|Fig. 1: Schematic of a rocket (top) and a scramjet (bottom).|
Hypersonic, air-breathing forms of propulsion, such as the scramjet (Supersonic Combustion ramjet), are actively being researched and developed, and such programs have been motivated by applications in defense (i.e., missiles) and space flight. [1,2] Hypersonic air-breathing missiles will be able to travel faster than conventional cruise missiles, providing obvious benefits in a defense context; however, the utility of air-breathing hypersonics is not as obvious in a space-flight context. To be useful for space-flight, air-breathing hypersonics must yield significant gains in vehicle payload capacity or significant reductions in cost-to-orbit. Reductions in cost are difficult to assess given the developmental state of scramjet technology, however, we can estimate the increase in payload for a scramjet assisted space-vehicle as compared to a conventional rocket.
Access to orbit is essential for the placement of satellites that we rely on for telecommunications, scientific studies, weather, etc., and rockets engines have been the only form of propulsion capable of and utilized for reaching orbit. Rockets combine a fuel (typically liquid hydrogen or kerosene, known as RP-1) with an oxidizer (usually liquid oxygen, known as LOX), producing hot combustion products, which are expanded through a nozzle to produce thrust. This thrust propels the vehicle to the altitude and speed necessary to sustain orbit.
Scramjets, also burn fuel and oxygen, but instead of carrying LOX on board the vehicle, these engines use oxygen present in the atmosphere to burn the fuel. Scramjets can only operate at supersonic speeds; they use the compression created by their flight to compress the incoming air, into which they inject fuel. The fuel and air mix, burn, and are expanded through a nozzle generating thrust. At supersonic speeds, low altitudes result in too much dynamic pressure for airframes to withstand (i.e., aerodynamic forces, like lift and drag, are too great), and at high altitudes, the air is too thin to provide enough oxygen to burn the required amount of fuel to accelerate the vehicle. Therefore, a scramjet cannot boost a payload all the way from the launch pad into orbit, but needs a rocket to both start and finish its flight. A reasonable operating window for a scramjet extends from about 10 km to 70 km.  For reference, the perigee (the minimum radius of an elliptical orbit) of a geosynchronous transfer orbit (GTO), a common destination for satellites, is around 185 km.  Therefore, for a scramjet-assisted space-vehicle, a rocket will need to provide thrust from 0-10 km, the scramjet will lift the vehicle from 10-70 km, and a rocket will finish the flight from 70km to orbit.
To estimate the expected increase in payload for a hypothetical hybrid scramjet/rocket vehicle, we can compare to an Atlas V 400, a Lockheed Martin built, RP-1/LOX fueled rocket. By making some assumptions about a typical launch of an Atlas V 400 to GTO, we can estimate how much oxygen is consumed during the rocket's flight from 10km to 70km..
An Atlas V 400 series rocket is a two-stage rocket without solid boosters. Its first stage burns RP-1, and its second stage burns liquid hydrogen. The first stage burns from the launchpad to about 120 km in altitude, so let us only consider the propellant carried by this first stage.  An Atlas V 400 weighs about 330000 kg, of which about 284000 kg is first stage propellant (LOX and RP-1).  The Atlas V 400 engine burns with a oxidizer-to-fuel ratio of 2.72, by mass, meaning that of all the propellant it carries, it carries 76000 kg of RP-1, and 208000 kg of LOX.  We can safely assume that an Atlas V burns all of its first stage fuel on its way to GTO. If we also assume a linear flight trajectory to orbit, and that the rocket burns its fuel at a constant rate during its flight, we can estimate that the Atlas V consumes 50% of its propellant from 10km to 70km (i.e., the operating window of a scramjet) during its first-stage ascent to 120 km.
If we assume that the scramjet follows the same trajectory to orbit as the Atlas V and it provides the necessary thrust from from 10 km to 70 km, the scramjet's ability to burn atmospheric oxygen translates into a weight savings of about 104000 kg of LOX, which is about 22 times more than the maximum payload of an Atlas V 400 delivering to GTO (4750 kg).  This payload increase should be considered to be quite optimistic, or a best-possible improvement. Our estimate is based on several questionable assumptions: the scramjet flies the same trajectory with the same velocity and throttle profiles as the rocket; the same amount and type of fuel is burned by both the rocket and scramjet; there is no mass added to the airframe by the scramjet; and the scramjet can convert fuel to thrust as efficiently as a rocket. These assumptions are not likely to hold true for a real scramjet, but even with a significant departure from our idealized scenario, we can still expect a significant payload increase (e.g., a 50000 kg weight penalty still corresponds to a 10 times increase in payload) for a hybrid scramjet/rocket space-vehicle.
© Victor Miller. The author grants permission to copy, distribute and display this work in unaltered form, with attribution to the author, for noncommercial purposes only. All other rights, including commercial rights, are reserved to the author.
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