Re: [Spaceships] getting into orbit without superscience?
 

Actually, some designs would leak radiation - in the form of fuel. You see, having a barrier between the energy source (fuel) and the propellant limits how hot you can run the rocket to what that barrier can handle. So some designs relied on careful flow design to keep the fuel from (mostly) leaving. Better performance at the cost of radioactive exhaust and consuming expensive radioactives at a much higher rate.

Re: [Spaceships] getting into orbit without superscience?
 

This is why *all* reaction engine designs have trouble getting into orbit. Chemical rockets already run hot enough containing the exhaust is an issue. So there's not a lot of space for going hotter (the requirement for better fuel efficiency), which combined with the issue that heating by something other than reaction in the fuel adds weight for both the heat source and heat exchange system, means there really isn't lots of room for improvement even in theory.

The truth is that even a lot of spacecraft engines and power plants that aren't "officially" superscience really are, it's just the required superscience is the somewhat invisible "requires parts that still work at temperatures too high for chemical bonds to exist".

Re: [Spaceships] getting into orbit without superscience?
 

Well, you can use hydrogen as reaction mass instead of combustion byproducts; that allows an ISp of around 1,400 without being any hotter than current rocket. The problem is that reactors can't generally run anywhere near as hot as a current rocket engine.

Re: [Spaceships] getting into orbit without superscience?
 

Free neutrons don't constitute any sort of long-term radiation hazard. All of the major components of Earth's atmosphere are still harmless after absorbing a neutron. The neutron hitting a nucleus but not being absorbed does transfer some energy but it's pennyante stuff. You're in much more danger from the heat of the exhaust.

Re: [Spaceships] getting into orbit without superscience?
 

GURPS clearly defines what it means by superscience, and it marks all equipment as to whether it is considered superscience or not.

"'Superscience' technologies violate physical laws... as we currently understand them" (page B513).

Superscience equipment has a "^" for its TL or, if the writer decides that it appears at a certain TL, the "^" appears after the TL number.

So the original question can be restated as: what is the most efficient way to get from the surface to orbit using only equipment that doesn't have a "^" on its TL? (And there is an implicit "also not magical" in there.)

Re: [Spaceships] getting into orbit without superscience?
 

Mean free path of a neutron in air near is something like half a mile at 1 atmosphere, so it's a bit of a direct radiation hazard and also can activate materials on the ground.

Re: [Spaceships] getting into orbit without superscience?
 

....and dispersing these enutrons over an area of half a mile in radius doesn't reduce the concentration to negligable numbers?

Re: [Spaceships] getting into orbit without superscience?
 

A ton of heat producing equipment (i.e. reactor) will heat a lot less reaction mass to those sorts of temperatures than a ton of chemical rocket engine can burn. In other words, you can have Isp or thrust (with current or near future technology), take your pick.

Re: [Spaceships] getting into orbit without superscience?
 

That's a separate problem: closed cycle solid power plants simply can't come close to the performance of open cycle gas combustion (i.e. a rocket). This is reflected (opaquely) in spaceships

A 5 ton (SM+6) chemical rocket produces 300 tons of thrust (3 MN) with an exhaust velocity of 3 mps, for a total power output of 7 GW.

A 5 ton (SM+6) fission power plant produces 1 EP, sufficient to power a beam weapon that emits 30MJ/10s or 3MW. There's probably inefficiency there, but even if you set beam weapons to 10% the rocket is more than 200x as powerful.

You can get around that, but it generally requires open cycle. Which means you're spraying fuel out the back.

Re: [Spaceships] getting into orbit without superscience?
 

The SABRE engine is also relevant. It has two modes, acting like a ramjet at low altitudes and switching to closed cycle mode at high altitudes. Wikipedia It works really well in Kerbal Space Program. It wouldn't be suitable for a trip around the solar system though, but you could fly crew and cargo to orbit and rendezvous with a craft constructed in orbit. Another drawback is that it requires an atmosphere with oxygen (or possibly some other oxidizer?).