DRACO: NASA and DARPA nuclear propulsion collaboration

[edzieba]

A cynical take is using DRACO to (re)establish "modern" knowledge/procedures/regulations while fully expecting it to DARPA fail, then some civilian company rolls in on the coattails. Like Atomos Space and their nuclear OTV.

Much the same way KRUSTY/kilopower was reestablishing modern space nuclear contexts, simply because the last time people got serious was "too long ago" in the eyes of too many people.

I'm not sure that's so much a 'cynical take' than the entire purpose of DARPA.

[edkyle99]

TBF. uranium reactors are safer in the case of a launcher crash than RTGs. Unfired (or just fired to criticality for a few seconds for the purpose of control characterization) is pretty much safe. The worst thing is (chemically) toxic materials, but such were and are launched in copious amounts already. It only becomes problematic after the reactor was fired for a few minutes and it now contains measurable amounts of fission products.

We probably shouldn't launch it into short life orbits nor orbits with a high risk of collision. But if the thing would end up in a orbit with 10000+ years decay, it's generally fine.

We just saw a Soyuz spring a leak, likely from a space debris hit.  That would not be a good look on an orbiting reactor.

 - Ed Kyle

[sebk]

TBF. uranium reactors are safer in the case of a launcher crash than RTGs. Unfired (or just fired to criticality for a few seconds for the purpose of control characterization) is pretty much safe. The worst thing is (chemically) toxic materials, but such were and are launched in copious amounts already. It only becomes problematic after the reactor was fired for a few minutes and it now contains measurable amounts of fission products.

We probably shouldn't launch it into short life orbits nor orbits with a high risk of collision. But if the thing would end up in a orbit with 10000+ years decay, it's generally fine.

We just saw a Soyuz spring a leak, likely from a space debris hit.  That would not be a good look on an orbiting reactor.

 - Ed Kyle

Only if the reactor was running or recently running (i.e. requiring active cooling). NTRs (and that's what discussed here) run for hours and cool down for hours too. After that the thing is passive so speck of dust is not going to make it RUD.

[InterestedEngineer]

Just saw a couple of ideas from Radical Moderate that might make NTR useful for e.g. Mars or deep space missions:

Not arguing for more dedicated structure. Suggesting that items that may have a use at the destination may be used to help structurally. Wasn't thinking about a pressurizable structure, just a bit of support from items that are eventually payload.

Were you think of truss elements that could be reassembled for other uses?

I'm prejudiced toward thinking about this as a nuke.  Odds are that a nuke dumps all of its payload, which makes arrangements for its own arrival at wherever it's going.  That makes it unlikely that anything gets reused other than the nuke, which wants to be as light as possible, since it probably has to do a pure braking burn if it's to be captured.

Second most likely is a hydrolox architecture that has to aerocapture the bejeezus out of itself, and you're not recycling any odds and ends from that, either.

There are obviously other architectures, but let's keep it to the irreducible minimum:  How light can we get away with for a departure burn?

Summarizing: 

(1) lightweight LH2 tank that is throwaway light and cheap.  This drops the LH2 tank to say 1% of prop weight instead of 10%.  Which puts the deltaV of a 100t NERVA system to 18km/sec
(2) A reserve tank for the NERVA stage to boost back to Earth for reuse (probably with a gravity turn around the target).  By the time it gets back it'll be nice and cool.


This is a little bit better than breakeven with a GTO refueled Starship, so any Isp improvements in the NTR make it outclass everything else again.

The reuse is a tad bit complicated though.  And still a hard time justifying the development cost when there's a ton of spare Starships floating around with cheap refueling.

Nuclear lightbulb with lightweight balloon tanks, OTOH gets into the 40km/sec deltaV range, which opens up new missions and high speed transfers.

[Robotbeat]

So, from where will they launch this nuclear reactor?  No reactor has ever been launched from Florida.  One was launched from Vandenberg, but that was during the 1960s and probably no one knew.  (It is still in orbit today, a few hundred km above our heads, leaking something...)   People get mad about all sorts of things these days.  I think we can predict that many will not like this idea.  If they couldn't find political support for NERVA back in the day, I can't see how this will fly.  And it isn't just something that affects the U.S..  It will have to overfly other countries on its way to orbit.

 - Ed Kyle

Lots of Pu238 has flown out of florida, and this reactor wouldn't start, maybe until it got out to like the Moon or something, so it would be *far* less radioactive than a typical probe RTG.

[sebk]

Just saw a couple of ideas from Radical Moderate that might make NTR useful for e.g. Mars or deep space missions:

Not arguing for more dedicated structure. Suggesting that items that may have a use at the destination may be used to help structurally. Wasn't thinking about a pressurizable structure, just a bit of support from items that are eventually payload.

Were you think of truss elements that could be reassembled for other uses?

I'm prejudiced toward thinking about this as a nuke.  Odds are that a nuke dumps all of its payload, which makes arrangements for its own arrival at wherever it's going.  That makes it unlikely that anything gets reused other than the nuke, which wants to be as light as possible, since it probably has to do a pure braking burn if it's to be captured.

Second most likely is a hydrolox architecture that has to aerocapture the bejeezus out of itself, and you're not recycling any odds and ends from that, either.

There are obviously other architectures, but let's keep it to the irreducible minimum:  How light can we get away with for a departure burn?

Summarizing: 

(1) lightweight LH2 tank that is throwaway light and cheap.  This drops the LH2 tank to say 1% of prop weight instead of 10%.  Which puts the deltaV of a 100t NERVA system to 18km/sec
(2) A reserve tank for the NERVA stage to boost back to Earth for reuse (probably with a gravity turn around the target).  By the time it gets back it'll be nice and cool.


This is a little bit better than breakeven with a GTO refueled Starship, so any Isp improvements in the NTR make it outclass everything else again.

The reuse is a tad bit complicated though.  And still a hard time justifying the development cost when there's a ton of spare Starships floating around with cheap refueling.

Nuclear lightbulb with lightweight balloon tanks, OTOH gets into the 40km/sec deltaV range, which opens up new missions and high speed transfers.

This is deep Sci-Fi specs. 10% is already pushing things (and it's 10% for just the tank, not the entire vehicle). 1% is not possible at any foreseeable future tech).

Space Shuttle SLWT ET hydrogen tank part reached ~11.5%. And this doesn't include propellant feed lines.  Note that ET is a pretty good approximation of an externally hanging jettisonable tank because it was one. Thanks to smart engineering hydrogen part of the ET essentially hung off the intertank which was the main element transferring flight loads between SRBs, ET and the orbiter. The switch to in-line stage tank in SLS increased that mass approximately 3-fold. Strip off the 1inch thick foam from the hydrogen tank part of the ET, replace it with MLI mat and you may get around 10% mass ratio hydrogen tank.

IOW. 10% is state of the art. 1% is pure Sci-Fi


One could argue that ET used 2 bar pressure while in-space propulsion needs less. But less pressure means:

- Higher boiloff rate, so your losses increase and your integrated vehicle fluids solution has more power, i.e. gets heavier
- Your tubing must be wider and your turbines bigger. Turbines must be already massive for pure hydrogen NTR. This is more mass as well.

You're also going to hit minimum gauge issues if you used anything else than pure metal.


Then there's engine mass itself. The pressure vessel scaling applies pretty well to 2/3 mass of the engines (3x less dense hydrolox is used by 2x worse TWR engines). at 13x (vs methalox) to 14.5x (vs kerolox) density your best hope for NTR excluding the reactor core is ~20x. But in an NTR about 2/3 of the mass is core, neutronics and shield (I'm excluding pressure vessel, as this would be replaced with chamber, injector and its manifolds and cooling). So 7x sounds like a realistic foreseeable future tech limit (and the talk about 30x TWR from some studies sounds like a similar BS to 21 days to Mars using VASIMR, i.e. usually it's achieved by sweeping smelly parts under the carpet). 0.2 whole vehicle TWR (payload included) is a limit below which Oberth effect for interplanetary injections diminishes too fast, engine must be 2.85% of the wet mass.


You also need an actual structure to hang the tanks from, plus quite a bit of the misc stuff (payload adapters, power, etc).
Together with engines and tanks it will be no less than 15% of the mass. And you want your payload to be at about 2/3 of the dry empty vehicle mass (to be comparable with Starship). So 10% of the total mass.

If  you drop tanks and payload at interplanetary speed and do propulsive capture you need 1/3 of your core structure mass (5% of total) to be braking propellant. So ~1.67% of total mass reserved for the capture.

Together 26.67% dry and 73.33% propellant. Mass ratio of 3.75:1.

But this is with propulsive capture from 5.5 month transit.

If you instead want faster transits like 3 months (the max achievable by chemical propulsion without heroic efforts) you now need 2.67 mass ratio for propulsive capture (chemical would just brake by 4km/s leaving the rest of 13.3km/s dV for aerodynamics), i.e. 13.33% of total mass reserved for capture. But payload released at such a speed (~13.3km/s) would have severe problems braking at Mars and for people it would be plain unsurvivable (Mars entry limit is about 9.5km/s for crewed vehicles; the combination of weak gravity, and tight curvature is nasty). So don't release the payload before capturing into at least HEMO. Now your braking fuel reserve goes form 13.33% to 40%.

So payload 10%, capture fuel 40%, capture tankage 4% (it's not negligible). The rest of the vehicle 5%. 59% together. 41% remains for TMI propellant and its drop tanks. 900s ISP doesn't close unless you inject from elongated orbit.

For 1000s ISP you need 36% for Mars capture, total gets less to 55%. And now you're able to do 5.5km/s TMI from LEO.

It's meh. It gives performance as chemical propulsion but with more hassle.

[grdja]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

And saying that some mentioned stuff is "solved" today is rather dishonest. Starship didn't have a test fire where it didn't lose parts of heat shield. We are years away from Starship being capable of safety and routinely doing LEO reentry. Full on Mars aerocapture and EDL with tile based heat shield may not be possible, and remember you need that same heat shield when you return to Earth.

[JayWee]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

It smells very similar to the "Isp obsession" people had with hydrogen. And it hails from the same age.
While I do support nuclear power, NTR seems wasteful. Total energy contained in the uranium vs fraction used to accelerate working gas.

And most importantely - I've yet to see a proper economic calculation - how much is the engine going to cost? Because that's *all* what matters.

[edzieba]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

It smells very similar to the "Isp obsession" people had with hydrogen. And it hails from the same age.
While I do support nuclear power, NTR seems wasteful. Total energy contained in the uranium vs fraction used to accelerate working gas.

And most importantely - I've yet to see a proper economic calculation - how much is the engine going to cost? Because that's *all* what matters.

Being a thermal system an NTR is more efficient than any electrical nuclear system (compare MWt vs. MWe for any reactor). Cost is also not the sole arbiter of utility (in space systems or otherwise).

[InterestedEngineer]

Just saw a couple of ideas from Radical Moderate that might make NTR useful for e.g. Mars or deep space missions:

Not arguing for more dedicated structure. Suggesting that items that may have a use at the destination may be used to help structurally. Wasn't thinking about a pressurizable structure, just a bit of support from items that are eventually payload.

Were you think of truss elements that could be reassembled for other uses?

I'm prejudiced toward thinking about this as a nuke.  Odds are that a nuke dumps all of its payload, which makes arrangements for its own arrival at wherever it's going.  That makes it unlikely that anything gets reused other than the nuke, which wants to be as light as possible, since it probably has to do a pure braking burn if it's to be captured.

Second most likely is a hydrolox architecture that has to aerocapture the bejeezus out of itself, and you're not recycling any odds and ends from that, either.

There are obviously other architectures, but let's keep it to the irreducible minimum:  How light can we get away with for a departure burn?

Summarizing: 

(1) lightweight LH2 tank that is throwaway light and cheap.  This drops the LH2 tank to say 1% of prop weight instead of 10%.  Which puts the deltaV of a 100t NERVA system to 18km/sec
(2) A reserve tank for the NERVA stage to boost back to Earth for reuse (probably with a gravity turn around the target).  By the time it gets back it'll be nice and cool.


This is a little bit better than breakeven with a GTO refueled Starship, so any Isp improvements in the NTR make it outclass everything else again.

The reuse is a tad bit complicated though.  And still a hard time justifying the development cost when there's a ton of spare Starships floating around with cheap refueling.

Nuclear lightbulb with lightweight balloon tanks, OTOH gets into the 40km/sec deltaV range, which opens up new missions and high speed transfers.

This is deep Sci-Fi specs. 10% is already pushing things (and it's 10% for just the tank, not the entire vehicle). 1% is not possible at any foreseeable future tech).

Space Shuttle SLWT ET hydrogen tank part reached ~11.5%. And this doesn't include propellant feed lines.  Note that ET is a pretty good approximation of an externally hanging jettisonable tank because it was one. Thanks to smart engineering hydrogen part of the ET essentially hung off the intertank which was the main element transferring flight loads between SRBs, ET and the orbiter. The switch to in-line stage tank in SLS increased that mass approximately 3-fold. Strip off the 1inch thick foam from the hydrogen tank part of the ET, replace it with MLI mat and you may get around 10% mass ratio hydrogen tank.

IOW. 10% is state of the art. 1% is pure Sci-Fi

Can you post this on the 'balloon tank thread'?  thanks

https://forum.nasaspaceflight.com/index.php?topic=58197.msg2454500#new

[InterestedEngineer]

(Mars entry limit is about 9.5km/s for crewed vehicles; the combination of weak gravity, and tight curvature is nasty). So don't release the payload before capturing into at least HEMO.

I did some basic geometry on an envelope and came up with 7 km/sec for aerobraking deltaV on Mars (in a single pass).

You got a better source than the back of the envelope I did?  I couldn't find one.

[InterestedEngineer]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

It smells very similar to the "Isp obsession" people had with hydrogen. And it hails from the same age.
While I do support nuclear power, NTR seems wasteful. Total energy contained in the uranium vs fraction used to accelerate working gas.

And most importantely - I've yet to see a proper economic calculation - how much is the engine going to cost? Because that's *all* what matters.

Being a thermal system an NTR is more efficient than any electrical nuclear system (compare MWt vs. MWe for any reactor). Cost is also not the sole arbiter of utility (in space systems or otherwise).

When cost drops by 3 orders of magnitude, it becomes the sole arbiter for a whole lot of existing mission concepts.

[InterestedEngineer]

Then there's engine mass itself. The pressure vessel scaling applies pretty well to 2/3 mass of the engines (3x less dense hydrolox is used by 2x worse TWR engines). at 13x (vs methalox) to 14.5x (vs kerolox) density your best hope for NTR excluding the reactor core is ~20x. But in an NTR about 2/3 of the mass is core, neutronics and shield (I'm excluding pressure vessel, as this would be replaced with chamber, injector and its manifolds and cooling). So 7x sounds like a realistic foreseeable future tech limit (and the talk about 30x TWR from some studies sounds like a similar BS to 21 days to Mars using VASIMR, i.e. usually it's achieved by sweeping smelly parts under the carpet). 0.2 whole vehicle TWR (payload included) is a limit below which Oberth effect for interplanetary injections diminishes too fast, engine must be 2.85% of the wet mass.

You also need an actual structure to hang the tanks from, plus quite a bit of the misc stuff (payload adapters, power, etc).
Together with engines and tanks it will be no less than 15% of the mass. And you want your payload to be at about 2/3 of the dry empty vehicle mass (to be comparable with Starship). So 10% of the total mass.

The model I used already uses the NERVA engine mass and then doubles it for shield mass.

Then double that again for the structure mass (so 91t dry mass for 1% tanks, 208t for 10% tanks)

It makes zero sense at 208t but almost makes sense at 91t.

Yes, need magic tanks.  See Mylar tank thread.

https://docs.google.com/spreadsheets/d/1ycrQlKql9fft9oLDoW1uP-rotK-Y5yVtH4T3S2ZKgk4

[InterestedEngineer]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

And saying that some mentioned stuff is "solved" today is rather dishonest. Starship didn't have a test fire where it didn't lose parts of heat shield. We are years away from Starship being capable of safety and routinely doing LEO reentry. Full on Mars aerocapture and EDL with tile based heat shield may not be possible, and remember you need that same heat shield when you return to Earth.

Engineers tend to not like solutions where the first order math doesn't make sense.

Starship is much farther along than any NTR rocket on paper right now.   The first order math makes sense.

[sebk]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

This is not extreme hostility. This is just reality check against BS talk about this being a solution for Mars travel. In the case for travel into deeper space (the Belt and beyond) it makes more sense. For example 4:1 mass ratio NTR with realistic ISP could get to Vesta in 9 months which is ~2 months better than chemical without heroic measures. Which is something. But it suffers the problem of requiring ISRU for return flight as well.

Without ISRU one could maybe get 4:1 to do 1 year leg to Vesta, one year stay and 1 year leg back to the Earth without ISRU. But it requires 3 years hydrogen storage.

In that regard, if one could get 300kW/t (300W/kg) electric power source, then one could build an electric propulsion ship which could get there and back on a single propellant load, too. We're getting there for solar panels at 1AU solar distance, by we're not there at 2.4AU distance. And we're not there for nuclear (Kilopower is 40x off the mark).

And saying that some mentioned stuff is "solved" today is rather dishonest. Starship didn't have a test fire where it didn't lose parts of heat shield. We are years away from Starship being capable of safety and routinely doing LEO reentry. Full on Mars aerocapture and EDL with tile based heat shield may not be possible, and remember you need that same heat shield when you return to Earth.

Sorry, but NTR isn't solved either. We are even more years away when any NTR (even demo) would fly. Starship Mars EDL is much firmer thing than some NTR flight. And there are no fundamental issues with EDL on Mars. We have good enough understanding to know that tiles are workable. Moreover there are multiple backup solutions. It's an engineering problem.

[sebk]

(Mars entry limit is about 9.5km/s for crewed vehicles; the combination of weak gravity, and tight curvature is nasty). So don't release the payload before capturing into at least HEMO.

I did some basic geometry on an envelope and came up with 7 km/sec for aerobraking deltaV on Mars (in a single pass).

You got a better source than the back of the envelope I did?  I couldn't find one.

The problem is not just aerobraking (or rather aerocapture), but g-load during such.

If you do ballistic entry (without lift) you could only fly so far before you punch through and either impact the surface or skip back to space. At 9.5km you will punch through and be still above escape velocity unless you put up with pretty high g-loads (the atmospheres have this "nasty" property of their density reducing exponentially with altitude, so as you enter the atmosphere the g-load is small but then suddenly grows exponentially as you get lower). It's hard to estimate exactly but it'd be somewhere in 6-15g range.

The other option is to actually follow the curvature of the planet, by generating (negative) lift. But to follow the curvature your acceleration must be:   -g_L((V/V₁)^2 - 1), where g_L is the local planet's surface gravity. So for 9.5km/s it's 3.38*(3.6^2 - 1) = ~40.4 i.e. ~4.12g. Now each vehicle has L:D ratio. For 0.5-0.6 L:D of something shaped like Starship the total g-load would be twice that at above 8g. That's a lot, too.

Either way you're in rather g regime, but survivable. But beyond said 9.5km/s things get really sketchy.

[john smith 19]

TBF. uranium reactors are safer in the case of a launcher crash than RTGs. Unfired (or just fired to criticality for a few seconds for the purpose of control characterization) is pretty much safe. The worst thing is (chemically) toxic materials, but such were and are launched in copious amounts already. It only becomes problematic after the reactor was fired for a few minutes and it now contains measurable amounts of fission products.

We probably shouldn't launch it into short life orbits nor orbits with a high risk of collision. But if the thing would end up in a orbit with 10000+ years decay, it's generally fine.

For orbital changes more like runs-for-minutes and stays cool for days.

IIRC NERVA was going to operate a few hours to push to mars.
We just saw a Soyuz spring a leak, likely from a space debris hit.  That would not be a good look on an orbiting reactor.

 - Ed Kyle

Only if the reactor was running or recently running (i.e. requiring active cooling). NTRs (and that's what discussed here) run for hours and cool down for hours too. After that the thing is passive so speck of dust is not going to make it RUD.

[grdja]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

This is not extreme hostility. This is just reality check against BS talk about this being a solution for Mars travel. In the case for travel into deeper space (the Belt and beyond) it makes more sense. For example 4:1 mass ratio NTR with realistic ISP could get to Vesta in 9 months which is ~2 months better than chemical without heroic measures. Which is something. But it suffers the problem of requiring ISRU for return flight as well.

Without ISRU one could maybe get 4:1 to do 1 year leg to Vesta, one year stay and 1 year leg back to the Earth without ISRU. But it requires 3 years hydrogen storage.

In that regard, if one could get 300kW/t (300W/kg) electric power source, then one could build an electric propulsion ship which could get there and back on a single propellant load, too. We're getting there for solar panels at 1AU solar distance, by we're not there at 2.4AU distance. And we're not there for nuclear (Kilopower is 40x off the mark).

And saying that some mentioned stuff is "solved" today is rather dishonest. Starship didn't have a test fire where it didn't lose parts of heat shield. We are years away from Starship being capable of safety and routinely doing LEO reentry. Full on Mars aerocapture and EDL with tile based heat shield may not be possible, and remember you need that same heat shield when you return to Earth.

Sorry, but NTR isn't solved either. We are even more years away when any NTR (even demo) would fly. Starship Mars EDL is much firmer thing than some NTR flight. And there are no fundamental issues with EDL on Mars. We have good enough understanding to know that tiles are workable. Moreover there are multiple backup solutions. It's an engineering problem.

What? I thought basic NTR was a solved thing since mid '60es. Both USA and USSR got their respective NTR engines to the test stands? And only issue was that there was no program that would use them and fly them?

[Robotbeat]

I'm confused by extreme hostility to nuclear propulsion out of a sudden. Shouldn't it still be greatly beneficial for places that are not Mars (and Titan) where you simply don't have ISRU access to methane?

This is not extreme hostility. This is just reality check against BS talk about this being a solution for Mars travel. In the case for travel into deeper space (the Belt and beyond) it makes more sense. For example 4:1 mass ratio NTR with realistic ISP could get to Vesta in 9 months which is ~2 months better than chemical without heroic measures. Which is something. But it suffers the problem of requiring ISRU for return flight as well.

Without ISRU one could maybe get 4:1 to do 1 year leg to Vesta, one year stay and 1 year leg back to the Earth without ISRU. But it requires 3 years hydrogen storage.

In that regard, if one could get 300kW/t (300W/kg) electric power source, then one could build an electric propulsion ship which could get there and back on a single propellant load, too. We're getting there for solar panels at 1AU solar distance, by we're not there at 2.4AU distance. And we're not there for nuclear (Kilopower is 40x off the mark).

And saying that some mentioned stuff is "solved" today is rather dishonest. Starship didn't have a test fire where it didn't lose parts of heat shield. We are years away from Starship being capable of safety and routinely doing LEO reentry. Full on Mars aerocapture and EDL with tile based heat shield may not be possible, and remember you need that same heat shield when you return to Earth.

Sorry, but NTR isn't solved either. We are even more years away when any NTR (even demo) would fly. Starship Mars EDL is much firmer thing than some NTR flight. And there are no fundamental issues with EDL on Mars. We have good enough understanding to know that tiles are workable. Moreover there are multiple backup solutions. It's an engineering problem.

What? I thought basic NTR was a solved thing since mid '60es. Both USA and USSR got their respective NTR engines to the test stands? And only issue was that there was no program that would use them and fly them?

Youve gotta decide on your standard. By the standard of “NTRs were solved in the 60s,” every tech involved with Starship is solved.

The NTRs of the 1960s still needed a bunch of certification to get to actual flight.

[FutureSpaceTourist]