Paul Allen's recent announcement that he'll be building a massive plane for air-launching rockets spawned a discussion thread over on Space.com...
I was a bit confused about the economics of the proposal, since it appeared that the plan is to air-launch a Falcon 9 derivative, but with a payload 10,000 lbs less to Low Earth Orbit than a ground-launch Falcon 9. Fellow commenters pointed out that the rocket appears to be a Falcon 5 (rather than a Falcon 9), and based on an article I found on wikipedia it appears that air-launch is achieving a 50% increase in the payload carried to LEO (from 9,000lbs to 13,500 lbs).
What I find amusingly disturbing about this discussion thread are the helpful statements that aren't easily verified... Statements such as:
There are many factors that differentiate the air-launch of a rocket from ground-launch of a rocket. The first factor that comes into mind is altitude...
Altitude, by itself, is a negligible factor. If you had a platform 1,000 miles high and stepped off the edge you'd fall right back to the ground. Orbits are dependent on velocity, not altitude (LEO velocities are on the order of 17,000 miles per hour).
The altitude factor has more to do with wind resistance. Rockets have to 'push aside' the air, so the denser the air, the more resistance. As the rocket ascends, the air density lessens, so our calculations need to take that into effect.
Another advantage of altitude comes from the nature of the rocket's engines - the effectiveness of an engine in converting fuel to velocity is called specific impulse, and the specific impulse for an engine at sea level is lower than the specific impulse of the same engine in a vacuum - on the order of 20% lower. The higher you go, the closer to vacuum, the higher the specific impulse.
One advantage of 'air-launch' is that you can fly your rocket above the majority of the atmosphere before you 'light the fuse'.
We should note at this point in the interest of full disclosure that space agencies haven't been too keen on building launch facilities on mountains to take advantage of the lower air density. Most launch facilities are at sea level... so air density doesn't seem to be a huge concern in practice.
The latitude of a launch site is much more important than its elevation... Achieving an orbit is dependent on the velocity of the rocket, and a rocket sitting on the ground at the equator already has a velocity of over 1000 miles per how thanks to the rotation of the Earth. The closer the rocket is to the equator, the more it benefits from the Earth's rotation.
One advantage of 'air launch' is that ability to vary the launch location. In the simplest case, you can fly your rocket to the equator to get the full benefit of the Earth's rotation before you 'light the fuse'.
The initial velocity of the rocket is tremendously important... Rockets, unlike planes, carry all of their propellant. Planes carry fuel, but they burn that fuel using atmospheric oxygen. Rockets have to carry their own oxidizers in addition to their fuel. Oxygen is actually quite heavy... so the relative initial weight of a 'fully fueled' rocket intended to achieve a specific velocity is much more than a comparable 'fully fueled' plane (with an air-breathing engine).
When a plane accelerates, it only has to accelerate it's own mass and the mass of the fuel. When a rocket accelerates, it has to accelerate it's own mass, the mass of the fuel, and the mass of the oxidizer. That's why we've seen a great deal of interest in air-breathing hyper-sonic vehicles... the eliminate the burden of accelerating that oxidizer.
Allen's air-launch proposal relies on legacy commercial jet engines, so the initial velocity of the rocket is likely to be increased in the neighborhood of 600 miles per hour compared to a ground launch... By using a Rocket Equation Calculator that I found on the web, I plugged in some numbers and came up with the following rough estimates...
To launch 1000 kilograms to LEO using a rather common rocket engine, the initial rocket weighs about 23,000 kilograms. A similar rocket launched at altitude from an aircraft traveling at 600 mph would only weigh about 12,500 kilograms.
So the poster's comment about needing 'half the rocket' holds true given what we know about Allen's proposal... Now let's look at the payload claim. Going back to my Rocket Equation Calculator and sure enough - My 12,500 kilogram rocket can loft 1000 kilograms to LEO if I start at 600 mph at a very high altitude, but only 540 kilograms if I start at sea level.
Very cool, and very satisfying to boot... but also very, very rough estimates. The devil's always in the details... and obviously some details are missing from these 'back of the envelope' calculations.
It would be marvelous if sites such as Space.com would add tools to their sites to help their reader's 'fact check' their discussions. The more confidence you have in 'the facts', the more meaningful your conversations can be.
On a related topic, Elon Musk's Why it's so hard to build a reusable rocket.
I was a bit confused about the economics of the proposal, since it appeared that the plan is to air-launch a Falcon 9 derivative, but with a payload 10,000 lbs less to Low Earth Orbit than a ground-launch Falcon 9. Fellow commenters pointed out that the rocket appears to be a Falcon 5 (rather than a Falcon 9), and based on an article I found on wikipedia it appears that air-launch is achieving a 50% increase in the payload carried to LEO (from 9,000lbs to 13,500 lbs).
What I find amusingly disturbing about this discussion thread are the helpful statements that aren't easily verified... Statements such as:
"launching from a carrier aircraft about doubles the payload, or alternately lets you use a rocket half the size"Let's do a fact check on that statement...
There are many factors that differentiate the air-launch of a rocket from ground-launch of a rocket. The first factor that comes into mind is altitude...
Altitude, by itself, is a negligible factor. If you had a platform 1,000 miles high and stepped off the edge you'd fall right back to the ground. Orbits are dependent on velocity, not altitude (LEO velocities are on the order of 17,000 miles per hour).
The altitude factor has more to do with wind resistance. Rockets have to 'push aside' the air, so the denser the air, the more resistance. As the rocket ascends, the air density lessens, so our calculations need to take that into effect.
Another advantage of altitude comes from the nature of the rocket's engines - the effectiveness of an engine in converting fuel to velocity is called specific impulse, and the specific impulse for an engine at sea level is lower than the specific impulse of the same engine in a vacuum - on the order of 20% lower. The higher you go, the closer to vacuum, the higher the specific impulse.
One advantage of 'air-launch' is that you can fly your rocket above the majority of the atmosphere before you 'light the fuse'.
We should note at this point in the interest of full disclosure that space agencies haven't been too keen on building launch facilities on mountains to take advantage of the lower air density. Most launch facilities are at sea level... so air density doesn't seem to be a huge concern in practice.
The latitude of a launch site is much more important than its elevation... Achieving an orbit is dependent on the velocity of the rocket, and a rocket sitting on the ground at the equator already has a velocity of over 1000 miles per how thanks to the rotation of the Earth. The closer the rocket is to the equator, the more it benefits from the Earth's rotation.
One advantage of 'air launch' is that ability to vary the launch location. In the simplest case, you can fly your rocket to the equator to get the full benefit of the Earth's rotation before you 'light the fuse'.
The initial velocity of the rocket is tremendously important... Rockets, unlike planes, carry all of their propellant. Planes carry fuel, but they burn that fuel using atmospheric oxygen. Rockets have to carry their own oxidizers in addition to their fuel. Oxygen is actually quite heavy... so the relative initial weight of a 'fully fueled' rocket intended to achieve a specific velocity is much more than a comparable 'fully fueled' plane (with an air-breathing engine).
When a plane accelerates, it only has to accelerate it's own mass and the mass of the fuel. When a rocket accelerates, it has to accelerate it's own mass, the mass of the fuel, and the mass of the oxidizer. That's why we've seen a great deal of interest in air-breathing hyper-sonic vehicles... the eliminate the burden of accelerating that oxidizer.
Allen's air-launch proposal relies on legacy commercial jet engines, so the initial velocity of the rocket is likely to be increased in the neighborhood of 600 miles per hour compared to a ground launch... By using a Rocket Equation Calculator that I found on the web, I plugged in some numbers and came up with the following rough estimates...
To launch 1000 kilograms to LEO using a rather common rocket engine, the initial rocket weighs about 23,000 kilograms. A similar rocket launched at altitude from an aircraft traveling at 600 mph would only weigh about 12,500 kilograms.
So the poster's comment about needing 'half the rocket' holds true given what we know about Allen's proposal... Now let's look at the payload claim. Going back to my Rocket Equation Calculator and sure enough - My 12,500 kilogram rocket can loft 1000 kilograms to LEO if I start at 600 mph at a very high altitude, but only 540 kilograms if I start at sea level.
Very cool, and very satisfying to boot... but also very, very rough estimates. The devil's always in the details... and obviously some details are missing from these 'back of the envelope' calculations.
It would be marvelous if sites such as Space.com would add tools to their sites to help their reader's 'fact check' their discussions. The more confidence you have in 'the facts', the more meaningful your conversations can be.
On a related topic, Elon Musk's Why it's so hard to build a reusable rocket.
