Efficient TL10 and TL10^ reaction engines in GURPS Spaceships


I’m not sure that Kirchoff necessarily applies - in particular, if landings have the potential to exceed launches (e.g. you’re setting up a new colony), there’s the option for a lot of cargo of building basic deadstick gliders to wrap round a small raft of containers.

To my mind one of the biggest lacks in Spaceships — which is a system I greatly admire, in general — is any rule for the level of armour needed to withstand re-entry (either generically, or with various profiles). The closest we get is the Atmospheric Flight box on p. 40.


Right. I mentioned that I was assuming Kirchoff because it doesn’t always apply, particularly to cargo and passengers: cheap back-loading is a thing. But it can apply either way: you can have expensive landings that use no fuel and cheap takeoffs that use a fortune worth of it,

I took a very quick look at soft landing systems, but it is hard to overcome the cost advantages of re-using a spaceship thousands of times and cheap, cheap water remass. Especially the cheap propellant, I eyeball the cost of soft landing as G$10,526 per SM+7 (15 ton) cargo system landed, which outcompetes everything except for fusion torches, but is dramatically undercut by fusion torches expelling water.

Soft-landing systems get more efficient as scale increases. I have this nasty image of 10,000-ton, 30,000-ton, 100,000-ton colony “ships” coming out of their JAFAL warp, de-orbiting with a passel of cheap chemical RATOs, and then splashing down in the seas of Tau Ceti III under parachutes the size of counties.

I’m with you about the steel armour. One of the things that GURPS does poorly is to make high-tech stuff more expensive and better. Mail was terrific armour, plate replace it because of being much cheaper, not by being more expensive and much better. The Industrial Revolution worked by making things much cheaper. New energy technologies generally drop in price by a factor of ten from introduction to maturity.


My somewhat silly vision has ferromagnetic re-entry vehicles being landed on suitable parts of the maglev rail network.

The Spaceships series in particular keeps prices rock solid, but improves performance at higher TLs. If you can buy a TL10 Isp-20 drive and a TL9 ISp-10 version of the same drive in the same shop, the latter is going to be cheaper…


Have you considered laser rockets from Spaceships 8, page 8? They seem generally good at “small frequent shuttle trips to surface and back”, though they also require a substantial on-ground power source and a field of small lasers.


Also, I assume you’ve already read through this:


And this:


Extrapolation based on airliner costs, roughly.


No. I’ll look at it now.

Not for a long time, and the page on Atomic Rockets has grown.

Basically, I want to d a lot better than assuming that spacecraft operations and economics are like jet liners, and I think with the specifics in GUPS Spaceships I’m in a position to do so.


Okay, done the analysis for laser-launch rockets.

  • The wingless design is optimised for Earth with two motors and 10 “tanks” of ablative plastic. Under the “heavy use” scenario it turns out to cost G$1,744 per SM+7 cargo system (15 tons, 20 passengers) per launch or landing, not including costs associated with the launch facility (amortisation, depreciation, operations).

  • The winged design is optimised for Earth with two motors and 10 “tanks” of ablative plastic. Under the “heavy use” scenario it turns out to cost G$1,057 per SM+7 cargo system (15 tons, 20 passengers) per launch or landing, not including costs associated with the launch facility (amortisation, depreciation, operations).

The high thrust is good, and the motors are cheap, which keeps the cost of the ship down. But the very modest specific impulse means lots of propellant, which crowds out the payload.


For a bit of a laugh I extended the analysis to TL7 chemical rockets. It turns out that with the trajectory I picked, and without taking into account that gravity drag would diminish as the tanks emptied (producing higher acceleration in the vertical phase and allowing further pitching down in the horizontal phase) SSTO is just possible—with zero payload. And even that is because the tanks-of-propellant requirement gets rounded up from 17.9 to 18. You could carry payload with a smaller control system or by splitting a system between two SM+6 tanks, 2 SM+5 tanks, and an SM+5 payload system.

Now I’m tempted to approximate a better trajectory with a series of sections involving equal delta-v and customised angle of flight. That was not the point of this exercise!


I’m a little wary of going too far with this while retaining Spaceships rules - why not use actual ISp and mass fraction values rather than the kludge that’s the tanks-of-propellant table? (Answer, because then you have to account for acceleration increasing too…)


Back when I was working on GURPS Weird War II, I did a rocketry table that attempted to use actual specific impulse and mass ratios.


And dead handy it has been. (I keep pointing out to people that if you want a good chemical rocket there’s only really one combination that gives you decent performance, and if people could just get over the whole HF exhaust thing…)

Let’s see. When GURPS Spaceships says “one tank gets you X miles/second”, it’s talking about 5% of vehicle mass.

Tsiolkovskiy says that dV=v ln m0/m1; m0/m1 = 1/0.95; v = dV / ln(0.95); so each mile/second of GURPS Spaceships performance is about 31km/s effective exhaust velocity.

units "1mile/second/ln(1/0.95)" m/s
	* 31375.329

Sanity check: a chemical rocket gives a nominal 0.15 mile/s per tank, so that’s a v sub e of 4700m/s, not wildly far from the 4400 of LO₂/LH₂.

Acceleration is harder to convert, but I think one has to assume the given figure is for a fully-loaded vehicle and increase it as fuel is burned.


The ForeSight spaceship rules had a drive mass and endurance table, implicitly including both motor and tank. When I came to build a spreadsheet to do all the fussy calculations (ForeSight had a formula for the mass of the structural frame) I decided to replace the table with Tsiolkovsky’s equation and treat tankage explicitly. It emerged that a lot of the drives implied superluminal specific impulses. So (for my own use) I replaced that chunk of rules entirely. I basically treated everything as a torch (i.e. fuel and fuel alone used as propellant), with TL6 drives based on the theoretical limit for fission fuels, TL7 on fusion fuels, TL8 on 1% antimatter, TL8.5 on 3% antimatter, TL9 on 12% antimatter, TL 9.5 on 25% antimatter, and TL10on 50% antimatter.

And that was all perfectly okay until that stinker Winchell Chung created his beautifully-researched Atomic Rockets website and let just anyone see what plausible speculation suggests for exhaust velocities and thrust-to-mass ratios.


Problem with a beanstalk is that satellites in low planetary orbit are very useful, and it’s hard to have both a beanstalk and low orbit satellites.


This is true. I suspect you get fewer, larger satellites, with active orbit maintenance and periodic reaction mass refills; then instead of lofting your own minisat, you rent space on someone’s small station. (I don’t think electrotether manoeuvres work for changing orbital plane, which is what you mostly need for beanstalk avoidance, but I may be wrong.)


I think you end up with no beanstalks. Low orbit satellites are just too handy to let something of marginal usefulness, great expense and great vulnerability get in their way.

There might be a few beanstalks in the setting. It would be kind of cool to have a bridge between two planets that were almost contact binaries. Though in that case you could just fly an air vehicle between them. What’s the novel about an invasion between worlds, carried by hot air balloon?