Chair Force Engineer

Monday, January 07, 2008

Winging It

Jon Goff knocks the ball out of the park with his most recent blog post on the benefits of air launch for single-stage rocket vehicles. Never before have I seen such a comprehensive and balanced discussion of what advantages and disadvantages are offered by this approach. It’s posts like this which demonstrate the educational power of the internet, when used properly.

I must admit some bias on the topic of air-launched rockets. When I was younger, and before I truly understood the ramifications of the rocket equation, I was enamored with winged spaceplanes. As I progressed through my undergrad classes, and as I read some insightful analysis, I began to understand how difficult it would be to incorporate all of the spaceplane-specific systems into the tiny structural mass budget of an air-launched RLV.

In the first semester of my senior year, my design professor forced my team into cloning the Pegasus launcher. While the design project was hardly original or creative, it at least allowed us to appreciate the decisions that were made in the design of Pegasus. My big takeaway from the design exercise was that air-launched vehicles would be tiny (due to the constraints imposed by the mothership,) and the velocity gains from air launch would be small.

That’s not to say that I find all air-launched RLV designs to be impractical. About a decade ago, AFRL had an internal concept known as BladeRunner. Unlike Pegasus and other designs that are volume-limited due to ground clearance concerns, BladeRunner would have been popped out the aft cargo doors of an Air Force transport. The wings would be simple scissor-wings on a pivot. BladeRunner had two stages, avoiding the mass penalties of the “assisted SSTO” approach. To this day, BladeRunner is one of the most sound RLV concepts I’ve ever looked at in detail. Perhaps I’m biased, having been given the BladeRunner briefing by one of the gentlemen who developed the concept. Not that it matters, as BladeRunner is a dead concept (unless some enterprising NewSpacer wants to take the idea and run with it.)

There are plenty of other ways to get around the performance penalties associated with an airlaunched SSTO. A fairly cheap one is to move some of the spaceplane’s propellants into a drop tank, much like the one used on the shuttle (which leads to the Soviet MAKS concept.) A more expensive concept is a custom-designed mothership that uses rockets or ramjets (perhaps both) to achieve speeds of Mach 3 or higher to provide a bigger velocity boost to the spaceplane (such as in the Blackstar fantasy.)