Air start temptation

The idea of ​​launching a spacecraft from an air carrier is regularly proposed as a way to radically facilitate humanity’s access to space. However, only one booster uses this principle. About what is advantageous and what difficulties creates an air launch, this post.

A bit of history

Rocket aircraft

The air launch was very successfully used in the USA after the war to study flight at high speeds and altitudes. Bell X-1, where the speed of sound was overcome for the first time in the world, was launched from a suspension on a B-29 bomber:

The decision was very logical - the use of rocket engines meant a small supply of fuel, which would not be enough for a full start from the ground. Model X-1 was developed - X-1A crossed the border in two Machs and investigated the behavior of the aircraft at high altitudes (up to 27 km). Modifications X-1B, C, D, E were used for further research.
The next big step forward was the X-15 rocket plane. He also launched from the air carrier B-52 bomber:

The powerful engine developed a thrust of 250 kilonewtons (71% of the engine thrust rocket Redstone), could reach speeds of 7000 km / h and a height of 80 km. It would seem that the United States has two roads into space - fast and dirty on Mercury capsules, Redstone and Atlas rockets, and longer, but much more beautiful on X-15, X-20 and subsequent projects. However, the "airplane" program was in the shadow of space flights, and, despite successfully achieved goals, did not receive such a brilliant development as the Mercury - Gemini - Apollo line

Neil Armstrong. He flew the X-15, but left the project on time.

Ballistic missiles

An alternative approach was the development of air-launched ballistic missiles. In the late fifties, when ballistic missiles required several hours to prepare for the launch, they lost to strategic bombers in flexibility and response time on combat duty. Bombers could spend hours patrolling around the borders of the enemy’s country, and, after the team, could strike for tens of minutes, or could also be quickly recalled. And ballistic missiles had the crucial advantage of not being able to intercept. The idea of ​​combining the merits of the two systems - the development of a ballistic missile for a strategic bomber. This is how the GAM-87 Skybolt project was born:

The first test launches began in 1961, the first fully successful launch took place on December 19, 1962. However, by this time ballistic missiles for the Polaris submarines, which could "patrol" under water for months, were delivered to the Navy. The US Air Force was developing a Minuteman solid-fuel rocket, whose performance was comparable to Skybolt, but the rocket was in the mine, ready for launch, which was much more convenient. The project has been closed.
On October 24, 1974, the Minuteman III rocket was dropped as an experiment from the cargo compartment of the C-5 transporter:

The test was successful, but the military did not see the need for such a system, and the project was closed.

Soviet "Spiral"

In the USSR, a notable project was one, but extremely interesting:

The system of a hypersonic plane propeller and an orbital aircraft was supposed to start from the runway, gain altitude up to 30 km and speed up to 6M (6700 km / h). Then the orbital plane, together with the accelerating stage on the fuel pair, fluorine / hydrogen disconnected and accelerated independently until it entered orbit. The project was launched in 1964 and officially closed in 1969 (although the orbital plane was "underground" tested as a technology tester of the future Buran). The saddest thing is (why - more on that below) that the scout plane was not built and tested.
I recommend to read more on the site Buran.ru.
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Modernity

Currently, there is one air launch vehicle, two realized projects of suborbital air launch aircraft and a model for testing hypersonic engines. Consider them in more detail:

PH Pegasus

The first launch - 1990, total 42 starts, 3 failures, 2 partial successes (orbit just below the required one), 443 kg into a low orbit. The air carrier is a modified passenger aircraft L-1011 . The separation from the carrier is made at an altitude of 12 kilometers and a speed not higher than 0.95M (1000 km / h).

SpaceShipOne

Air launch suborbital aircraft. It was developed for participation in the Ansari X-Prize competition , made 17 flights in 2003-2004, of which the last three are suborbital space flights to an altitude of about 100 km. Despite optimistic promises “about 3,000 people will be able to fly into space in the next 5 years” the project was actually stopped after winning the X-Prize, and for ten years no space tourists along suborbital trajectories flew.

SpaceShipTwo

Air launch suborbital aircraft. Developed for ten years instead of SpaceShipOne. Currently, test flights are underway, the maximum height reached in February 2014 is 23 km.

X-43, X-51

Unmanned vehicles for testing hypersonic engines.

The X-43 was originally developed as a scale model of the future X-30 spaceplane. He made three flights. The first in June 2001 ended in failure due to errors in the calculations, which led to a loss of stabilization of the acceleration stage. The second, in March 2004, was successful, a speed of 6.83M was achieved. The third flight took place in November 2004, the speed of 9.6M was reached for 12 seconds.

The X-51 was designed for slower (~ 5M), but longer flights. He made four flights - a relatively successful first in May 2010 (200 of the planned 300 seconds for 5M), two unsuccessful, and fully successful (210 seconds for 5M, as planned) in May 2013.

Unrealized projects

There are also unrealized projects: MAKS , HOTOL , Burlak , Vehra , AKS Tupolev-Antonova , Polet , Stratolaunch , S3 .

Air start profitability calculations

The Pegasus PH gives us a very convenient opportunity to determine the degree of profitability of the air launch. The fact is that the Minotaur I PH has, as the third and fourth steps, the second and third steps of the Pegasus, it displays the same payload, but starts from the ground. Comparison of the masses seems to be noticeable in favor of the Pegasus - the air launch missile weighs 23 tons, and the ground launch weighs 36 tons. However, in order to fully compare these launch vehicles, it is necessary to calculate the reserve of the characteristic speed, which is given by the rocket stages. On the material Encyclopedia Astronautica ( data for Pegasus-XL , data for Minotaur I ) were calculated stocks of the characteristic speed of steps for the same payload:

Google Docs Billing Document
The result was very curious - due to the air start, 12.6 percent of the characteristic speed is saved. On the one hand, this is quite a noticeable benefit. On the other hand, it is not so much to cause the explosive growth of air start systems.
Pay attention to the hypothetical comparison with the "Spiral". If Pegas were standing on the Spiral plane, then the separation would occur at a speed of ~ 1800 m / s and an altitude of 30 km, which would save at least 2000 m / s of the characteristic speed. By the same principle there is a comparison with the "Minotaur". Notice how the benefit has increased. From this it follows that the benefit of the air start is most determined by the carrier — the greater the speed and height of the separation, the higher the gain.

General reasoning about the advantages and disadvantages of the air start

Virtues

Reduction of gravitational losses . The greater the initial velocity, the smaller the initial pitch angle of the rocket. Gravitational losses are considered as integral of the function of the pitch angle, therefore, the smaller the pitch to the horizon, the smaller the loss.

Model plot of pitch angle. The area of ​​the curved trapezoid (painted in red) is the gravitational loss.

Reduction of aerodynamic drag losses . Pressure decreases with height exponentially:

At an altitude of 12 km, where the Pegas starts, the pressure is about 5 times less than at sea level (~ 200 millibar). At an altitude of 30 km - already a hundred times less (~ 10 millibar).

Reduced back pressure loss . The rocket engine works more effectively in a vacuum, where there is no external pressure that prevents the expansion and rejection of fuel. The UI of one engine on the surface is less than in a vacuum, so starting in a discharged atmosphere will reduce back pressure losses.

Jet engine has a higher specific impulse . Since the oxidizer is taken “for free” from the ambient air, it does not need to be carried with you, which increases the specific impulse of the system at the expense of the aircraft carrier.

The ability to use existing infrastructure . The air launch system can use existing aerodromes without the need for launch facilities. But systems for preparing for the launch (assembly and testing complex, fuel component warehouses, flight control building) are still needed.

The ability to start with the desired latitude . If the aircraft carrier has a significant range, you can start from a lower latitude to increase the carrying capacity or move to the desired latitude to create the desired orbit inclination.

disadvantages

Very poor scalability . The rocket that brings 443 kg to the LEO weighs a comfortable 23 tons, which can be attached / hung / put on the plane without any special problems. However, rockets that put at least 2 tons into orbit are beginning to weigh already 100–200 tons, which is close to the carrying capacity of existing aircraft: the An-124 lifts 120 tons, the An-225 - 247 tons, but it is in a single copy, and the new ones airplanes are virtually impossible to build. Boeing 747-8F - 140 tons, Lockheed C-5 - 122 tons, Airbus A380F - 148 tons. For heavier missiles, you need to develop new airplanes that will be expensive, complex and monstrous (as on the KDPV).

Liquid fuels will require media modification . Cryogenic components will evaporate over a long time of take-off and climb, so you need to have on the carrier a supply of components. Especially bad with liquid hydrogen, it evaporates very actively, you will need to carry a large stock.

Structural strength problems of the payload and the launch vehicle . In the West, satellites are often developed with the requirement to withstand only axial overloads, and even horizontal assembly (when the satellite lies "on its side") is unacceptable for them. For example, at the Kourou cosmodrome, the Soyuz launch vehicle is transported horizontally without a payload, put into the launch structure and attach the payload already there. As for the aircraft carrier, even takeoff will create a combined axial / lateral overload. I'm not talking about the fact that in an unstable atmosphere so-called. "Air pockets" can seriously shake the complex. Carrier rockets also did not pay for flights “on the side” in the filled state; for sure, not one existing oil-fuel launcher can be simply loaded into the cargo hatch and thrown into the stream for launch. It will be necessary to make new rockets, more durable - and this is overweight and loss of efficiency.

The need to develop powerful hypersonic engines . Since efficient media is fast media, conventional turbojet engines are poorly suited. L-1011 gives only 4% of the height and 3% of the speed for the Pegasus. But the new powerful hypersonic engines are on the verge of the current science, such have not done. Therefore, they will be expensive and will require a lot of time and money to develop.

Conclusion

Aerospace systems can be a very effective means of delivering cargo to orbit. But only if these loads are small (probably not more than five tons, if predicted, taking into account progress,), and the carrier - hypersonic. Attempts to create flying An-225 dual-type monsters with twenty-four engines or some other super-heavy example of the victory of technology over common sense are a dead end at the current level of our knowledge.

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