How would you build a nuclear propulsion lander?

[SomeGuy123]

So you've got your space pwnboat, and you'd like to send a lander down to a world like Earth or Mars without leaving a crapton of expended parts around.  

Frankly, I don't actually see this ever coming up - with an interstellar mission, you'd just send landers 1 way with heat shields and parachutes or retrothrusters with enough fuel for a suicide burn.  You'd then use your advanced technology, required for an interstellar mission in our universe, to build an industrial complex on the planet you landed on capable of manufacturing brand new rockets and/or an expanding self replicating robot swarm.  

Still, you want a "shuttlecraft" like they have on Star Trek, but you only have technology we know is possible with known physics in our universe today.  It needs to be able to land on a planet and return without an excessive amount of pieces lost, so you need high ISP.  One way to get this is nuclear.

The problem is, where do you stick the reactor?  A reasonable lander design is wide, with a low center of gravity, and you want to put the reactor in the center because it is really heavy.  Except...how do you protect the crew from getting fried by the radiation without weighing it down with 100 tons of radiation shielding? (the airborne nuclear reactor experimental aircraft had something like this in shielding, too tired for cite but it was either 100 or 50 tons)

 

 

[Rakaydos]

Leave the reactor on the mothership, and have the lander utilize a microwave/thermal turbojet engine design. With the mothership directly overhead, beaming power down, a squat, wide lander uses it's best cross section to receve power with, and (assuming an atmospheric body) you use your surroundings as an unlimited supply of reaction mass.

Edited December 3, 2015 by Rakaydos

[SargeRho]

As Rakaydos said, leave the reactor on the mothership.

Or use some sort of reactor that doesn't produce lasting radiation. A shut-down fission reactor would continue to emit a lot of radiation, the same doesn't hold true for a fusion reactor, or some types of antimatter reactors. It'd be wise to land behind mountains though, you don't want to gamma-fry your colony during your approach

[PB666]

Well in the game i modded a super sized lvn, made a bunch of light weight xlong mgs and bulit a cage around the lvn. put a flat oval fuel tank on top and on the down facing side of the oval two lander pieces and all the science stuff. But .....my kerbals don't breed and are fairly resistent to radiation poisoning, but i was able to hop four times on the mun before refueling. With two ships i managed to get all the mun science. 

I think NASA prolly has the imgur photo marked as the lander design not to show to congress, ;^).

The problem with lvn is that you need many, they are very heavy, very hot, and cannot carry much weight (mass times local gravity, and you really need a thrust/mass to land that is 3 times the surface gravity of the celestial you are landing on or you will waste fuel. 

they are really something you put on a super huge tank and only use when you burn window can be fraction of an hour in length. Thats not a lander. 

[RainDreamer]

I would say this will be depending on the place you want to land on and whether it has atmosphere or not. If we are going to a place like venus, for example, we can have the reactor float high above and connected with a tether, detach when ascend and collect later on. Though I still think beaming energy down from orbit with mothership/energy satellite seems like the way to go.

[Bill Phil]

You're going to need a better reactor. A small-ish one, but still producing lots of power. Maybe a gaseous core reactor, if that's possible. It has a greater energy density. Then you'd only need about 10 cm of lead casing all around it to reduce the gamma dose by 1000 times. In the direction of sensitive cargo, add more shielding.

Edited December 3, 2015 by Bill Phil

[wumpus]

You don't.

If you try landing on a planet with an atmosphere, the fuel suddenly becomes a huge problem (nevermind the nuclear issues already mentioned may be insurmountable).  I've already gone over the fuel issue in space.  In an atmosphere (temperature >30K) things get hairy fast.  First and foremost, if the LOH goes >~30K you have a big explosion.  Try closer to 20K at all times (a slury) to keep leaks to a minimum (H2 leaks through everything).  Obviously, this isn't an issue with bringing a lander home to Earth, but do you want all the real and imagined nuclear issues in bringing a nuke back to Earth?

Oddly enough, I'm working my way up to this issue in KSP.  I'm taking a party (8 or so) of scientists for some xp gathering and the main rocket uses nukes.  Being a cheapskate, the rocket has solid boosters and then switches to nukes at stage 2 and goes to Mun, Minmus, and beyond on just stage 2.  Being even more of a cheapskate, this huge nuclear stage 2 is coming home to Kerbal.  Best guess is that it will require asymmetric landing gear to land on its side (I've favored wide landing gear in the past, but mostly in versions without re-entry heating.

Edited December 4, 2015 by wumpus
greater than 30K. Greater is the problem.

[PB666]

10 minutes ago, wumpus said:

Oddly enough, I'm working my way up to this issue in KSP.  I'm taking a party (8 or so) of scientists for some xp gathering and the main rocket uses nukes.  Being a cheapskate, the rocket has solid boosters and then switches to nukes at stage 2 and goes to Mun, Minmus, and beyond on just stage 2.  Being even more of a cheapskate, this huge nuclear stage 2 is coming home to Kerbal.  Best guess is that it will require asymmetric landing gear to land on its side (I've favored wide landing gear in the past, but mostly in versions without re-entry heating.

The thing is that you often need to get the fuel tank into orbit, then fuel it via ports. If you have infinite time ahead of your launch you can send your tank via ion-drive and solar, even using an photon-impulse drive (CFG mod a light) to get the tank into orbit at the destination. It only needs solar panels, a small drive, do a little oberth affect around some planets, a reaction wheel. If you have it use a mech-jeb unit, and whatever antennas are needed. Tanks can also carry empty science stations and solar panels so . . . .

[AeroGav]

If I understand correctly, the OP is asking about what the nearest thing to a space opera style shuttlecraft that we'd have a high likelihood of being able to build within the next 20 years , assuming unlimited R&D and using nuclear power.   The short answer is not very like one at all, but if you've solved the problems of artificial gravity and FTL travel I imagine such a shuttlecraft is trivial.  Anyway, it's a fun question so let's run with it..

 

Nuclear, the only known technology is fission.   That means radiation - to solve the heavy shielding issue, you only shield the side of the reactor facing the crew compartment.   Under power, that reactor is going to spit instantly lethal doses of X rays for considerable distance on the unshielded side, and even shut down the residual radiation means you'd have to approach it via the shielded side.  However, so long as it doesn't melt down, the radioactive materials should remain within the reactor itself, so once it's flown past to a safe distance, the atmosphere it passed through will be uncontaminated.

I'd use that reactor to drive a nuclear turbojet at lower altitudes, higher up it can power a conventional nuclear thermal rocket.  Build all of that into a spaceplane, and you'd at least have a re-usable ground-orbit shuttle with comfortable delta-v reserves.   You never see sci fi heroes fretting about launch profiles and delta v on a routine flight, nor boiloff, so I presume the propellant is going to be room temperature storable like maybe, plain water.

Note this depends on landing on Earth where there are runways.    If visiting a more primitive society , things get much harder.    A seaplane perhaps?  Could you possibly design an airframe that can fly slow enough to land on water while having enough go a high mach numbers?   At least you could desalinate the seawater readily at hand to refill the tanks.   

What about a helicopter with folding rotor blades for the final landing phase?   trouble is helicopters have much lower max speed & altitude capability than fixed wing, the nuclear thermal rocket stage is going to need more delta v, more fuel, therefore be heavier, then will it be able to achieve a high enough TWR?

Thrust lift alone, like a VTOL aircraft, with current tech solid core fission reactors, even unshielded, no way to do that in earth gravity I'd have said.   Especially given all the fuel you'd be carrying for the climb to orbit.

Edited December 3, 2015 by AeroGav

[Red Fang]

I wouldn't unless I really have to. 

That said, if you need a lot of dV and don't have an atmosphere to fight, you can get 0.5 mass ratio for a NTR 6500m/s rocket stage. That should be more than enough for propulsive Mars landing and takeoff without refueling (propellant issues aside). Using NTR has the benefit of spewing very little fallout in the form of hot particles, so it would take only a couple of days, (maybe even hours if you don't have to stick around the lander for long!!! ) to be safe to leave the lander. A couple of tons of water around the crew compartment should provide sufficient shielding against gamma particles from the landing fallout, while the propellant itself and a shadow shield should protect you from the engine.  Atomic rockets has a detailed article on a more powerful NSWR called "Polaris". Quick calculation says that you can make a 100t SSTO by strapping 3x Timberwind75 NTRs using 70t of LH, with a payload in the range of 10-15t provided you are prepared to put up with some fallout.  Thermal protection for atmospheric landings and long spool-up periods for NTRs would probably render this design non-deployable from orbit. 

That made thinking of what to do on a planet with an atmosphere that provides significant lift and drag, ie. enables flying. 
If you manufacture your spacecrafts off-world, you might be able to land an empty lander after it discards heat shield, top it up with propellant and launch in as many stages as you need. That seems quite expendable. Using nukes, it might be possible to make a reusable one, which would require a runway, a base and a dedicated launch aircraft. My math just about adds up, but it's back-of-the-napkin calculation...