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My only claim to expertise is playing a lot of Kerbal Space Program, but I'm not sure this is practical. I don't think an air-breathing tow plane could fully re

My only claim to expertise is playing a lot of Kerbal Space Program, but I'm not sure this is practical. I don't think an air-breathing tow plane could fully replace a rocket first stage. Even if it can lift a launch vehicle to the same altitude as a rocket first stage, velocity is the more important component of getting to orbit. A Falcon 9's first stage gets it to 11,000kph, about 3 times the top speed of an SR-71, the fastest jet aircraft.

zelos 11y ago
You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. The Tsiolkovsky equation suggests that's going to be a big saving: http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation And yes, my only qualification is Kerbal as well ;-)
- lmm 11y ago
  > You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. Sure, but the altitude gain is almost "free" - you need to accelerate by something like 6 km/s laterally (number pulled out of somewhere, it's 2.3 in kerbal space program but the planets are smaller there) and 1 km/s vertically (depending on ascent profile, 0 for an instantaneous ascent and infinite for a slow hover upwards, but it ends up something like that), so by Pythagoras it's sqrt-37 = really not a lot more than the 6 kms/s you'd have to spend if you could start at 100km altitude. Intuitively we assume rockets go "upwards" partly because that's the part we see, but actually the gravity turn occurs very early and most of the acceleration is closer to horizontal. E.g. the less-crazy of the space shuttle abort plans if something went wrong early in flight was to land at Shannon.
  - berntb 11y ago
    It is an hour, someone much more knowledgeable than me should have answered? OK, I'll give it a go. Rocket engine nozzle designs are optimized depending on the external pressure. If the rocket engines can start at a higher altitude, it is a nice win. http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheric_use http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheri... Also, it is quite a lot of extra drag at sea level, so starting from higher up is another benefit in that way. It is not obvious if those advantages is larger than the weight of the wings and the weight of robust wheels etc to start horizontally with all that propellant weight. This is an alternative design: http://www.ai.mit.edu/projects/im/magnus/bh/analog.html http://www.ai.mit.edu/projects/im/magnus/bh/analog.html
- pjc50 11y ago
  Another KSP observation: air resistance is much more of an issue at sea level, and it lowers specific impulse.
PaulHoule 11y ago
The original plans for the space shuttle involved the development of two vehicles, the smaller of which might have been partially or completely airbreathing. There have been many conceptual spaceplanes (e.x. Sanger) that work the same way. If you look at the patents, towing is the big difference. Other spaceplane concepts have involved the big plane carrying the little plane, but it is easier to tow another plane than it is to carry it. The "prior art" that hasn't been mentioned yet is the Japanese "Baka Bomb" of WWII which flew as a towed glider until it got near a target, then the pilot would switch on rocket engines for the terminal part of the flight.
ethbro 11y ago
The critical benefit, as I understand it, is that you don't have to carry the mass of an oxidizer with you. Ergo, you "get" non-zero altitude and velocity for much cheaper than an equivalent rocket first stage (including its own oxidizer) would give it to you. And in a towed/mated application, you're not forced to carry the subsequently useless mass of air-breathing engines with you either. Also, the design constraints of air-breathing engines tend to be a lot looser than rocket engines. Which buys you increased reliability.
- teraflop 11y ago
  Not having to carry oxidizer with you is part of it, yeah. But in a vacuum, it's not just reaction energy that's a limiting factor -- it's reaction mass. Once you get into space, the only way you can gain momentum is by ejecting material that you brought with you. But in an atmosphere, you have a free supply of mass to push against. I don't know how to estimate the behavior of a jet engine, but plugging in the numbers for a Cessna 172 shows that the mass of air flowing through the propeller per second is several hundred times higher than the amount of air actually used by the engine.
  - ethbro 11y ago
    I wasn't thinking about it that way. Good perspective! For props, I imagine this would be a fundamental difference, as they're not ejecting mass, no? They're expecting energy to pull themselves through something more fluid-like.
    - teraflop 11y ago
      Not sure if I'm understanding you right, but this isn't really a fundamental difference between piston "prop" engines and turboprop "jet" engines. The mechanical design of the combustion chamber is very different, but in both cases, the combustion produces mechanical energy, which spins a fan, which propels air backwards.
      - ethbro 11y ago
Shivetya 11y ago
however isn't this the idea used behind the Virgin Air space plane? Granted it won't make it to true orbit but that is only a modification of the rocket they are carrying up
Gravityloss 11y ago
This has a few advantages that are not apparent immediately: 1) Rocket engines work a lot better at low ambient pressure. You get more thrust for the same fuel flow. You can fly with a bigger nozzle, increasing this benefit. This is probably bigger than the altitude and speed benefits in subsonic airlaunch. 2) Air launch is flexible since you can fly a long distance to a secluded spot over the ocean and can launch in any direction. You could fly polar flights from Florida. Or low inclination orbits from California. Both of which are impossible for fixed launchers. You can fly towards an orbit's ground track to get flexibility in launch time as well. 3) If you have a problem with the early rocket flight stage, you have enough altitude that gives you more emergency options. For a ground takeoff rocket, the early part of the flight is very dangerous. Reaching orbit is really hard. It is extremely hard with a single stage vehicle. The problem is exponential in nature. The required mass ratio is proportional to e^(required delta vee / exhaust velocity) For a reference design, with 3500 m/s exhaust velocity and mission delta vee of 10 km/s, you'd need a mass ratio of 17. That means the vehicle is 16 parts fuel and 1 part rocket and payload. Helping a single stage with an aerotow to 0.3 bar ambient pressure, 10 km altitude and 300 m/s speed might just help it enough. Say if delta vee is reduced 10% and average exhaust 10% as well, the required mass ratio suddenly drops to 10! To demonstrate what that means: if you could make the ground launched reference rocket plane to fly to orbit with a 3,5 ton empty weight and 0,6 ton payload, (70 ton wet mass), the aerotow version could replace 2,9 tons of propellant with payload, meaning 6 times the useful payload. These calculations are not very accurate, and are very sensitive to the reference design's payload but they give you an idea of the nonlinearity.
- msandford 11y ago
  Also since you're flying in a regular vehicle above the weather, weather related concerns basically disappear. You can build a hanger big enough to house the vehicles so if you have a 30 minute good weather opening you can use that to get from the ground to the air above the clouds. And since you're towing with presumably a big airplane you could probably loiter for a few hours (maybe more) which would mean that the 30 minute good weather window doesn't need to be all that precise. Even less so if you're willing to use a tanker to refuel the tow aircraft. Further, since you're flying -- not stationary -- you have a lot of freedom to shift the launch point a bit if your weather window moves too far forwards or backwards. If it's two hours too early you can take off early and fly east. If it's too late you can take off late and fly west. 6x the payload for the same rocket is pretty darn attractive!

zelos 11y ago

You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. The Tsiolkovsky equation suggests that's going to be a big saving: http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation http://en.wikipedia.org/wiki/Tsiolkovsky_rocket_equation And yes, my only qualification is Kerbal as well ;-)

lmm 11y ago
> You're eliminating the first part of the flight, so the rocket just has to provide the delta-V for orbital insertion, not lift the fuel to do that from sea level. Sure, but the altitude gain is almost "free" - you need to accelerate by something like 6 km/s laterally (number pulled out of somewhere, it's 2.3 in kerbal space program but the planets are smaller there) and 1 km/s vertically (depending on ascent profile, 0 for an instantaneous ascent and infinite for a slow hover upwards, but it ends up something like that), so by Pythagoras it's sqrt-37 = really not a lot more than the 6 kms/s you'd have to spend if you could start at 100km altitude. Intuitively we assume rockets go "upwards" partly because that's the part we see, but actually the gravity turn occurs very early and most of the acceleration is closer to horizontal. E.g. the less-crazy of the space shuttle abort plans if something went wrong early in flight was to land at Shannon.
- berntb 11y ago
  It is an hour, someone much more knowledgeable than me should have answered? OK, I'll give it a go. Rocket engine nozzle designs are optimized depending on the external pressure. If the rocket engines can start at a higher altitude, it is a nice win. http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheric_use http://en.wikipedia.org/wiki/Rocket_engine_nozzle#Atmospheri... Also, it is quite a lot of extra drag at sea level, so starting from higher up is another benefit in that way. It is not obvious if those advantages is larger than the weight of the wings and the weight of robust wheels etc to start horizontally with all that propellant weight. This is an alternative design: http://www.ai.mit.edu/projects/im/magnus/bh/analog.html http://www.ai.mit.edu/projects/im/magnus/bh/analog.html
pjc50 11y ago
Another KSP observation: air resistance is much more of an issue at sea level, and it lowers specific impulse.

PaulHoule 11y ago

The original plans for the space shuttle involved the development of two vehicles, the smaller of which might have been partially or completely airbreathing. There have been many conceptual spaceplanes (e.x. Sanger) that work the same way. If you look at the patents, towing is the big difference. Other spaceplane concepts have involved the big plane carrying the little plane, but it is easier to tow another plane than it is to carry it. The "prior art" that hasn't been mentioned yet is the Japanese "Baka Bomb" of WWII which flew as a towed glider until it got near a target, then the pilot would switch on rocket engines for the terminal part of the flight.

ethbro 11y ago

The critical benefit, as I understand it, is that you don't have to carry the mass of an oxidizer with you. Ergo, you "get" non-zero altitude and velocity for much cheaper than an equivalent rocket first stage (including its own oxidizer) would give it to you. And in a towed/mated application, you're not forced to carry the subsequently useless mass of air-breathing engines with you either. Also, the design constraints of air-breathing engines tend to be a lot looser than rocket engines. Which buys you increased reliability.

teraflop 11y ago
Not having to carry oxidizer with you is part of it, yeah. But in a vacuum, it's not just reaction energy that's a limiting factor -- it's reaction mass. Once you get into space, the only way you can gain momentum is by ejecting material that you brought with you. But in an atmosphere, you have a free supply of mass to push against. I don't know how to estimate the behavior of a jet engine, but plugging in the numbers for a Cessna 172 shows that the mass of air flowing through the propeller per second is several hundred times higher than the amount of air actually used by the engine.
- ethbro 11y ago
  I wasn't thinking about it that way. Good perspective! For props, I imagine this would be a fundamental difference, as they're not ejecting mass, no? They're expecting energy to pull themselves through something more fluid-like.
  - teraflop 11y ago
    Not sure if I'm understanding you right, but this isn't really a fundamental difference between piston "prop" engines and turboprop "jet" engines. The mechanical design of the combustion chamber is very different, but in both cases, the combustion produces mechanical energy, which spins a fan, which propels air backwards.
    - ethbro 11y ago

Shivetya 11y ago

however isn't this the idea used behind the Virgin Air space plane? Granted it won't make it to true orbit but that is only a modification of the rocket they are carrying up

Gravityloss 11y ago

This has a few advantages that are not apparent immediately: 1) Rocket engines work a lot better at low ambient pressure. You get more thrust for the same fuel flow. You can fly with a bigger nozzle, increasing this benefit. This is probably bigger than the altitude and speed benefits in subsonic airlaunch. 2) Air launch is flexible since you can fly a long distance to a secluded spot over the ocean and can launch in any direction. You could fly polar flights from Florida. Or low inclination orbits from California. Both of which are impossible for fixed launchers. You can fly towards an orbit's ground track to get flexibility in launch time as well. 3) If you have a problem with the early rocket flight stage, you have enough altitude that gives you more emergency options. For a ground takeoff rocket, the early part of the flight is very dangerous. Reaching orbit is really hard. It is extremely hard with a single stage vehicle. The problem is exponential in nature. The required mass ratio is proportional to e^(required delta vee / exhaust velocity) For a reference design, with 3500 m/s exhaust velocity and mission delta vee of 10 km/s, you'd need a mass ratio of 17. That means the vehicle is 16 parts fuel and 1 part rocket and payload. Helping a single stage with an aerotow to 0.3 bar ambient pressure, 10 km altitude and 300 m/s speed might just help it enough. Say if delta vee is reduced 10% and average exhaust 10% as well, the required mass ratio suddenly drops to 10! To demonstrate what that means: if you could make the ground launched reference rocket plane to fly to orbit with a 3,5 ton empty weight and 0,6 ton payload, (70 ton wet mass), the aerotow version could replace 2,9 tons of propellant with payload, meaning 6 times the useful payload. These calculations are not very accurate, and are very sensitive to the reference design's payload but they give you an idea of the nonlinearity.

msandford 11y ago
Also since you're flying in a regular vehicle above the weather, weather related concerns basically disappear. You can build a hanger big enough to house the vehicles so if you have a 30 minute good weather opening you can use that to get from the ground to the air above the clouds. And since you're towing with presumably a big airplane you could probably loiter for a few hours (maybe more) which would mean that the 30 minute good weather window doesn't need to be all that precise. Even less so if you're willing to use a tanker to refuel the tow aircraft. Further, since you're flying -- not stationary -- you have a lot of freedom to shift the launch point a bit if your weather window moves too far forwards or backwards. If it's two hours too early you can take off early and fly east. If it's too late you can take off late and fly west. 6x the payload for the same rocket is pretty darn attractive!