Space Nuclear Propulsion | Hacker News Reader

Space Nuclear Propulsion

nasa.gov

24 pointsneverminder20 comments

This is the only viable future for space travel. In orbit assembled space ships with nuclear thermal propulsion. Travel time to Mars with conventional chemical propulsion takes just way too long.

I think nuclear electric propulsion is probably a viable option as well.

Indeed. Probably a combination of both nuclear-thermal and nuclear-electric or ion drive I think it's called otherwise. The nuclear thermal would provide initial boost, then ion drive can do continuous acceleration half way and then deceleration the other half way. That would get a ship to Mars in a fraction of time.

There are non-nuclear alternatives, particularly inward of the asteroid belt.

PV in space can be made very thin. The absorption length for photons in CdTe, for example, is just 0.1 microns. Without having to be mechanically robust against wind and rain, great gossamer PV arrays could have very high power/mass ratios. These could drive plasma engines with high Isp.

For crewed missions, wouldn't the low trust lead to very large transit times anyway, in a Mars mission scenario?

That depends on the power/mass ratio of the system.

Lower acceleration systems can also be used to preposition chemical fuels for use by crewed vehicles.

None of that has anything to do with reducing travel times to Mars unless your entire payload is on the order of a couple of pounds.

That's like replying to someone saying it takes too long to drive from New York to Seattle, by saying that we could build an efficient 1000 mile per gallon car, that travels at .01 miles per hour. How efficient the vehicle is isn't the slightest bit useful to solve their complaint.

A high thrust to weight ratio when the weight is a couple of pounds isn't useful. What's useful is having a huge amount of thrust that's large enough to shove multiple tons of mass at high accelerations.

Yeah, no, that's nonsense. There's nothing preventing anyone from scaling up such systems. Remember, construction in space was already stipulated.

How large would such a construction need to be to accelerate 100 tons at 1g? Maybe someone could do the math for us. I assume it's on the order of dozens/hundreds of miles long per dimension and would be completely infeasible compared to just using an engine with high thrust to begin with.

[Edit]

Here's some rough math.

From wiki, assume a typical ion engine can produce 150mN of thrust from 4,000 W of power input.

Using a space station solar panel as an example of solar collection in space, each space station solar panel is 420 square meters in size and produces 31,000 W of power.

One space station solar panel would then provide (31,000 W / 4,000 W) * 150 mN = 1,162 mN, or .001162 N of force.

The force required to accelerate 100 tons at 1g requires 996,402 Newtons of force.

To generate that much force, you would then need 996,402 N / .001162 N = 857,488,812 space station solar panels worth of power.

As one space station solar panel is 420 square meters, then that requires 857,488,812 * 420 square meters = 360,145,301,040 square meters of solar panels.

Assuming square construction, each side would need to be 600,121 meters, or 373 miles long.

I assure you, just using high thrust engines makes infinitely more sense than building a pv-based ship scaled up so far that the ship's dimensions are nearly 400 miles long on each edge. At least for any time soon ..

Why would you need to accelerate anything at 1 g? That's a ridiculously high acceleration for getting to Mars. What matters more is the total delta-V, and if it can deliver it in time short compared to the transit time to Mars.

High Isp solar electric systems would not exploit the Oberth effect (likely they would start in high Earth orbit) so they don't have a high acceleration need from that.

If you want to accelerate to 15 km/s in 1 week, that's 2.5 milligees.

Accelerating/decelerating at 1G the entire journey would be the perfect scenario. Not only that would be the shortest travel time, but it would maintain gravity inside the ship all the time. If this is not the ultimate goal being worked towards, then we may as well just give up now. Nuclear is where it's at - it's the most efficient weight to power ratio generation known to man.

It's about as realistic as propelling the vehicle with unicorn farts. In particular, the kinds of nuclear propulsion being discussed in this thread could not do it. Solid core nuclear thermal rockets using hydrogen have an Isp of about 1000, so they could accelerate a vehicle at 1 gee for less than an hour.

The power/weight ratio of nuclear rockets actually sucks, compared to chemical rockets. Conveying heat through a solid/fluid interface is awkward and slow compared to just making it in situ by combustion.

NASA: “Here are some idea around Nuclear propulsion in space”

HN: > It's about as realistic as propelling the vehicle with unicorn farts.

The idea I was responding to there was not NASA's.

Another realistic and cost effective scenario would be Von Braun Wheel or O'Neill Cylinder stations this orbit: https://en.wikipedia.org/wiki/Mars_cycler

> Without having to be mechanically robust against wind and rain,

What about micro meteorids?

Very sparse, and in a properly designed PV cell a hole wouldn't matter.

TIL, always thought it would. I stand corrected!

I can really see this happening with Malten Salt Reactors finally getting traction. China already has built a demonstrator and is now building a full-scale version of an MSR Reactor and now finally the US is building a demonstrator as well.

https://www.thecooldown.com/green-business/us-nuclear-test-r...

No, molten salt reactors are not the right technology for nuclear propulsion. The idea of a nuclear thermal rocket engine is to heat up very light molecules to very high temperatures, and so to achieve higher exhaust velocities than chemical rockets. If you plug a higher exhaust velocity in the rocket equation, you end up needing less fuel mass for the same cargo mass. In practice, the best nuclear thermal rockets achieve a lower temperature than chemical rockets, but they can dedicate it to heat only hydrogen (H2), rather than the combustion products in chemical rockets (such as H2O or CO2), so overall the exhaust velocity can be approximately twice as high.

Still, temperature is quite important, you want the core of the reactor to run as hot as possible. You are limited by the fact that you don't want the core to disintegrate. The NERVA project [1] achieved temperatures in excess of 2200 K.

Molten salt reactors are designed to reach about 1000 K. That gives up most of the benefit of using a nuclear reactor. You would still beat chemical rockets, but only by 25%, not by a factor of 2. Why would you do that? If you build on the NERVA project and use TRISO fuel (which was not available at the time) you can end up with a specific impulse of more than 1000 s, which is 2.2 times higher than what the best chemical rockets can deliver, and 2.85 times higher than SpaceX Starship.

[1] https://en.wikipedia.org/wiki/NERVA#Reactor_and_engine_test_...

Reply on news.ycombinator.com