New Spacex Engine has to be nuclear

[raketa]

1/Starship to reach any part of solar system need nuclear power plant
2/Nuclear rocket engine give more delta V for available fuel.
3/New nuclear engine could be very low trust engine, using just for everything with exception of landing and launching from planets.(Hydrogen could be produce from methane by steam reforming)
4/Power plant could be used also for producing electricity, replacing solars in distances beyond Mars
5/Cooling of power plant could be done using skin of rocket, that is design to accept and radiate heat during landing
6/Power plant will be turn off during launch and landing, to cool down for landing heat wave

[sferrin]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

[Hominans Kosmos]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

[cferreir]

Good documentary on the NERVA project

[sevenperforce]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Depends on how much dV you need.

[rsdavis9]

There is also nuclear electric with ion thrusters

[raketa]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

Nuclear fuel could be deliver by Dragon to orbit and install to SS over-there. Dragon launch abort system, could help to approve it such approach.
Spacex didn't need to rush this development, requirement will become imminent in 10-15 years.

[raketa]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Biggest issue is weight of radiator.
Starship skin could serve as radiator.
SS will be glowing in darkness in infrared. Could not be use to attack alien fleet :-).

[raketa]

There is also nuclear electric with ion thrusters

I think direct conversion of heat propulsion is  more efficient and also way to cool down reactor, that will be one the issue deal with.
Ion thruster could  use just electricity and you have to deal with 95% heat to radiate. If most heat is expel and use for propulsion, it will easier to radiate through skin of SS.

[tleski]

Let's say SpaceX decides they want to develop a nuclear upper stage (or interplanetary tug).  Logistically, CAN they?  Wouldn't the government (and not just the US's) throw up a bunch of red flags?  Maybe it would be similar to the way companies like GE can design/build/sell nuclear reactors? 

I guess it depends on the type of fuel. Much easier to get permission to use low enriched uranium compared to high enriched. In the Space Show podcast linked below Dr Christopher Morrison is discussing the issues (including the regulatory ones) related to nuclear propulsion:
https://www.thespaceshow.com/show/31-mar-2019/broadcast-3292-dr.-christopher-morrison

[DistantTemple]

My Vision for SpaceX's ship development towards the 2040's

OK When will nuclear be needed and what for?

What for: It will be needed to go further into the solar system, more quickly. It will be needed for much larger ships.
When: After initial success on Mars. So by 2028 some expectations and prototypes should be appearing.

How will SpaceX get there?

Initially they will have to work closely with NASA. This risks being slow and expensive. SpaceX could set up a Neclear lab, and employ experts in the field. However strong government oversight would still be there, but ISTM that this stage is needed to get a foothold, and gain core competencies.

Because SX will be the only space transport company able to offer NASA a cheap and flexible platform for trials of nuclear engines, NASA will cooperate and develop partnership arrangements.

Once humans land on Mars in 2024 SX rovers, and satellites will start prospecting in earnest for uranium. (Also metals, and rare ores etc etc). If Mine-able Uranium is found on Mars, SX will be able to set up a large operation without contaminating Earth. Reactor components can be shipped from earth, and fuelled in Mars Orbit.

Marian (or Luna maybe) Uranium will be the critical resource, with a big SpaceX push to discover and mine it. However Steel and making stainless steel on mars will be an important secondary thrust.

Initial ships will be 9m SS cylinders re-purposed. Once prototypes are functioning in 2030, larger cylinders will be fabricated in orbit. Maybe Earth orbit, but once stainless steel is made on Mars, then fabrication will be in Mars orbit. (2035) It will be automated, use cold vacuum welding ("perfectly" clean (no oxidation etc) cut edges brought together, make an outstanding join, with no heat deformation etc), and there will be little limit on size, as there will be no gravity deformation issues, and momentum and kinetic energy will replace weight-being-lifted as the main"force" to cope with in aligning and assembling the vast structures. A 50m body will only be limited by the supply of steel and the time to "weld" it. These ships will be mass produced by 2038

The next limitation will be boosting Martian stainless into Mars orbit. Then a space based stainless factory will be assembled form one of these "50m" tubes, in orbit, and sent to make steel from asteroids. (2040) At this point it will also start the first town there. The factory will ingest parts of asteroid, and generate spaceships, and other metals, and minerals etc.

With this vision in mind, in the 2020's SX will have sent unmanned probes to begin to identify resources.
As soon as there is a nuclear engine, (2024) an out-of-solar system probe will be launched, to satisfy GS's ambition.

SpaceX will also put up (space based) telescopes, arrays and other instruments, largely for prospecting, and furthering their own goals, but also allowing pure research, through academic collaboration. On-orbit ship manufacture and nuclear engines will allow more powerful instruments to be sent relatively quickly to all parts of the solar system and beyond.

There is a significant risk that politics, war, pollution, disease, sea level rise, economics, etc could vastly increase the pressure and attractiveness of living off-earth. Conversely if Earth is cherished, removal of Nuclear activities, and boosting metals from Earths surface, could create an economic raison d'etre for the path I have described.

These developments will not require as many 10's and 100's of $Bns as it might seem, as each stage is a rational development of SpaceX from the previous.

Edit: I forgot to include developments in AI robotics, and Nuralink. These will speed up my timeline significantly. Space will be "unregulated" real estate for manufacturing, once BEO. Therefore as long as you protect your humans, an Elonesque rush to develop and manufacture will be standard.
Edit2: bold uranium on Mars

[envy887]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Water NTR gets you hydrogen ISP but at 3-4x the bulk density. Maybe an advantage but there are challenges with shielding the crew and cooling the reactor.

[DistantTemple]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Water NTR gets you hydrogen ISP but at 3-4x the bulk density. Maybe an advantage but there are challenges with shielding the crew and cooling the reactor.

A ship assembled in orbit can have the reactor on a truss 10's or 100's of metres away from the crew.

[50_Caliber]

There already is a plasma thruster that would fulfill the next-generation role for advanced propulsion -VASIMR-https://www.nasa.gov/audience/foreducators/k-4/features/F_Engine_That_Does_More.html
It would need a MW-class nuclear reactor to power it though.

However; with orbital refueling, this will give every bit as much performance needed for a while without any expensive R&D programs to get the propulsion system in orbit. Considering the incredible cost reduction that a functioning SS/SH system will provide, the economic case for nuclear propulsion may be a tough one to make for a while. Though if anyone has a chance at making such a system within a reasonable economic framework, it would be SpaceX.

[raketa]

There already is a plasma thruster that would fulfill the next-generation role for advanced propulsion -VASIMR-https://www.nasa.gov/audience/foreducators/k-4/features/F_Engine_That_Does_More.html
It would need a MW-class nuclear reactor to power it though.

However; with orbital refueling, this will give every bit as much performance needed for a while without any expensive R&D programs to get the propulsion system in orbit. Considering the incredible cost reduction that a functioning SS/SH system will provide, the economic case for nuclear propulsion may be a tough one to make for a while. Though if anyone has a chance at making such a system within a reasonable economic framework, it would be SpaceX.

Again use just electricity and what you will do with heat of nuclear power plant. It is better use heat to use for propulsion. IF reactor is use just produce electricity, it could run in lower input and heat could be dissipated through skin.

[raketa]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

Water NTR gets you hydrogen ISP but at 3-4x the bulk density. Maybe an advantage but there are challenges with shielding the crew and cooling the reactor.

I think Spacex will able to push it here and make comparable to solar.
They could build  Power plant and  engine in test version where heat from nuclear fuel could replace temporally by non nuclear source and lower  government red tag at the beginning of development.

[butters]

Elon's comment about an 18m diameter system as his current vision for what comes after Starship would argue against nuclear propulsion being something that he is interested in pursuing. He's obviously thinking that huge refillable propellant tanks is the way to see the solar system. Maybe there will develop an engineering constituency within SpaceX which will someday convince Elon that nuclear propulsion is worth serious consideration, but at present it seems quite far from what he has in mind.

[kendalla59]

Doesn't nuclear propulsion schemes largely negate your specific impulse gains with dry mass growth?

I've been trying to get more information on this topic. Nuclear engines have a thrust to weight ratio that is about 1/20th that of chemical rockets. But the chemical rockets need to carry a lot more propellant mass. In the end the nuclear rocket wins in a theoretical calculation for a Mars mission.

But there are many other considerations:
1. The total cost of placing the propulsion system into space.
2. Maintaining cryogenic H2 for a long duration.
3. Materials properties when irradiated with neutrons over time.
4. Materials properties when exposed to hot H2 and hydrogen plasma over time.

The basic need is to either carry or acquire energy and reaction mass during the flight. Lasers can be used to transmit energy to the spacecraft while in flight. Reaction mass refills could perhaps be propelled separately to the spacecraft using railguns or chemical rockets.

To carry the entire reaction mass and energy needed for the flight, imagine a spacecraft surrounded by, say five shells of rocket stages. The craft itself has a single engine with propellant. That would be surrounded by four engine stages with propellant. The next layer say has twelve engine stages, then 36, then maybe 108 (!!). Each engine/propellant stage will be able to separate and fly back to Earth (or Mars) for recovery and re-use.

Being able to continuously accelerate the spacecraft produces some amount of "gravity" and also reduces the flight time to days instead of months.

[rakaydos]

Given the current regulatory environment and Shotwells interest in eventually taking SpaceX actually interstellar, I suspect they would leapfrog Nuclear to focus on Antiproton Capture.

[sanman]

Nuclear seems to have a lot of risk and liability associated with it, even just for development.

On the other hand, Elon has been promoting that meme of "Nuke Mars", and it may be that he's only half-joking about it. (Why doesn't he just drop a comet/asteroid onto Mars?)
But none of the technology he's already invested into coincides with nuclear.