Perhaps the most striking thing about launching spacecraft using conventional ELVs is how inefficient and costly it is. Typically only about 1% of the mass that sits initially on the launch pad reaches orbit and is usefully employed to fulfill the mission objective. The remaining 99% is jettisoned either on the ascent or in orbit. What can be done about that? Well, a great deal. Rocket scientists want to develop a launch vehicle with operating characteristics similar to a civil aircraft: a launcher that takes off from a conventional runway, delivers a payload to orbit, and returns to a runway landing without jettisoning big lumps of itself on the way. If this can be achieved, then the cost of access to orbit would be significantly reduced, which would accelerate the exploitation of space in the existing areas of application satellites and scientific research. If this revolution can be achieved in a way that increases the reliability of launchers to match civil
airplanes, the exploitation of space as a potential holiday destination also becomes a reality.
However, the principal obstacles to achieving this vision of the future are the technical challenges that it poses, which go hand-in-hand with the large costs that would be incurred in the development of such a new generation of launch vehicles.
What are the technical challenges? The sort of launcher we are envisioning is referred to as a single-stage-to-orbit (SSTO) vehicle, and if you do the calculations to see if we can reach orbit with this type of vehicle, you find that it is just beyond our reach in terms of our current rocket technology. It almost feels like God created the planet, in terms of size and gravity field, so that it would be just that little bit too difficult to reach orbit with a SSTO vehicle using our current level of launcher technology. However, this paranoiac notion is tempered by the fact that this situation only applies now—to our capability at the turn of the 21st century. As time goes on, technology will develop, and it seems likely that the SSTO launcher will fly sometime in the next few decades, probably under the banner of a military research program.
What do we need to do to inject a useful payload mass into orbit using a SSTO vehicle? There are a number of approaches to the problem that can be taken, and a successful solution would probably combine all of them. The principal measures that can be taken include the following:
• An improvement in vehicle structural efficiency
• The enhancement of the process of optimization of the ascent trajectory to reduce the level of gravity and drag losses incurred
• The redesign and improvement of the performance of launcher's propulsion system
We discuss these measures in terms of the level of technical challenge they pose.
Structural efficiency: A conventional launcher typically carries hundreds of metric tonnes of liquid propellant and oxidizer, contained within large tanks distributed throughout the various stages of the vehicle. The safe containment of this mass of liquid in a harsh launch environment (of high acceleration and vibration levels) is a challenge to the structural designer of the launcher. The crux of this challenge is to manage this safe containment with the minimum of structural mass, so as not to compromise the launcher's ability to inject a good payload mass into orbit. A measure of the structural efficiency of a launcher is the ratio of the mass of its structure to the mass of fuel on board. Currently values of this ratio are on the order of 0.1, which means that for every 100 metric tonnes of fuel on board, we need typically 10 metric tonnes of structure to safely carry it. The challenge for structural designers and material scientists is to reduce the value of this ratio—equivalent to improving structural efficiency—in order that the mass saving in launcher structure can be invested in increasing the mass of the spacecraft payload.
Trajectory enhancement: The process of optimization of the ascent trajectory would probably mean adopting a horizontal runway takeoff and a fairly gentle climb to orbit (to reduce gravity loss). The vehicle would then spend more time in the lower, denser part of the atmosphere, potentially causing an increase in drag loss. To overcome this, significant technical effort would have to be invested in the optimization of the shape of the launcher to make it more aerodynamic over the prescribed flight path. The objective would be to ensure that overall losses are reduced
Propulsion system: The improvement of the performance of the launcher's propulsion system is probably the most difficult technical challenge in the development of a SSTO vehicle. With a conventional launcher system, the oxygen needed to burn the propellant in the rocket engines is carried, usually in liquid form, in large massive tanks. The overall performance of the launch vehicle can be enhanced significantly if we can devise a way of reducing this mass by extracting the required oxygen from the atmosphere. This is, after all, how a conventional aircraft operates, burning its fuel using the oxygen coming in through the engine intakes. This type of propulsion is referred to as air-breathing. There are some difficulties here, though, perhaps the most obvious one being that as the launcher climbs to near-orbital altitudes there is no usable atmosphere left from which to extract the oxygen. Another issue is perhaps a little more subtle, and has to do with operating air-breathing propulsion systems at high speeds. To understand this, we need to think about fast aircraft and the jet engines they use to sustain high-speed flight. Usually people use the word supersonic to suggest high speed, but this actually means that the aircraft is traveling at a speed in excess of the speed of sound. At sea level, the speed of sound is around 340 m/sec (1115 feet/sec), which is about 760 mph. An airplane traveling at this speed does seem fast, but it is actually traveling quite slowly compared to a launch vehicle, which needs to reach speeds of 8 km/sec (5 miles/sec) to reach a LEO, which is of the order of 18,000 mph.
Another way of addressing how fast an aircraft travels is to compare its speed with the local speed of sound using a Mach number. An aircraft moving at the speed of sound is said to be traveling at Mach 1, and one moving at, say, three times the sound speed, at Mach 3. Thinking in this way, ramjet-powered airplanes can reach speeds up to about Mach 5, which is a quite impressive 3500 to 4000 mph. At such high speeds, the air is rammed into the intake, which compresses the air sufficiently to be able to dispense with much of the mechanical complexity that you normally find in jet engines. Consequently, the ramjet is a relatively simple device—effectively a tube with an intake at one end, an exhaust nozzle at the other, and a combustion section in the middle. The inflowing air is mixed with fuel (for example, kerosene) and ignited. The pressure produced by the high-speed flow into the intake compresses the air and fuel mixture, and effectively directs the explosively ignited gas out of the exhaust nozzle to produce thrust.
One critical attribute of the ramjet is that the intake air flow has to be managed to reduce its speed to a subsonic level in the combustor, in order to ensure that combustion takes place. If it were otherwise, the air-fuel mixture would not be there long enough for the burning of the fuel to take place. The reason why this is critical is that the process of slowing the incoming air actually produces a kind of drag force on the engine, which is why the maximum operating speed of a ramjet is limited to about Mach 5. Given that we require higher speed operation for an air-breathing launcher propulsion system, this factor seems to be a bit of a problem.
To attempt to overcome it, an air-breathing propulsion system called a supersonic combustion ramjet, also known as a scramjet for short, has been proposed to potentially increase flight speeds up to around Mach 15. As the name implies, the scramjet is similar to the ramjet, but combustion takes place in the air-fuel mix while it is flowing at supersonic speeds within the engine. As a result, some of the limitations of the ramjet are overcome, but further technical challenges are posed, not the least of which is the problem implied above about sustaining the engine combustion in such a high-speed flow. Another issue with a scramjet-powered launcher is that the entire underside shape of such a vehicle needs to be designed and optimized as part of the propulsion system. The underside forebody becomes part of the engine intake system, ensuring high-speed flow into the engine, and the underside aft section becomes part of the engine exhaust jet designed to maximize the resulting thrust.
As if all this wasn't difficult enough, another challenge posed by operating at such high speeds in the atmosphere is the heat generated in the launcher's structure caused by atmospheric friction. The vehicle's forebody and the leading edges of the wings will reach very high temperatures, leading to a requirement to develop appropriate cooling techniques and materials, so that the vehicle does not fall apart due to this extreme heating.
Recently, experimental flight test programs have been established, by both civilian and military agencies, to attempt to demonstrate high-speed flight using scramjet propulsion, and to look at the challenges of the hypersonic
aerodynamic design of such vehicles. Figure 5.10 shows an artist's impression of a scramjet-powered vehicle in flight, in this case from NASA's X-43 experimental aircraft program. At the time of this writing, however, these programs have had limited success, demonstrating scramjet-powered flight up to speeds of around Mach 10, with powered flight sustained for only a short period, of the order of tens of seconds.
Regarding the fully reusable, single-stage-to-orbit launch vehicle with aircraft-like operations, we can now see that the vehicle propulsion has to operate in a variety of different ways in order to accommodate the ground-to-orbit flight envelope. The takeoff from an airport runway would require the use of conventional jet engines, to take the speed to about Mach 2 or 3 when ramjet operation becomes effective. At around Mach 5, the engines would need to switch operation to scramjets, taking the launcher to around Mach 15 and to an altitude where the air is too thin for continued air-breathing operation. The last boost to orbit speed and height would require the engines to operate as rockets. To achieve this combined-cycle operation for the propulsion system, while limiting the overall mass of the engines, poses difficult technical challenges—so much so that many rocket scientists have expressed doubts that a single-stage-to-orbit manned vehicle, taking off from a conventional runway, will ever be achievable. But then perhaps this is an overly pessimistic view. After all, commercial air transport was a similar pipe dream a century ago.
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