NASA Punts Over SLS With A Nuclear Rocket Solution For The Journey To Mars
Keith’s note: NASA announced a new Mars nuclear rocket ship thing today. Note that neither the symbol “$” or the abbreviation “FY” appear anywhere. Apparently this has to do with sending humans to Mars without SLS which Bill Nelson now says won’t happen until the late 2030s. I guess the NASA PAO art department will now need to come up with yet another infographic on how/when we’ll send people to Mars – with the dates all pushed to the right of course. Ask yourself this – if NASA took more than a decade and billions in cost overruns just to use existing stuff to make SLS how confident are we that they can create something wholly new like a nuclear propulsion system on time and under budget? They say: “NASA and the Defense Advanced Research Projects Agency (DARPA) announced Tuesday a collaboration to demonstrate a nuclear thermal rocket engine in space, an enabling capability for NASA crewed missions to Mars. NASA and DARPA will partner on the Demonstration Rocket for Agile Cislunar Operations, or DRACO, program. The non-reimbursable agreement designed to benefit both agencies, outlines roles, responsibilities, and processes aimed at speeding up development efforts.” more
11 responses to “NASA Punts Over SLS With A Nuclear Rocket Solution For The Journey To Mars”
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If nuclear thermal propulsion gets us to Mars in months instead of years, this helps a lot on the radiation problem for astronauts. For this reason the slower SLS/Orion will not support a sustained Mars program except for unmanned cargo. Nearly twenty years ago there was the Prometheus program for fission-powered ion drive propulsion but then defunded with no results, so we can probably expect to wait at least another 20 years for operational NTP, after maybe a Mars manned flyby with SLS.
Chemical propulsion doesn’t take years to get to Mars, and NTRs don’t get us there any faster, due to the need to propulsively brake into orbit and the crappy mass ratios that come with nuclear propulsion and liquid hydrogen propellant. DRA 5.0’s NTR option had a nearly 7 month transit, compared to the ~4 months Starship should be capable of.
I’m close to being as skeptical as you are about NTP but I’m not quite prepared to declare that it doesn’t make sense at some point.
There are solutions to the “brake into orbit” problem. The simplest one is to detach the payload (presumably a crewed lander) before capture. Then you let the payload do direct EDL/aerocapture, and you leave the NTP and just enough prop to do the minimal propulsive capture. Then it can aerobrake into low orbit at its leisure over months or even years.
The structural mass fraction problem is never going away, but it does get less serious as you scale up. A Starship-sized NTP doesn’t make much sense, but something that was 10x Starship-sized might.
Hydrogen is obviously a big pain, with its own structural mass fraction problems. However, since most propellant is used at departure, it might be possible to develop an expendable “flimsy” tank, which won’t hold the LH2 for more than an hour or so, but that’s more than enough for a departure burn. Getting such a flimsy tank into LEO is an unsolved problem, and how much mass you can save by making it flimsy is an arm-wave at best. If you could put a huge amount of LH2 into a Kapton-like bag (and there’s some research being done with structural films at cryo temperatures) , with a pressure of a few kPa, that would do nicely. It’ll obviously boil like crazy, but if you can fuel and go quick enough, it’s about as close to a zero SMF contribution as you’ll get. The low flight pressure means that you need massive multi-stage turbopumps to avoid cavitation. But that’s almost a rounding error compared to the mass of the reactor.
By far the biggest problem is that the downward lift needed to stay long enough in the atmosphere to do effective aerocapture or direct EDL goes up as the square of the entry speed. Beyond about 12km/s of arrival v∞, the acceleration will kill your crew, even if you can figure out how to deal with the thermal problems. So any hypothetical NTP that wants really short transit times has to reserve a hefty chunk of delta-v to do enough propulsive braking to get below that arrival speed.
The hydrogen for that arrival burn **can’t** be in a flimsy tank, and it has to be storable for a couple of months at least. But maybe there’s a way to build a dual-fuel NTP that can also use liquid methane which is a lot denser than LH2 and has a higher boiling point. You get lousy Isp, but it’s an arrival burn, which makes prop efficiency less important.
All that said, this is still science fiction. If there’s really a pony in there, you’re gonna have to dig through a huge manure pile to find it.
faster speeds mean Mars capture will be more “interesting”.
a brand new rocket + a new propulsion system = development time measured in decades (not years) …
unless more innovative management is steering the ship.
I reckon SpaceX will have a patio set up with drinks waiting for the NASA crew …
That’s precisely what’s happening with this demonstrator system. DARPA is responsible for the ship and for overall management of the project. As with Commercial Space, this will be a learning opportunity for NASA.
Someone pointed out to me a while back that the limit on how fast you can aerocapture has nothing to do with thermal protection; it has to do with how much acceleration is needed to stay in the atmosphere, which in turn is limited by the picky little detail of not killing the crew.
To aerobrake/aerocapture/direct EDL/whatever, staying inside the atmosphere long enough to reduce to at least orbital speed requires generating lift **down** towards the spherical surface of the planet. In other words, you have centripetal acceleration. (neededAcceleration = (entrySpeed)²/(radiusOfMars + entryAltitude.) If you put the limit that a crew can tolerate for several minutes at something like 5 gee and assume that your spacecraft is flying at an altitude of 100km, then the crew’s spacecraft can’t arrive with more than 12km/s of excess hyperbolic velocity (aka v∞), which translates to an entry speed of about 13km/s. (For reference, most Mars probes’ entry speed has been under 7km/s.)
Beyond that speed, it doesn’t matter if your heat shield is made of unobtanium. You’re still going to smush your crew. So even if you had a spacecraft with 20km/s of delta-v (unlikely, even for a nuke), for any departure v∞ that results in an arrival v∞ that’s above that critical 12km/s value, roughly half of the excess delta-v has to go into braking the spacecraft **before** it hits the atmosphere.
Bottom line: 45 days to Mars is very, very unlikely, even for a nuke with 900s of specific impulse. I’d believe it can be done in 70-80 days if you’re willing to spend the money. But you can do 100 days to Mars pretty easily with a big enough chemical rocket (yes, like Starship). So… how much is it worth to chop off about a month of travel time?
The rocket design and simulation are straightforward. The hard part is political. How do you convince of politicians to support a science project through multiple election cycles? Particularly one with serious safety concerns?
I think it should be tested in lunar orbit.
I’d’ve thought most of the safety issues where between here and lunar orbit. But of course, no one would design a new rocket and propulsion and then fire it at Mars on mission 1. There’ll be a host of technology demonstrators and testing, culminating in an unmanned flight to Mars (with cargo and sensors and telemetry sent back to Earth).
I wonder why they chose to represent the flight to Mars with a “Gordian knot” ?
Nuclear propulsion is the technology of the future! Always has been, always will be…
Though I’ve not heard too much recently, Franklin Chang Diaz was developing a system that could get a spacecraft to Mars in 3-5 months. His propulsion system was well along in testing years ago. He just needed a power source.