Hacker News Board Hacker News, browsed like a classic message board
Live data from Hacker News

Space Nuclear Propulsion

nasa.gov

Post reply on HN
Print view View original

11–20 of 21 posts

Re: Space Nuclear Propulsion

#11

Post by pfdietz »

thegrim33 wrote: post #9
pfdietz wrote: post #8

Earlier quoted context omitted.

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. Us…

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.

Re: Space Nuclear Propulsion

#12

Post by neverminder »

pfdietz wrote: post #11
thegrim33 wrote: post #9

Earlier quoted context omitted.

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. Us…

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 accelera…

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.

Re: Space Nuclear Propulsion

#13

Post by whaleofatw2022 »

pfdietz wrote: post #4
neverminder wrote: post #2

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.

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.

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

What about micro meteorids?

Re: Space Nuclear Propulsion

#14

Post by pfdietz »

neverminder wrote: post #12
pfdietz wrote: post #11

Earlier quoted context omitted.

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 accelera…

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.

Re: Space Nuclear Propulsion

#15

Post by pfdietz »

whaleofatw2022 wrote: post #13
pfdietz wrote: post #4

Earlier quoted context omitted.

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.

> 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.

Re: Space Nuclear Propulsion

#16

Post by Apofis »

neverminder wrote: post #2

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 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...

Re: Space Nuclear Propulsion

#17

Post by Teever »

neverminder wrote: post #12
pfdietz wrote: post #11

Earlier quoted context omitted.

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 accelera…

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.

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

Re: Space Nuclear Propulsion

#18

Post by mdekkers »

pfdietz wrote: post #14
neverminder wrote: post #12

Earlier quoted context omitted.

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…

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

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

Re: Space Nuclear Propulsion

#19

Post by whaleofatw2022 »

pfdietz wrote: post #15
whaleofatw2022 wrote: post #13

Earlier quoted context omitted.

> 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!

Re: Space Nuclear Propulsion

#20

Post by pfdietz »

mdekkers wrote: post #18
pfdietz wrote: post #14

Earlier quoted context omitted.

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…

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.
Post reply on HN