A launch vehicle I recall from home. I don't remember the name, but I'm sure "nuclear" was in there somewhere.
Here's what I told our friend Sofia about it.
left: overview of the entire launch vehicle. since i don't know mellanish anymore i don't know what it's called. Drawing a blank. Anyway. It's a nuclear rocket. The first stage is a cluster of solid rocket motors which gets the rocket flying fast and high and out of the thickest part of Gymnome's atmosphere, where the solid core nuclear thermal rocket can take over.
The rocket uses a water tank for propellant during stage 2A (the initial burn), because although the efficiency (Isp) is low for a waterlogged NTR, the thrust is as high as conventional hydrogen/oxygen chemical rockets but in a much smaller tank size. (this is because SCNTRs and Chemical rockets are both constrained in temperature to about the same amount, and so the thrust power is the same and all that can change is the mass flow rate.)
Once the rocket has accelerated up such that it no longer needs to worry about falling down, the vehicle can switch to Stage 2B, and start running hydrogen through the NTR. The thrust is lower, but the velocity gain per unit mass of fuel is much higher due to the fact that the hydrogen can be accelerated to a higher speed than the water. (with all that pesky oxygen).
It was noticed at some point that these rockets were really well suited for orbital refueling. This is still fairly early all things considered so that kind of infrastructre had been thought about a lot, but not actually built. The first time it was tried was in a sort of self contained way.
on the right you can see a diagram of the upper stage with a different payload, which I think was maybe used OH! I thought it was an ice giant probe or something but I bet this was from after the Zaldans left and the space program was rushing to figure out why, so they needed something to zip over to Oldsky around Omen real fast. Maybe. Point is, you need a small payload going very fast.
normally with an LH2/LO2 chemical rocket you would just build a small upper stage that can fit on top of the lower stage, and use that to kick the payload. But for whatever reason they decided to make good on the theoretical refuelability use case of the rocket stage in a goofy self-contained way.
They just added a hydrogen tank (which was actually straight off of an LH2/LO2 chemical rocket upper stage), and plumbed it into the water tank. They plumbed it into the water tank instead of the LH2 tank because, well probably partly just to show that they could. But also because of the ullage gas (shown in transparent orange in the diagram). Ullage gas, usually helium, fills up the empty part of a fuel tank to keep the propellant from boiling away as it's used. There's separate spheres of helium used for the job. With propellant still inside of the main hydrogen tank, but the water tank totally emptied (aside from the little reservoir used for cooling the reactor on startup and shutdown), it would be easier to force the hydrogen into the water tank than to simply refill the hydrogen tank.
at least, that was the theory.
it didn't work, but i don't recall why. If I had to guess, some component was overlooked when testing the water tank and associated plumbing's ability to work with frigid liquid hydrogen.
Anyway investigating Oldsky to figure out the disappearance of the Zaldans had to wait for the next Omen apparition.
