Oxidizer to fuel ratio (O/F) Figure 3.4. The less oxidizer carried, the lower the mass ratio.
propulsion (about 6). There is a discontinuity in the oxidizer to fuel ratio curve between rocket-derived propulsion (value of 6) and where airbreathing rockets begin, at a value of 4. Based on the definition of fuel weight to OWE in equations (3.1), the values from Figure 3.3 result in a fuel weight to OWE ratio of approximately 1. That is, for all of these hydrogen-fueled propulsion systems, the fuel weight is approximately equal to OWE. The mass ratio is decreasing because the oxidizer weight it decreasing as a direct result of the oxidizer to fuel ratio. So, using hydrogen fuel, an all-rocket engine can reach orbital speed and altitude with a weight ratio of 8.1. An airbreathing rocket (AB rocket) or KLIN cycle can do the same with a weight ratio about 5.5. A combined cycle rocket/scramjet with a weight ratio of 4.5 to 4.0, and an air collection and enrichment system (ACES) needs 3.0 or less. So an airbreathing launcher has the potential to reduce the mass ratio to orbit by one-half. It is clear that results in a significantly smaller launcher, both in weight and size.
What that means is that, for a 100-ton vehicle with its 14-ton payload loaded, an all-rocket requires a gross weight of 810 tons (710 tons of propellant) and a 1,093-ton (10.72-MN) thrust propulsion system. With oxidizer to fuel ratio reduced to 3.5 the gross weight is now 600 tons (500 tons of propellant) and a smaller 810-ton (7.94-MN) thrust propulsion system. If the oxidizer to fuel ratio can be reduced to 2, then the gross weight is now 200 tons (100 tons of propellant) and a much smaller 270-ton (27-kN) thrust propulsion system. For the same 810-ton gross weight launcher with a oxidizer to fuel ratio propulsion system of 2, the vehicle weight is now 405 tons with a 67-ton payload.
SSTO is shown because it requires the least launcher resources to reach LEO. Hydrogen is the reference fuel because of the velocity required for orbital speed: any other fuel will require a greater mass ratio to reach orbit. A two-stage-to-orbit launcher will require two launcher vehicles, and can have a different mass ratio to orbit (depending on fuel and staging Mach number), but the effect of increasing airbreathing speed is similar. Since the ascent to orbit with a two-stage vehicle is in two segments, the lower-speed, lower-altitude segment might use a hydrocarbon fuel rather than hydrogen. The question of SSTO versus TSTO is much like the aerospace plane versus Buran arguments. The former is very good at delivering valuable, fragile cargo and crew to space complexes, while the TSTO with the option of either a hypersonic glider or a cargo canister can have a wide range of payload types and weight delivered of orbit. It is important to understand that they are not mutually exclusive, and in fact in all of the plans from other nations and in those postulated by Dr William Gaubatz both SSTO and TSTO strategies were specifically shown to have unique roles.
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