Could air-launched space planes reach escape velocities?

Question

Rockets like the Ariane 5 weigh hundreds of tons, but with about 85% of that weight being fuel, the payload fraction is only about 3% (~10-20 tons).

Virgin Galactic is building suborbital space planes, mostly for tourism purposes. They fly at about Mach 4, too slow to escape earth.

Now, I wonder if an air-launched spacecraft in the next 20 years realistically could really fly us to the moon - that is, could they reach escape velocities?

As side question in case they can: Would they be more or less fuel efficient than standard rockets such as the Saturn V? How much payload would be realistically being transportable?

Answer 1

Believe it or not, we could have done this 50 years ago, if government funding hadn't been pulled from a project at the last minute. Frustratingly, after years of work by scientists, engineers and technicians, the Boeing X-20 Dyna-Soar project was cancelled just after work had started on the actual spacecraft.

Here's an artist's impression of the X-20:

The X-20 was the result of a military program that aimed to develop an orbital spaceplane to be used for bombing and reconnaissance. It was designed to be launched into orbit and to stay there for a short while. Despite its small size - only 35 feet long - it would reach orbital speeds after launch, in theory. It managed to get to Mach 18 during practice glide tests.

The X-20 was not designed to be air launched, but to be launched on top of a Titan III missile. However, a similar design - a precursor to the X-20, if you will - called Bomi was designed to be launched like this. Here$^1$ is a comparison of Bomi (on the left), the X-20 (the two on the far right), and Robo, a related project:

_{(source: astronautix.com)}

There were two versions of Bomi: a suborbital one, with a maximum speed of Mach 4, and an orbital one, with a maximum speed of - well, orbital velocity. The latter is probably the one you're interested in. It would have been 23 feet long and would have had a payload of 34,000 kilograms - enough for two nuclear bombs.

Both versions would have been launched on some sort of launcher - the larger vehicle which Bomi is shown attached to. This design might also be changed depending on whether the flight was to be orbital or sub-orbital.

Bomi was eventually cancelled as funding was pulled for Dyna-Soar (the X-20), which then suffered the same fate. But Dyna-Soar got past the glide-test stage (being dropped from a B-52), and almost actually made it to space. Had the resources been moved to Bomi, it could have succeeded.

Could Bomi have escaped Earth orbit? With a bit of work, it could have. Think of how various rocket families have evolved. Different types can fulfill different missions. The Saturn V was the end result of smaller, suborbital and orbital rockets. If Bomi had been developed to the extent of the Apollo program, I think it's very likely that it could have made it out of Earth orbit.

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$^1$ This image appears to be in the public domain, as stated here.

Answer 2

Now, I wonder if an air-launched spacecraft in the next 20 years realistically could really fly us to the moon - that is, could they reach escape velocities?

• Air launch to LEO: Done now

• Air launch to lunar orbit - yes, but at 20%-25% of LEO payload

• Air launch to Moon and back to LEO: Yes, but with about 5% of LEO payload

• It is easy to overlook some practical realities when enthusing over paper-based systems.
  Ratio of air-launched vehicle mass to winged return-to-base Mothership mass must not be overlooked. The Mothership size sets an upper limit to the space-vehicle mass. Increases above heavy lift aircraft payload masses may be possible with eg balloons, but this calls for some extremely specialised systems. Looking at the figures below it looks like manned lunar return to earth's surface is an unrealistically high expectation for air launched systems. Small unmanned craft to lunar orbit are practical.

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The answer is "yes, obviously" as you can build a small-er lunar launcher than is usually used and you can conceivably build a means to air-launch it. eg Balloon launch can allow very substantial mass and has been proposed in various studies.

The existence proof of the general concept comes in the form of the several "Orbital Sciences Corporation" air launched orbital vehicles. These are only used for LEO (low earth orbit) insertion but escape velocity would be achievable given a suitably small payload.

The material below gives examples of what could be realistically achieved based on existing small air-launched LEO satellite launchers and the as it was then 2013 proposal from Orbital Sciences, Burt Rutan and Paul Allen.

This demonstrates that a not insignificant air launch could deliver about 800 to 1000 pounds to lunar orbit - more with utter leading edge fuels and systems or even larger 'motherships'. This is uncomfortably smaller than what you realistically want to deliver one person to lunar orbit and back. While scaling is possible it's not looking attractive for multi-person lunar return flights.

The advantages of aerial launch is not altitude gain as such but the significant gain in reduced air resistance, and the small gain in velocity. While the air launch velocity is a minor fraction of orbital velocity, a ground based launcher must add the initial velocity while supporting the maximum mass against gravity. This is minor compared to air resistance losses, but useful. Air resistance halves about every 15,000 feet, and drag is inversely related to air density. And drag is proportional to velocity squared - so if you can start slower and higher it can help significantly. You will ultimately need very substantial "horizontal" velocity to orbit, but initially, getting up out of the thick lower atmosphere with minimal losses is extremely important. The "mothership" has wings and air breathing engines and fuel is cheap compared to the cost of carrying it to high altitude and high velocities, so an air launched system provides gains in launch vehicle costs and capabilities in situations where it is reasonably possible to build a large enough "mothership". For small LEO payloads it's eminently viable (and used), for very small one way lunar payloads its doable, but for lunar return, the mothership logistics start to become formidable.

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Here is a video of the air launch of an XL Systems "Pegasus". This shows the action from just before launch until stage 1 burnout.

The "next stage" of this capability as of May 2013 is shown here.
Stratolaunch and Orbital – The Height of Air Launch . How this has been modified by more recent events I know not but this showed what was being planned in 2013 so is pertinent to your question.

This launcher proposed a 13,500 pound payload to LEO.
That's not vast - but definitely provides useful payload

The assignment of relative delta V's and fuel requirements to missions is too complex to allow simplistic answers that cover more than specific examples, but as a really really rough indication, the "delta-V" from LEO to lunar orbit is very approximately 40% of that required to reach LEO from the earth's surface. The table below provides velocity changes needed for various orbital and location transitions. This gives 3.9 km/s as the delta V needed from LEO to lunar orbit.

The basic formula for calculating velocity change for a rocket is the (not surprisngly) "rocket equation: -

• V = Isp x g x ln(M2/M1)

  Isp = specific impulse of fuel
  M2 = start mass
  M1 = end mass g = gravitational constant (~~= 10 m/s/s)

Call M2/M1 = mass ratio = MR.

Using a modest by modern standards Isp of 300, to produce a delta-V of say 4000 m/S requires a MR of about 3.7 or end mass ~= 1/3.7 = 27% of total.
So ABOUT 25% of the above 13,500 pounds could be delivered to lunar orbit
=~ 3375 pounds = 1.5 tons
~= 1.5 tonnes :-)

This in turn could return about 840 pounds to LEO and a rather lesser amount back to earth. The table below is from this Delft university page

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Related:

Pegasus launcher pictures with links

OSC Pegasus - 44 launches since 1990.

Pegasus XL - 443 kg to LEO so ABOUT 100 kg to lunar orbit.

NASA Pegasus mission 2014

OSC Facebook page

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Inner system delta V chart

From **Wikipedia - Delta-v budget
and also used in this stack exchange post

Answer 3

Start your mental model by assuming a rocket flight path. A velocity/altitude vs time chart for the Space Shuttle:

_{(source: aerospaceweb.org)}

Jet engines have better $Isp$ than rockets. Let's put jet engines on our rocket. The Falcon 9 puts out about 1.1m lbs of thrust, so we can use a GE-90 to add 120,000 lb, doubling the acceleration at sea level. Elon Musk said that the Falcon 9 costs about \$54m per entire rocket. The GE-90 costs about $24m. Oops. We added 50% cost to the system (not including integration or developing a recovery system) and the thrust drops off rapidly with altitude.

Let's use an F-414 instead. It costs about \$4m and can be useful up to about Mach 2 with a properly designed inlet, and the speed really helps us develop ram pressure, which feeds the afterburner ramjet style. We get 26,000lbs of thrust for only \$4m and longer burn, better but not stellar. The rocket we're lifting still has to be gigantic, so we're not that well off yet.

Pure ramjets present the dilemma of dead weight at liftoff, adding more rocket at the slowest phase of acceleration, so maybe we can't win there either. Ramjets only overtake rockets in $Isp$ at about Mach 0.5 and can't generate full thrust for a while because they will blow air out the front if they add too much fuel until ram pressure is high enough.

So.. air-breathing engines don't generate a ton of thrust per dollar and have a low speed range. Wings lift at a rate of about 16:1, so we can use our engines to accelerate slowly and fly to 40,000ft and Mach 1. This won't save a ton of rocket weight because it's at about 1/25th of it's final velocity and one minute of drag. Let's say we cut weight 20% and only need to carry 900,000lb.

A 747-8 carries 308,000lb of cargo and costs about \$350,000,000. Let's say that costs and cargo scale linearly, we're at least looking at a \$700,000,000 launcher, a far cry from \$54m, amortized over the number of launches, but so is the development cost, which for the 747-8 was \$3.7bn. Again, scaling linearly, we need about \$8bn to spread over a lot of launches. SpaceX recently raised \$1bn from Google and Fido, not quite enough.

There lies the dilemma of launching payloads with air-breathing aircraft. Either you need a vastly cheaper, higher thrust-to-weight jet engine that develops thrust at zero velocity, or you go back to rockets and recovery techniques like ULA and SpaceX are developing.

Many have tried to assume longer air-breathing flight paths at ever increasing speeds, but you start using scramjets, pre-coolers, heat management, and it never seems to get smaller, perform over a large enough envelope, or reach a high enough speed to matter for the eventual rocket anyway.