BARD PAPER: SARA2001

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Commercial Airbreathing Shuttle

design by Bruce Alan Macintosh

TL-8 versions of these designs would look almost identical; if anyone is really interested in the subtle differences I can post them.

The one major difference to published TNE rules is of course the delta-V to orbit. The TNE rule-book number-of-G-hours to orbit, as has been noted, are completely wrong. Delta V from earth's surface to a 150km orbit is 8.00 km/s. (Not 0.64 G-hours, which is 22.5 km/s.) Getting back from orbit requires much less delta-V, too; all you have to do is change your orbit so it scrapes the atmosphere more, and then aerobrake (like the Space Shuttle does.) This raises total delta-V to 8.04 km/s. If you start near the equator delta-V goes down; 7.64 km/s is the figure I used, appropriate for a 30-degree inclination takeoff. For a size 9 world of normal density delta-V is 8.52 km/s, for a size 7, 6.72 km/s.

These are delta-V in a vacuum and assuming very high instantaneous acceleration. Real atmosphere and gravity complicate this somewhat. FFS talks about non-CG spacecraft using 1-g of their thrust to negate gravity until they reach orbit, which is spectacularly nonsensical, even by GDW standards. *Anything* that's moving is in an orbit; a thrown baseball is in an orbit - just a *very* long and skinny elliptical orbit that happens to intersect the Earth's surface. The trick for a rocket is to make sure your orbit never actually reaches the ground. For finite accelerations this means some amount of delta-V is spent going upwards as the spacecraft lifts off, which is less efficient than a pure Hohmann transfer orbit. In addition, you lose delta-V due to atmospheric friction. Together these add about 1.5 km/s to the delta-V requirements for a classical rocket, and require that the takeoff acceleration be significantly higher than 1G.

The shuttle designs below are horizontal-takeoff designs which use their airbreathing AZHRAE engines for takeoff and to climb to high altitudes (and, in the case of the first design, for some extra delta-V.) This is similar to the (now-cancelled) X-30 Aerospace Plane. This approach has been pretty much abandoned in the real world, but FFS makes it easier to design than most other single-stage-to-orbit designs (see below.) It also means that the acceleration required of the engine is only 1G (or even less.) I've allowed 0.6 km/s delta-V for remaining atmosphere drag (8.1 km/s round trip to 150-km orbit from 30 degrees latitude), which is typical for the Pegasus air-launched rocket. There's a huge range of operational parameter space for these vehicles depending on the atmosphere, density and diameter of the world they're operating on; I've designed for an Earthlike world.

Terminology for those who don't have FFS: AZHRAE is the acronym for a combination turbojet/ramjet/rocket engine. HRF is hydrogen rocket fuel. HCD is hydrocarbon distillates (petrochemical fuels.) EAPlaC is the incredibly efficient solid-rocket-like TL-9 engine in FFS, normally used in missiles.

FFS engines have low thrust-to-weight but also lower fuel consumption than the Real World. This is actually an advantage for craft like this one which have quite low accelerations. Using more realistic engines (see the URL above) bites heavily into the payload; designs are available if anyone is really interested.

One can argue that craft that aerobrake should have higher armor values than normal craft, but one can also argue that AV=1 and especially the internal structure is far heavier than real world craft needed, so I've just left it at AV=1.

EAPlaC