Chair Force Engineer

Saturday, November 22, 2008

Air Launch Revisited

The Selenian Boondocks blog has, since its inception, been one of the best sources for discussion of space transport ideas on the internet. Some of the ideas have been downright inventive, while others have only been shot down after much serious discussion. Nevertheless, it represents the kind of thinking we need to truly solve the problems of making space access routine and affordable. While already a great blog under the meticulous care of Jon Goff and Ken Murphy, Selenian Boondocks just got even better with the addition of John Hare to the team. Hopefully he will dispel the notion of a “Hare-brained” scheme as a bad thing. (Pardon the pun.)

My eye was recently caught by a discussion of an optimized carrier aircraft to serve as the first stage for a reusable launch vehicle. During Jon Goff’s previous air-launch discussions, I came out in favor of using unmodified military cargo planes for air-launching small vehicles with scissor-wings. While I still favor this idea based on tight development budgets, discussion of an optimized mothership has merit if somebody is willing to pay for one.

It should be noted that the most successful mothership in recent memory, the Lockheed Tristar operated by Orbital Sciences, was chosen because it was cheap to obtain second-hand, and because it had fairly tall landing gear which allowed for adequate ground clearance. While the option exists for captive-carry on top of the mothership, it has fallen out of favor because of incidents like the D-21 collision with its Blackbird mothership. While the idea worked for the Enterprise glide testing, it’s probably a much different story if the separation involves igniting a rocket.

John Hare’s idea for a “
flying wing” mothership does have some historical basis. One of the Northrop-Grumman concepts for Space Launch Initiative, released in 2002, used a six-engined flying wing as its first stage. A large winged rocket would provide most of the delta-V from subsonic cruise to orbit, and the crew would ride in a small lifting body. The biggest problem with the design is the amount of money required to develop it. The second biggest problem is whether the large winged rocket could accelerate from Mach 0.8 to orbital velocity, especially when burdened with the structural requirements of its own wings and landing gear. Problem three is how you’d get the winged rocket back to base after it’s expended all of its propellants.

One of the big assumptions here is that the mothership need only carry the vehicle to a speed around Mach 0.8. In fact,
Boeing twice patented a two-stage system with a supersonic mothership. The 1982 iteration used eight turbojets, while the 1993 version used six. Both designs made use of a Space Shuttle Main Engine for acceleration during climb, prior to staging. The mothership would be nearly 200 feet long. The problem is that multiple studies have shown that subsonic launch is a pretty efficient solution. The challenges of supersonic flight erode the benefits of a higher separation speed. Supersonic launch doesn’t break even with subsonic launch until hitting mach 3 or higher. The Boeing idea isn’t bad if your goal is to ignite a scramjet, but it seems like engineering and budgetary overkill for the space launch problem. The primary benefit of the Boeing patents was their addition to the lore of mythical craft like Aurora and Blackstar.

The challenge of a custom air-launch system is the need to develop both the rocket and the mothership aircraft. Systems that have utilized existing aircraft for motherships have been able to spare themselves the development of one element of the system. But the inverse paradigm has not been attempted: use of a newly-designed mothership aircraft to boost the performance of an existing rocket. Structurally, this may not be feasible due to the changes to bending and other loads encountered by the change to air launch. At the same time, it has been studied before (such as the Crossbow concept) and is worthy of further consideration. Even though the payload boost may be small, it reduces the facilities costs and the weather-related delays that have accompanied traditional ground-launched rockets. Air-launch makes space launches more flexible in terms of launch azimuths, launch sites, and reduced launch delays. The concept's advantages should draw the interest of the Defense Department as a solution to the problems of Operationally-Responsive Space.