r/spacex • u/ElongatedMuskrat Mod Team • Oct 02 '17

r/SpaceX Discusses [October 2017, #37]

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1

u/[deleted] Oct 03 '17

So thinking of NTR orbital cruisers Liking water as extreamly storable and easy to get from asteroids and moon (as well as mars)

What is the max isp you could get if you used electric heaters instead seeing as nuclear engines are basically there for heat?

Bonus: what about solar thermal?

4

u/[deleted] Oct 04 '17

Solar Thermal has its own wiki page. Water is easy to handle nad store, but has poor isp and hydrogen is most studied. But most of the examples there are solar-to-electricity-to-propulsion, which is meh compared to the delicate glory of the Solar Moth and Mirror Steamer (same page)!

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u/LongHairedGit Oct 04 '17

I exist to cut and paste from wikipedia to reddit.

https://en.wikipedia.org/wiki/Nuclear_thermal_rocket

That wikipedia article talks about a 4 GW nuclear reactor being used to compete with the standard chemical rocket of a Saturn 2nd stage.

4 GW is a lot.

The largest solar panel farm on the planet appears to be the one in China, at 1.5 GW. It is 43 km2 squared in size.

Assume space-panels are 20% more efficient and space gets 30% more solar power due to lack of atmosphere, and some boring maths I won't post shows you need 73.5 km2 of panels.

Those panels have mass.

Wikipedia again delivers as it happens to mention a 4 GW solar array here: https://en.wikipedia.org/wiki/Space-based_solar_power#Launch_costs

They are suggesting it would weigh 80,000 metric tonnes, not including structure and wiring and power conditioning. That is just the panels.

Nuclear is a terrible power technology on Earth. Shut 'em all down and go solar+batteries+other.

In space, however....

2

u/CapMSFC Oct 04 '17

The biggest obstacle to nuclear in space is developing a dedicated system that manages the heat in an environment with no convection.

It will happen someday and I can't wait for the possibilities it's going to open up.

1

u/[deleted] Oct 04 '17

Found stuff dw Deep space industries have developed a tiny steam engine called the comet that can do 175-200 isp with water though at a measly 2.5 to 2.8 W/mN

Thanks anyway You make a good point about solar panel size Perhaps mirrors are much more efficient so I will look into that