe, much heavier) used in the Soviet RORSAT (radar sat) program, e.g. Kosmos 954 that crashed in Canada, used thermionic conversion to turn heat into electricity. This has the advantage of no moving parts - but is relatively low efficiency.

Thermionic converters (not nuclear) only had their first prototype in 1957, and one clearly needs time to develop them, increasing efficiency, finding what might work with your reactor, etc.

So. A reactor powered ion engined mission to the Outer Planets is probably only possible iOTL in the mid 70s by the Soviets, and that if they wanted to throw a bucket of money at it.

How early could you push that forward? With a PoD of, say the end of WWII?

Hmmm... With luck you could get the thermionic converter a few years earlier. Ion engines and building the nuclear powered satellite would take a bit. ... Plus you need satellite reliability to be good enough for a multi-year voyage.

Late '60s?

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The United States could certainly be a contender here. Fission-powered electric thrusters were heavily studied in the United States in the 1950s and 1960s for both robotic and crewed spacecraft, but due to lack of funding for the kind of spacecraft that would require them never really got into physical testing. If you found some way to change that, then the United States could certainly launch some kind of nuclear-electric spacecraft sometime in the late 1960s or 1970s; they launched the SERT-I ion thruster test in 1964 and the SNAP-10A space nuclear reactor in 1965, so the basic building blocks existed and had been tested in space, so about a TRL of 6 at that time. Putting them together should not have been all that hard...

The main barrier to any actual use, on a technical level, is the primitive state of electric thrusters at the time, not the nuclear side. All thruster designs, especially in the United States, at the time used mercury or cesium for propellant. Admittedly, these are dense and easily ionized metals, but they're also toxic, difficult to handle, explosive (for cesium), and, perhaps most importantly, foul up sensors on the spacecraft and create a cloud of ionized metal particles that can interfere with some scientific measurements. Obviously this reduced interest from the scientific community in electrically-propelled spacecraft. It wasn't until sometime in the 1980s or early 1990s that the idea of using a noble gas as a propellant was developed, leading to the current generation of xenon thrusters.

The main problem with the use of nuclear technology in space, in any case, has never been about international agreements but about the utility and technical feasibility of doing so (and the utility at a given level of technical feasibility). Funds for the kind of spacecraft for which nuclear propulsion of any type would be useful have never been available, so, aside from the NERVA program which was essentially developed for political reasons, there was never any significant interest in developing spacecraft that relied on nuclear sources for energy (aside from RTGs, due to their much lower cost). Nuclear propulsion simply wouldn't have helped anything that NASA was doing (or the Soviet space program). If you can find a way to have a larger space program, then nuclear propulsion techniques could be developed to support the larger missions it might undertake.