Viewing a single comment thread. View all comments

dittybopper_05H t1_j243gto wrote

Reply to comment by amitym in What if we kept pursuing nuclear spacecraft propulsion? by rosTopicEchoChamber

You've got a major misconception there.

XE Prime, the last NERVA engine developed, weighed 40,000 lbs. Launch mass of a shuttle orbiter was 240,000 lbs, and it could loft 53,000 lbs to LEO. Dimensionally, you could have easily fit XE Prime in the cargo bay of a shuttle orbital vehicle.

So no, it's not "like in the same mass neighborhood". You're off by a whole order of magnitude.

Plus, we stopped development of NERVA in 1973. It is now almost 2023, fifty years later.

Today, we have advanced materials and manufacturing techniques like additive manufacturing that weren't available in the early 1970's. That means that we could make an NTR much lighter and more efficient than we could back then.

1

amitym t1_j245ihm wrote

I'm all for it if it's true. But from everything I've seen, 40 years of R&D after the original NERVA project netted all of a 5% increase in Iₛₚ. Sure you can make one that's ⅓ of mass, but that doesn't help much if you will still end up needing 3 of them. You know?

0

dittybopper_05H t1_j2531cx wrote

That's just it: There really hasn't been any actual R&D after NERVA shut down. There has been (to my knowledge, anyway) *ZERO* hardware built since the late 1960's.

Everything since then has just been paper studies.

1

amitym t1_j25cff8 wrote

I mean paper studies still count as "R" if not "D..."

But leaving that aside. Let's get back to the original point. There is no application for nuclear thermal propulsion right now. Everyone is all about surface-to-surface reusable transatmospheric launchers and low-earth orbit delivery. If we were ready to build something in the 50T range to act as a permanent shuttle between Earth orbit and Lunar orbit, I don't see why we wouldn't happily use NERVA as we had already developed and tested it. It seems already there. In terms of being ready for that application.

1

dittybopper_05H t1_j25ohpo wrote

Even the paper studies (such as they are) haven't been well-funded.

There actually are several different possible applications for NTR. Getting to Mars and back in half the time it takes by chemical rocket is a good one. Getting there and back quickly makes the problem a whole lot easier. It boils the difficulties down to ones we've largely already solved.

Even just having the ability to put a probe to, say, Uranus or Neptune without having to do a whole bunch of gravity assists, and still get there in a reasonable amount of time, is good reason. Those two planets are very unexplored, and Triton may have a subsurface ocean. But we won't really know until we go there.

Plus, even having unmanned stations on the moon (like very large radio telescopes on the far side, like Arecibo and FAST, built into craters) means we'll need to haul a bunch of stuff up to the Moon. Having NTR-propelled craft means we'll be able to send more up there, and we could refuel them in Earth orbit. When we need to retire them, send them on a trajectory into the Sun.

1

amitym t1_j25qr2o wrote

So what you're saying is that permanent heavy orbit to orbit shuttles are the application?

1

dittybopper_05H t1_j28l6lg wrote

Earth orbit to lunar orbit, sure.

But you could also also use them for other things. The engines are about twice as efficient as chemical rockets for solid core designs, and since the fuel is generally hydrogen and no oxidizer is needed, that simplifies refueling, and also means you can loft that much more fuel per launch, because hydrogen is the lightest element, all other things being equal.

But you could also use something like that for maintenance of geosynchronous satellites, something we simply don't do now. And for even higher missions. Imagine being able to service the Webb Space Telescope like we've done with the Hubble Space Telescope. Having a near-Earth tug capable of getting astronauts to the Moon and back would also allow missions like that.

I kind of get the impression that you're not really imagining the possibilities here. Kind of like looking at a Wright Flyer in 1904 and asking "What use is it?", not seeing that something like that just opens the door for further development and that the jobs will be attracted to the application.

Heh, kind of reminds me about how the "killer app" for personal computers back in the early 1980's was organizing your recipes.

In short, if you build it, creepy ghost players will emerge from the maize.

1

amitym t1_j2a246f wrote

No, I'm kind of making fun of you because you are saying the same things I am.

1

Shrike99 t1_j25jiv9 wrote

>Sure you can make one that's ⅓ of mass, but that doesn't help much if you will still end up needing 3 of them. You know

The three final engine designs in the SNTP program all had thrusts of ~45,500lbf - about 80% of NERVA's 55,500lbf, while weighing between 3000lb and 4200lb - only about a tenth of NERVA's 40,000lb.

The best all around design was probably the partial flow expander, which offered an isp of around 935s and weighed 3300lb for 45,700lb of thrust. Two of those would weigh 6600lb - only 1/6th as much as a single NERVA, while producing ~65% more thrust and ~11% better isp.

1