Launch Assist Technologies
As we have seen, a completely reusable launch vehicle with a good payload capacity—preferably single-stage-to-orbit—is very difficult to achieve with purely onboard chemical propulsion systems. In this chapter we will evaluate technologies that either act
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Launch Assist Technologies As we have seen, a completely reusable launch vehicle with a good payload capacity - preferably single-stage-to-orbit - is very difficult to achieve with purely onboard chemical propulsion systems. In this chapter we will evaluate technologies that either act externally or use electromagnetic energy to enhance the overall launcher performance.
3.1 Reduction Of Required 41) Referring to the exponential factor determining the payload mass fraction in Tsiolkovski's eq. (1.7), a small decrease of the required d1) to reach orbit will increase payload capability overproportionally. For example, let's assume a standard L0 2ILH2 engine with an Isp of 450 seconds, and a du = 8,000 mls to reach LEO orbit. Then the payload mass fraction would be 16.3% of the total launcher mass. If we now reduce the required velocity increase by only 300 mis, the payload mass fraction would rise to 17.5% which is an increase of about 7%. This can be achieved by several means: (i) Launching from an aircraft with an initial velocity; (ii) Providing an initial boost with a chemical/electromagnetic catapult; (iii) Launching outside the atmosphere on top of an ultra-high tower. 3.1.1 Aircraft ASSisted Launch
Launching from an aircraft produces two advantages: first, an initial velocity and second, if the altitude is high enough, a reduction of the drag losses encountered in the atmosphere. One example is the Pegasus booster rocket from Orbital Sciences launched from the L-lOll aircraft (Fig. 3.1). Typical aircraft performance of a Boeing 747 aircraft is a maximum cruising speed of 0.85 Mach (= 255 mls) and an altitude of 13 km. Although drag reduction at this altitude is not so important, the initial boost when separating from the aircraft is close to our example at the beginning of this chapter. M. Tajmar, Advanced Space Propulsion Systems © Springer-Verlag Wien 2003
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3 Launch Assist Technologies
Fig. 3.1. L·1011 aircraft with Pegasus booster (Courtesy of NASA)
Fig. 3.2. space Shuttle Enterprise on top of Boeing 747 (Courtesy of NASA)
The Pegasus rocket (launch mass 22 t) is very small compared to a real SSTO launcher (e.g., Space Shuttle orbiter weights 104 t, the complete launcher 2000t). Hence launching a big rocket would greatly increase complexity and costs. Also separation would be much more difficult since the launcher must be carried on top of the aircraft and not below as with the small Pegasus (see Fig. 3.2). Supersonic speeds and higher altitudes are also a costly factor. However, for small rockets/payloads, aircraft-assisted launch is an option which is worthwhile studying. 3.1.2 catapults The idea of launching payloads into orbit using a giant gun dates back to the Jules Verne story Voyage from Earth to the Moon in 1885. Not only classical guns, also
3.1 Reduction of Required Ll1J
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gas guns, electromagnetic accelerators or other catapult concepts are in principle capable of bringing payload into orbit-or providing an initial boost velocity to reduce the launcher's Llu requirement. Obviously,
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