Carnival of Space 36 at Cat Dynamics

One of the best recent Carnivals of Space is being hosted at Dynamics of Cats blog. Among the many excellent entries, one posting that particularly caught my eye was: the Air Launched Single Stage to Orbit.

Staging is one of the single highest risks of failure for existing launch vehicles. Additionally, with a TSTO, now you're really designing three vehicles, not just one. A first stage, an upper stage, and a combined entity. You now have to come up with abort modes for all the different configurations.

Probably one of the biggest headaches for TSTOs is how to recover and reuse the first stage. Getting to orbit is only a little bit about going up, and mostly about hurtling yourself sideways fast enough to "throw yourself at the ground and continually miss". Doing so entails gathering quite a bit of horizontal velocity with a first stage, which means that the first stage gets quite a bit of horizontal distance between it and the launch site by the time it releases the upper stage....

Starting, as SpaceShipOne did, at a reasonable altitude gives several distinct advantages over ground launch (the following list comes from Dan DeLong, with some thoughts from me [in brackets]):

1. The airplane carrier contributes to the overall altitude and velocity...
2. Meteorological uncertainties are mostly below launch altitude. Propellant reserves can thus be less....
3. Total integrated aerodynamic drag losses are less, as the launch is above much of the atmosphere....
4. Max Q is less, which reduces structural mass, and may allow lower density thermal insulation....
5. Engine average Isp is increased because the atmospheric back-pressure effect affects a smaller fraction of the trajectory....
6. Engine expansion ratio (non-variable geometry assumed) can be greater because overexpansion is less problematical....
7. Wing area can be smaller because the wings do not need to lift the gross weight at low subsonic speed. Air launch Q is greater than runway rotation Q.
8. Wing airfoil shape need not be designed to work well at high gross weight and low subsonic speeds.
9. Wing bending structure need not be designed for gross weight takeoffs or gust loads....
10. Thrust/weight ratio can be smaller because the low initial trajectory angle does not have large gravity losses. This allows a smaller engine, propellant feed, and thrust structure mass fraction. I found 1.25 at release to be about optimum. This is a bigger advantage in air launching because total integrated aerodynamic drag losses are less and the trajectory need not get the orbiter out of the thick stuff as fast....
11. The lower mass/(total planform area) yields lower entry temperatures. I assumed inconel foil stretched over fibrous blanket insulation for much of the vehicle undersurface. Titanium over blankets, or no insulation worked on the top surface. Payload bay doors peaked at 185 F....
12. Mission flexibility is greater.... The ability to move the launch point also potentially opens up longer launch windows. Lastly, being able to move the launch point allows options like operating out of an airport closer to "civilization" while still launching out of an area with low population density, like say over an ocean or a desert...
13. landing gear for an air-launched SSTO can be designed based on landing weight instead of takeoff weight. This is a big deal for SSTO designs.

Selenian Boondocks

Affordable access to space is the main obstacle to the coming "space rush." The inner solar system (from the asteroids inward) contains material wealth beyond human imagination. But more important, space--like the oceans, the polar regions, and the seafloor--represents a new frontier. Humans desperately need frontiers in order to express one of their most endearing characteristics--the ability to solve existential problems while being stressed beyond reason by the conditions at hand.

The die.off generation--represented prominently within the Peak Oil, CAGW, and Population Crisis camps--wishes to return to a stone age Earth with a population of a hundred million or so humans, maximum. You may already be an unwitting member of the Human Extinction Movement. If so, you will wish to hurry over to one of the two links above without delay. For the rest, who actually have the gumption to face existential problems head-on, it is never too early to begin working.

Hat tip: Brian Wang