Launching Spacecraft From Aircraft

Embedded Geek writes: "New Scientist has an article on a proposed launch scheme named 'Bladerunner' (presumably, someone is a P.K. Dick fan) that would use a pneumatic launcher to shove a launch vehicle out the back of a military transport aircraft at high altitude (40,000 feet/12,000 meters). As with all the new systems (such as this one) the goal is to reduce launch costs to more reasonable levels (to about $6K/kilo from today's $11-44K). An existing Pegasus system uses dedicated B-52s with the vehicle slung underneath, but Bladerunner would be an improvement by not requiring dedicated planes (the launcher could be set up on a transport in 24 hours) and also could accomodate larger vehicles (since it wouldn't be slung underneath)."

Comments

Pegasus uses Lockheed L-1011, not B-52s

[richie2000]

According to the Pegasus link in the submission, the Pegasus system uses a modified Lockheed L-1011 airliner jet named Stargazer, not B-52s. Methinks someone has overdosed on CNN again...

Re:do the math.

[FTL]

> In any case the energy savings by lifting the payload to 20km are minimal at best. Most of the advantage comes from being weather independant, due to being above the clouds.

Nice point. However, I'd think that there are more important saveings than a 20km lift and weather independance:

• Equator. You get to launch exactly on the equator, instead of having to burn fuel in dog-leg maneuvers to get there.
• Horizontal speed. An old German V2 can get to space, but it can't get to orbit. A launch is a little bit of up, and a whole lot of sideways. If you dump the rocket out of the plane while traveling at Mach 0.75, that's 3% of your velocity taken care of.
• Engines. The real reason rockets stage is to swap out engine nozels. The bells that work at sealevel are ill-suited to vacuum operation. By launching above most of the atmosphere you can just use a single stage.
• Friction. A good portion (numbers anyone?) of the energy of a lunch is devoted to plowing through the air. Something that's not an issue when you start 20km up.

Re:Half the cost is first 40K feet?

[Buran]

I don't get it. How do you cut the price in half just by getting the first 40,000 feet out of 200 miles free?

Most of the fuel use by a modern rocket occurs just getting off the launch pad. Current launch systems work by placing the rocket in a vertical position on the launch pad before liftoff, and the rocket lifts straight up before performing what's called a "roll maneuver" (this is unmistakable on the Space Shuttle, but "regular" cylindrical rockets do it as well) to get into the correct attitude for the rest of the flight. This is an incredibly energy-inefficient method, but the rockets are designed to work this way.

Consider also that most launches take place from sea level (the Sea Launch converted oil platform is a perfect example) where the earth's atmosphere is thickest, causing a lot of reistance that has to be overcome by the force of the rocket motors. At 40,000 feet, the atmosphere is far thinner (consider that humans have to wear breathing masks above 10,000) so there's less fuel used just burning through the lower atmosphere.

There is also a velocity bonus that comes from launching this way. A rocket lifting off from the earth's surface is only getting a "free" boost from the speed at which the ground moves at that latitude, explaining why the equator is the best latitude for launches and why Sea Launch tries to get as close to it as possible. An air-launched rocket like Pegasus gets the "free" velocity bonus from the launching aircraft in addition to that from the earth's rotation.

So... let the aircraft, which costs far far less to operate (we don't have rockets in every garage, but I know a few pilots) do most of the hard work and then let the rocket literally piggyback on that. (Yes, I know full well that Pegasus rides under the L-1011!)

Half your mass can be spent in 40,000 feet.

[Spamalamadingdong]

Buran didn't get it quite right, so I'm going to see if I can briefly but accurately sum up why this is an improvement.

• When a rocket takes off from the ground, it is throwing away gas at many times the speed of sound while it's moving very slowly. If you calculate the amount of energy which actually accrues to the rocket versus what disappears as heat and noise with the exhaust gas, the efficiency is dismal. Launching from an aircraft allows the rocket to begin operating at a much higher efficiency; indeed, the air-launched rocket starts at a speed and altitude that the ground-launched rocket may have to burn half of its mass to reach.
• Nozzles cost about the same, but a nozzle with a bigger bell can expand the gases more and get more thrust out of them. More thrust for the same fuel means more payload to orbit, and costs go down. You can't use a large-bell nozzle on a launch from the ground because the gases would be over-expanded, separate from the nozzle walls and cost you badly in efficiency and thrust. This means that the rocket launching from high altitude has an advantage which goes well beyond starting a bit higher.
• The payload at the end of a rocket burn is an exponential function of the delta-V (the more speed you have to put on, the more of your vehicle has to be fuel and the less is payload); getting a 550-600 MPH or so head-start helps a lot. So does the aerodynamic lift of the wing, which is effectively "vertical thrust" that comes for a fraction of the fuel required to produce the same with rocket fuel.

Hope that helped.