guest blogger john hare
I had an interesting conversation with Jon last month about the problems with air launching rocket ships. The various flavors of air launch involve some form of altitude and velocity loss as the rocket ship drops away from the mother ship before it can light it’s engines. In most cases, it also requires wings and other aero surfaces to correct the flight path to the desired vertical. These aero surfaces are not only dead weight from Jon’s point of view, they also induce airframe stresses on the rocket ship that it really doesn’t need.
The most important things for air launch are altitude, attitude, and airspeed, in about that order of importance. A conventional air launch seems to compromise somewhat on all three. Another point Jon didn’t care for is that the rocket cannot light it’s engines until well clear of the mother ship. Ignition or other engine related failure could easily result in loss of vehicle instead of an abort and return to base to examine the problem.
In my opinion, which is not shared by many that I am aware of, the White Knight series are perhaps the best high altitude airplanes in the world by the best designer in the world, but isn’t the optimum mother ship for space craft. The space craft they carry suffer from all the problems mentioned above, with only some mitigation by launching from extreme altitude.
I believe that the best mother ship is one that works for the requirements of the rocket vehicle. The rocket should be able to light the engines and confirm a healthy burn before separation. The release attitude should be such that the rocket ship is vertical or nearly so and doesn’t need any aero surfaces or the loads they impose. And it needs to release from the highest altitude possible at the highest airspeed possible.
I suggest that conventional air launches have been done backwards. The rocket should be lit before separation, and separate from the top of the vehicle at both high subsonic speed, and high altitude. Most of this can be accomplished with a change in operational technique rather than building a brand new super duper mother ship.
The rocket ship is mounted centerline bottom of an aircraft. At 25,000 feet or so, the mother ship has reached maximum altitude at the loaded condition of full tanks and rocket ship payload. The rockets ignite and feed from tanks on board the mother ship as the two vehicles accelerate and begin to climb. By 30,000 to 40,000 feet, the vehicles begin to roll inverted as you would see in a Shuttle launch. At 50,000 feet, the pair is climbing at 70 degrees and high subsonic airspeed. Then the mother ship lights some RATO type rocket engines and the rocket ship throttles back so that they are accelerating at the same rate as they separate. The switch to internal fuel supply for the rocket ship is confirmed stable at separation. With both vehicles accelerating at the same rate, they fly apart in parallel formation so that neither exhaust impinges the other. The rocket ship rolls from a 70 degree climb to vertical while the mother ship rolls from a 70 degree climb to the horizontal.
When sufficiently clear, the rocket ship throttles back up for the climb with full tanks and 250+ meters per second velocity at extreme (for aircraft) altitudes. The mother ship reaches horizontal at nearly ‘coffin corner’ altitude for a clean ship with low fuel and no load. Coffin corner is the term I believe applies to an altitude where the aircraft can’t go faster for engine or airframe reasons, and can’t go slower without stalling and falling out of control with a strong possibility of no recovery.
I believe this method would impart considerably improved performance to a rocket ship compared to the conventional approach. It would not require the rocket ship to have aero surfaces.(VTVL friendly) It would allow abort to base in case of rocket engine problems. It would not require developing the best high altitude aircraft in the world.