Airships and Launch Systems
The use of airships in launching to LEO conjures up images of a "rockoon" or a rocket that is lifted by balloon to a certain altitude before being fired into space.
Although a small rocket can be lifted to heights over 30,000 metres, the method is still of limited application since there is very little if any control over the balloon after it is released from the ground.
Proper airships can be controlled quite accurately - even though large dirigibles are not very manoeuvrable, and could provide a stable platform for the launch of rockets on a precise orbital alignment. However, traditional airships have a very low maximum altitude, typically below 3000 metres, hence they do not provide a significant delta-V advantage compared with launching from the ground. Low flying airships are also prone to damage from bad weather, and are not usable except in conditions even more restricted than for a ground launch.
Recent developments in advanced airship design have have seen great improvements in the possibilities for their use as transport aircraft and as high altitude platforms.
There are now working designs of transport airships which use a combination of aerodynamics and buoyancy to create lift, and which can fly at up to 7000 metres.
These could be used as a launch platform - perhaps with some modification to achieve 10,000 metres altitude, although they would still be within the troposphere and hence prone to significant weather restrictions.
As the launch height of a rocket increases, there are a number of advantages:
- Aerodynamic drag: is reduced as the air density decreases exponentially with altitude.
- Gravity drag factors: are reduced as the craft has less height to gain to make orbital altitude and does not need to spend as much time rising vertically to clear the launch platform and to rise through the denser air of the troposphere.
- Rocket efficiency: increases at higher altitude as it is related to the difference between the combustion chamber pressure and the external atmospheric pressure.
The specific impulse of the rocket will increase exponentially with altitude towards the maximum value which occurs in the vacuum of space.
Also the rocket engine used can be optimised for high altitude flight, which gives an additional efficiency gain.
- Weather factors: for rockets launched from the stratosphere, particularly over 20,000 metres, weather systems are much more predictable and have very little turbulence.
A high altitude air launch also has the advantage of reduced insurance - or even no insurance if the launch is in international airspace over the middle of an ocean. This is not insignificant since launching a commercial booster anywhere near a population centre may require insurance of up to 50 percent of the entire cost of the flight.
An air launch at 10,000 metres could reduce the delta-V budget of an equivalent equatorial launch by as much as 0.5km/s, from 9.3km/s to 8.8km/s, but would still be subject to some of the limitations of a ground launch.
Launched at 25,000 metres in the stratosphere a rocket would save another 0.5km/s from it's delta-V budget and would therefore require only 8.3km/s for an equatorial launch.
It would also benefit from the advantageous weather factors mentioned above.
There are currently plans being developed by airship designers to build high altitude platforms that could support a payload of 50 tons at a height of 50,000 metres in the mesosphere.
From there, flyback boosters could be launched to LEO with very little delta-V budget overhead for an equatorial launch.
Unfortunately, the engineering required for such platforms may not be realisable in the near term.
Even if it were, such platforms would be approximately 4 kilometers in diameter and would not be very manoeuvrable at all.
They would have to drift slowly around the polar latitudes, from where there is an inherent LEO delta-V budget overhead of at least 0.5km/s.
There is also a significant radiation hazard at that location, adding to the logistical problems of servicing and resupplying a platform at such a high altitude.
Even for airships to operate in the lower stratosphere, at more than 20,000 metres, the engineering challenges are substantial with current technology.
The problem is essentially that the lifting capacity of airships decreases proportionally with air density, which decreases exponentially with altitude.
Even at 20,000 metres an airship has to be very large to carry a small payload, and will generally be very light and fragile and difficult to manoeuvre, making low level operations near the ground very difficult, even in perfect weather conditions. It is likely that high altitude platform operations above 20,000 metres will be carried out by craft that are essentially autonomous and which either spend their entire working life at high altitude, or at least operate there for extended periods of up to several years.
High altitude platform craft operating in the lower stratosphere is the goal of several current serious design projects.
Such vehicles may be operating within 5 to 10 years carrying payloads of about 2 tons initially, rising to about 10 tons with the type of craft envisioned for communications and high altitude telescope applications.
For commercially useful rocket launch facilities, a high altitude platform will need to have a much greater net lifting capacity, at least 50 tons. This is possible and could be achieved in the same time frame as the smaller scale projects, if development begins soon. A high altitude platform could be designed with a volume of 1,000,000 cubic metres which for hydrogen would give a total lifting capacity of over 300 tons at 25,000 metres. For a craft designed to spend its entire working life at about that height, flying essentially in a straight line with only slight and gradual changes in heading and altitude, lack of rapid manoeuvrability would not be a drawback.
Such a craft could be in the form of a flying wing with a wingspan of about 1.5km and a rigid centre section of about 150m in length and a few meters in breadth.
This would serve as a launch platform and a landing strip for a wheeled vehicle which would make horizontal take-offs and landings.
If this proves too difficult to achieve, a vertical take off and landing fly-back booster could be used from a circular or hexagonal shaped blast proof launch and landing area.