Executive Summary



What is assisted launch?

     Most space experts are surprised to learn that the Space Shuttle burns half its fuel just to reach 1000 mph (Mach 1.3), because it struggles to push through the dense lower atmosphere with a full fuel load.  After reaching Mach 1.3, air resistance is minimal and half its fuel mass is gone, so it can zoom to Mach 24 and into orbit.  Therefore, until some magical new technology arrives, the solution to cheaper space launch is to give spacecraft a quick boost during liftoff.  There are several options, as Glenn Olsen explains at his website linked in our Reference section.  Pneumatic launch like those used by modern roller-coasters is promising, but inclined rocket sled launch has already been proven hundreds of times at test sites.  For an example of the value of ground-based assisted launch, we'll use the newest expendable rocket, the Lockheed-Martin Atlas V, which was first launched on August 21, 2002 (below). Aviation Week and Space Technology described its liftoff:

   "At liftoff, the Atlas V weighed 737,547 lb.--125 tons heavier than the Atlas III. With only a 1.2 thrust-to-weight ratio, the heavy vehicle climbed slowly taking an agonizing 11 sec. to clear its umbilical tower. Once momentum had been established 4 sec. into the flight, the engine was throttled down slightly to 99%.  At 17 sec. and 800 ft. altitude the RD-180 was gimbaled to begin the vehicle's pitch and roll program.  At 100 sec. just after passing the critical Max-Q point, the engine was throttled down again to 95%. As propellant was depleted and the rate of acceleration increased, thrust was then modulated to hold a maximum of 5gs for Hot Bird payload structural limits."

      So the Atlas V burns at full thrust for 10 seconds just to get 200 feet off the ground and up to 53.6 mph; using the formula v=sqr[2as].  At 20 seconds, it is 1000 feet high and traveling just 77.3 mph.  The Atlas V first stage booster carries 627,000 lbs of fuel and burns for 236 seconds, which means it consumes 2657 lbs of fuel a second.  Therefore, it consumes 53,000 lbs of fuel during the first 20 seconds just to reach 77 mph.

     The maximum payload for the basic Atlas V "501" is just 9000 lbs to GTO.  Therefore, a 77 mph ground-based assist for a modified Atlas V could put 53,000 lbs more mass into space.  This doesn't mean that all the fuel savings result in greater payload into orbit since the second stage now has more payload to propel.  Nevertheless, this small boost will result in 3000 - 8000 lbs more payload into orbit depending on final altitude and velocity. 

     Sky Ramp advocates have discussed varied means of ground-based assist for existing vertical launch systems.  One suggested a LOX tower with long thick umbilical cord to pump LOX into the rocket for several seconds during liftoff.  Once suggested a massive metal spring under the pad that could heave the rocket upward at launch.  One suggested a deadweight attached to a pulley at the top of a tall tower dropped at launch to pull the rocket upward.  These are all interesting yet tricky proposals which NASA should investigate.  They will conclude that whatever method is used, a ground-based assisted launch can increase the payload of existing rocket systems several times.

Why not use aircraft-assisted launch?

    Since a ground-assisted boost is so valuable, why not use large aircraft to fly spacecraft much higher prior to launch?  The main problem is size.  The largest American aircraft is the U.S. Air Force C-5B with a maximum payload of 270,000 lbs, compared the launch weight of the Atlas V of between 735,000 - 2,120,000 lbs, depending on the configuration.  Therefore, a new aircraft at least three times larger than a C-5B is needed to launch a sizable rocket.  Small rockets like Pegasus are air-launched from commercial aircraft, but the actual boost is small since the launch altitude is limited to 20,000-30,000 feet for large, fully-loaded aircraft because their wings begin to lose lift and jet engines become starved for oxygen.  A massive rocket-powered spaceplane would not have this disadvantage, but would be costly to build and maintain, just like the Space Shuttle system.

     Since numerous 20,000+ foot mountains exist around the world, it is easier to haul large spacecraft up high and launch it  where the air is half as dense as sea level.  This is helpful, but the key element of assisted launch is upward velocity, which a mountaintop launch and even aircraft launch cannot provide because most  velocity gained from horizontal launch is lost in doing work against air in changing the velocity vector to near vertical.  An aircraft can perform a dive and pitch up maneuver for launch, but must be even larger and heavier to withstand the extreme stress on its airframe.  The US Air Force is currently funding a plan to shove a small rocket out the back of a C-5B and firing it from 28,000 feet as it falls.  This will result in a NEGATIVE assisted launch which requires more fuel than shooting it off a mountaintop.

      Shooting a spacecraft upward off a rocket-powered sled a better alternative.  A rail system has no constraints on size and weight.  An RLV shot up a 45 degree Sky Ramp and off 13,000 foot peak at Mach 2 will propel the RLV mass up to 54,000 feet, even before the RLV fires its engines; using the formula v^2=2as where v=vosin45 and vo~700m/s. and a~9.8m/s^2  we get s=12.5km~8miles + the 13000 feet starting point.   So ramp launch is like air launch, it just flings the RLV twice as high as any fully loaded transport can fly.  It is also much safer and much cheaper than building and maintaining a massive aircraft or rocket-powered spaceplane.  There are other bogus space launch concepts that are proven failures, yet continue to be advocated and funded, like maglev launch and reusable boosters.

       Any workable method of ground-based assisted launch will be a breakthrough in space flight.  NASA recognized this recently when it studied maglev assisted launch.  While it learned that current maglev remains grossly underpowered for space launch, it failed to note that simple rocket sleds can provide the same assist.  Most space experts dismiss the value of a Mach 1-2 launch assist because they assume this just saves 4-8% of the fuel needed to reach Mach 24.  They fail to recognize the massive energy required to push a fully loaded spacecraft through the dense lower atmosphere and are surprised to learn the Space Shuttle burns half its fuel to reach Mach 1.3.  

One objection is that a ramp does not allow launches to different azimuths.  However, there is more to space than putting satellites into orbit. Putting tourists into orbit and supporting a space station only requires one azimuth. Future space travel concepts usually involve staging and assembling sections of interplanetary spacecraft in orbit. All these goals require reliable and low-cost space launch, and only to a specific azimuth. Until mankind develops a new propulsion method, ground-based launch assist is the only promising method of advancing space flight.

2008 Sky Ramp Technology