Ramjets have no moving parts. They are essentially hollow tubes containing an inlet-diffuser, combustion chamber with fuel injectors, and exit nozzle. They produce zero static thrust, and so a ramjet-equipped vehicle cannot taxi or take off
Fig. 8.2 Ramjet missile showing simple tubelike construction (courtesy NASA)
under its own power. Therefore some means must be found to accelerate the vehicle to its operating speed before the ramjet can be started, because it is the forward velocity that provides the ram air (Fig. 8.2).
A typical ramjet is designed specifically for subsonic, low supersonic, or high supersonic operation, up to Mach 5. These three designs are exclusive, and do not typically overlap. They differ mainly in the way the inlet-diffuser and nozzle areas are constructed, which depends on vehicle speed. As ram air enters the inlet, it passes through a diffuser section, which slows down and compresses the air, creating a subsonic pressure barrier. The airflow inside the ramjet is always subsonic, even if the vehicle itself is flying at supersonic speeds. High-pressure fuel is injected and ignited in the combustion chamber, and the resulting hot exhaust gases are accelerated through a nozzle. The ram air pressure barrier ensures that the exhaust escapes in the correct direction. A flame holder, situated just forward of the nozzle, stabilizes combustion. The main advantage of ramjets is that they are simple, reliable, and fast. More important, they do not require an onboard supply of oxidizer, typically the heaviest item in any space launch vehicle. The main disadvantage is that the faster the operating speed of the ramjet, the more it must be accelerated by some independent means before it can be started (Fig. 8.3).2
Scramjets, despite their name, can never scramble off a runway under their own power, for they suffer from the same basic limitations as ramjets do. They are, in fact, supersonic combustion ramjets, in which the incoming airflow remains supersonic as it passes through the combustion chamber. This allows the vehicle to fly at much greater speeds, up to Mach 15, while using air as an oxidizer and a propellant. As with ramjets, scramjets have to be accelerated to some specific speed - about Mach 5 in this case - before they will begin to operate. Neither ramjets nor scramjets are very good at accelerating themselves, because of a relatively low
thrust-to-weight ratio combined with large drag forces at their high speeds of operation. Pure rockets have thrust-to-weight ratios of around 60, while those of ramjets and scramjets have values of 2 or 3.3 This is important, because it means rockets are much better at accelerating a vehicle from the ground to spaceflight speeds - from Mach 0 to Mach 25 - than are ramjets or scramjets. Also, ramjets work only in Earth's atmosphere, whereas rockets work anywhere - in atmospheres with no oxygen as well as in regions with no air.
Do subsonic or supersonic combustion ramjets have a future in advanced spaceplanes? As with other concepts, the idea is to get into orbit as efficiently as possible. This means reducing both drag and gravity losses as much as possible. Ballistic rockets minimize drag by launching vertically and punching through the thickest layers of the atmosphere as quickly as possible. They minimize gravity losses by accelerating quickly to orbital velocity before gravitational forces can erode the vehicle's trajectory. The Space Shuttle takes only 8'/2 min to reach orbit from a standstill on the launchpad. Its trajectory is specifically tailored to minimize drag by launching vertically (Fig. 6.2) and momentarily throttling down the main engines at "max q" - maximum dynamic pressure. At the same time, it minimizes gravity losses by getting into a desired orbit as quickly as practical and before its propellants run out. If the Shuttle were to keep its liquid-propellant engines throttled down for much of the flight, they would burn for a longer period, but gravity would pull the Shuttle into a lower orbit during the burn. The difference between the orbit it was aiming for and the orbit it ended up in would be the gravity loss.
If that orbit were inside the atmosphere it would quickly decay, and the Shuttle would return to Earth sooner than expected.
How would scramjet-powered vehicles minimize drag and gravity losses? The drag on supersonic combustion ramjets is enormous, which is one reason they do not accelerate as quickly as pure rockets. This drawback is offset by the fact that ramjets do not need to carry their own oxidizers. Furthermore, they use the atmosphere as the main propellant. Air is 23.1% oxygen by mass, with the balance being made up of the inert gas nitrogen and trace amounts of carbon dioxide, water vapor, argon, etc. The bulk of the atmosphere is nitrogen, which serves as some 75% of ramjet propellant, even though it does not contribute one bit to the combustion process. This is a trick up the sleeve of the ramjet: the propulsive potential of the nitrogen-rich atmosphere. So this trick somewhat offsets drag losses, since the ramjet vehicle needs to carry fuel only. Gravity losses are much less of a problem, because ramjets can be integrated with lifting bodies, and aerodynamic lift inherently cancels gravity. As long as such a vehicle can generate lift, gravity losses vanish completely. The price for this lift, of course, is drag, as in any aircraft. These two factors are inseparable, and hypersonic drag is a factor that cannot be ignored. Drag goes up exponentially with velocity, although it declines somewhat with air density. But velocity trumps air density in this case, and even at high altitudes, drag causes reduced performance and severe heating.
Scramjets are specifically designed to operate in the upper atmosphere, at an altitude of 100,000 ft or above. If they run out of air, they cease to operate. And if they have not reached a certain speed, they would not operate at all. These are the greatest drawbacks. Their greatest advantage is that they can use the atmosphere as an oxygen source and as a propellant at hypersonic speeds, and provide lift for the vehicle while doing so. But this opens up a whole new range of problems associated with drag, excessive heat loads, and the inevitable search for new materials and designs that can alleviate those concerns. At this point, scramjets lose their simplicity, because complex methods and materials must be used to keep the vehicle from melting. These measures include using advanced materials and often use active cooling methods, such as circulating cryogenic propellants through the vehicle's skin.
For all its merits, the air-breathing scramjet engine operates best at a constant high speed and high altitude inside the atmosphere. It may well have a future in advanced hypersonic airliners, but its future in spaceplanes is still open to debate. If ramjets are used in future space vehicle designs, the flight into space would consist of four phases: (1) an initial acceleration by nonramjet booster to ramjet speed, (2) ramjet acceleration to scramjet speed, (3) scramjet acceleration to Mach 15 at the top of the atmosphere, and (4) rocket acceleration to orbital velocity. This degree of operational complexity, combined with the design challenges of high drag and thermodynamic loads, makes the development of scramjet-powered spaceplanes extremely ambitious, and certainly costly. As in most aspects of advanced spaceplane design, it is fruitful to compare it to other methods and techniques. First, though, let us look at two examples of advanced scramjet technology, and see what lessons they can teach us.
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