The greatest asset of the advanced spaceplane will be its engines. They will be far more versatile and powerful than anything before, using elements of the turbojet and aerospike rocket in their designs. These attributes will make them, and the spaceplanes they power, very reliable and efficient. As we have seen, engine and vehicle reliability is what is required to make spaceflight routine and safe. You have already read about these engines in the last chapter; so we will not say anything more about them here.
Beyond the value of its engines, the advanced spaceplane will use advanced flight and fuel management (FFM) techniques. Flight and fuel management refer to the proper utilization of the atmosphere for lift, oxygen, and working mass, as well as choosing the optimal path and velocity profile through the atmosphere. By keeping the ascent to orbit as efficient as possible, the least amount of fuel - typically liquid hydrogen - will be needed to reach space velocity. Let us now look at each aspect of FFM to see what information we can glean with current knowledge.
The first and most obvious element of FFM is aerodynamic lift. Spaceplanes will use the atmosphere to assist them in ascending to space height. This is fairly straightforward, and the best understood aspect of spaceplane operations. Airplanes use the atmosphere in this way all the time, although every airplane has a ceiling beyond which it cannot climb. Winging its way up to around 50,000 ft, the advanced spaceplane can remain subsonic, flying just like a commercial airliner. An excellent subsonic lift-to-drag ratio is employed to minimize drag and fuel consumption during this stage of the flight. In this way, aerodynamic lift provides much of the altitude that the spaceplane craves, all for the price of a modest amount of fuel.
The next element is oxygen, required for the operation of any engine. Since the atmosphere is almost one-quarter oxygen by mass, running the advanced engines in the atmosphere is a convenient way to avoid having to carry that oxygen along inside the ship. From the standpoint of engine operation alone, the spaceplane should be able to stay in the atmosphere a long time.
The next item on the FFM list is working mass, which the atmosphere also has in unlimited abundance. This working mass is the sum total of the atmosphere, composed mainly of inert nitrogen, but also trace elements, in addition to the oxygen that we have already considered. What is so important about this working mass? Well, it serves as the propellant, the stuff that propels the machine forward, as long as the flying machine remains in the atmosphere. Like the oxygen for the engine, the working mass does not have to be carried onboard as long as the vehicle is flying through it.
The turbine in a typical jet engine powers a compressor that acts like a huge vacuum cleaner, sucking in air, compressing it, and eventually shooting it out the rear as a jet of continuous thrust. The turbine uses fuel (typically a kerosene derivative) and atmospheric oxygen while the compressor utilizes the working fluid of the surrounding air mass. Again, from the standpoint of working mass, it is expedient for the spaceplane to remain in the atmosphere as long as desired.
At some point, the advanced spaceplane must accelerate to orbital speed. But increasing speed also increases drag, the arch-nemesis of all aircraft. Supersonic speeds also result in severe heating, another serious barrier to overcome. At some point, therefore, the spaceplane must exit the atmosphere to remain flight efficient, and this will probably occur long before reaching orbital speed.
An optimum velocity profile during ascent to orbit involves deciding how fast to fly at every altitude, and at what angle to climb, in order to minimize onboard pro-pellant usage. Subsonic cruise-climb is used during the first part of the flight, because this achieves about 10% of orbit insertion altitude and minimizes drag both during this stage and the later zoom to orbit. This ascent profile is similar to that of a modern airliner or strategic bomber, and takes the spaceplane from the runway up to some 50,000 ft. At this point, still climbing, the spaceplane applies full thrust, accelerates to supersonic speeds, and pitches up to around a 40-degree angle of attack. The air-breathing turborocket remains effective until speeds reach the low hypersonic region - Mach 6 or 7. The pilot now switches off the helium loop; the compressor shuts down, and the spaceplane goes to pure rocket thrust. By now, it has gone completely ballistic, is leaving the sensible atmosphere, and has only its onboard propellants to reach orbital speed. The spaceplane pitches over to more nearly horizontal, settles into the proper angle of attack to counter remaining gravity losses, and aims for its orbital insertion point downrange.
By using subsonic lift up to 50,000 ft, and then employing a 40-degree angle of climb thereafter, gravity losses are kept to a minimum, because aerodynamic lift is used to the maximum practical extent. At the same time, atmospheric drag and ther-modynamic loads are kept at bay by climbing subsonically in the lower atmosphere, and by using a greater rate of climb during the supersonic zoom to orbit. In other words, by using this optimum velocity profile, both gravity and drag losses can be kept low, while making full use of the atmosphere for lift, oxygen, and working mass.
Another aspect of FFM involves refueling, which actually refers to aerial propel-lant transfer or aerial propellant tanking (APT). By pulling up behind a tanker aircraft, a spaceplane can take on its load of oxidizer-propellant at altitude rather than taking off from a runway fully loaded (Fig. 9.17). An oxidizer is typically the heaviest item onboard any space launch vehicle, and so departing the ground without it would have obvious benefits. The oxidizer to be transferred could be liquid oxygen (LO2) or it could be hydrogen peroxide (H2O2), which looks and handles much like water, although it is highly sensitive to impurities. Alternatively, the spaceplane might generate onboard oxygen during subsonic flight by liquefying ambient air, separating out the oxygen component, and storing it internally. This last method involves much longer subsonic flight times and requires onboard lightweight air-liquefaction equipment. All of these are possible technologies for the advanced spaceplane.
Lightweight composite materials will make up much of the advanced space-plane's structure. It is vitally important that the design be both lightweight and very strong, in order to achieve the necessary mass ratio for single-stage-to-orbit flight. One element of the design will combine a high strength aeroshell with the thermal protection system. By using a double-walled honeycomb structure, the spaceplanes
of the future will have highly reliable thermal protection systems and strong reentry bodies, both vital for Earth orbital and Lunar return reentries. This design was first considered for use on the X-33 testbed, before its ignominious cancellation.
For wingless rockets, the mass ratio required for SSTO spaceflight is about 8.5, using high-energy propellants and altitude-compensated rocket engines. This means that no more than 12% of the vehicle's gross lift-off weight can be composed of structure, engines, crew, and cargo. The other 88% must be propellants only. Applying the rocket equation, and assuming a specific impulse of 435 s, we can calculate the following result.
Ay = (435 s) (32.174 ft/s2) ln (8.5) Ay = 29,950 ft/s
This result is equal to 5.67 miles/s, or 9.13 km/s, significantly higher than low Earth orbital velocity. When drag and gravity losses are taken into account, we find that this is about right. This calculation is based on pure rocketry, with no FFM, lifting optimum velocity profile, or APT involved. When all the tricks of the trade of the advanced spaceplane are used, it will be possible to reduce this mass ratio significantly, thereby increasing the payload fraction to practical levels. We saw an example of this in the last chapter.
Advanced spaceplane infrastructure will involve ground infrastructure in the form of spaceports, aerial infrastructure in the form of airborne tankers, and space infrastructure in the form of orbiting propellant depots, space stations, and Lunar bases. This infrastructure is already well on the road to developing, since existing airports will become future spaceports. Aerial tankers already exist as well, and in-flight refueling procedures are well-practiced and routine. With the establishment of the first low Earth orbital "gas stations," together with the first spaceplane "customers," spaceflight activity will rapidly increase.
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