Launch Vehicle Lingo

In order to understand many of the subtleties regarding launch vehicle design it is useful to understand many of the terms used in the engineering analysis and evaluation of these systems. Below are a few of the most important definitions.

Ascent profile - The shape of a launch vehicle's trajectory with reference to the surface of the Earth. The optimum ascent is one in which gravity is allowed to naturally rotate (pitch) the vehicle's velocity vector toward the horizon in such a way that this vector becomes aligned with the horizon (or another prescribed direction) just as the vehicle's upper stage speed reaches the required speed.

Bipropellant system - A propulsion system that uses two chemical propellants that are stored separately. For example, the Space Shuttle Main Engines (SSME) use liquid hydrogen (LH2) and liquid oxygen (LOX), the highest energy propellant combination in our inventory. Several other launch vehicle engines use LOX and RP (kerosene).

Combustion chamber - The part of a rocket engine in which propellants are burned to produce combustion gases which then pass through a nozzle to provide thrust. Combustion chamber pressure is a key parameter in determining the level of efficient propellant utilization. The higher this chamber pressure, the higher the propellant performance, i.e., specific impulse, Isp-

Delta-V - Mathematical terminology for a change in velocity associated with a maneuver. An orbital change or plane change cannot be done without a Delta-V maneuver. The mathematical relationship between ?V and the propellant expended to achieve this maneuver is known as the Rocket Equation.

ELV - Expendable launch vehicle, i.e., one flight only. When compared to reusable designs these vehicles have the advantage of:

+ Injecting a payload into a variety of orbits or high trajectories for Earth escape
+ Having relatively low dry mass values
+ Relatively high recurring and low non-recurring costs
+ Simple subsystems and tanks

Heat shield - For recoverable stages, this is a coating or surface which offers protection from excessive heat such as that experienced during reentry into the atmosphere. Reentry heat shields must be designed to withstand heating in specific parts of the vehicle, whereas ascent heat protection areas may not coincide with those during reentry.

Max-Q - An abbreviation for maximum dynamic pressure. This is a major design driver because it defines the maximum axial load (compressive force) on the vehicle.

Max-Q-Alpha (a) - An abbreviation for the maximum value of the product of dynamic pressure (Q) and angle of attack (a). This is a major design driver because it defines the maximum lateral load on the vehicle during ascent.

Multistage rocket -A launch vehicle with more than one stage. Today's launch vehicles have multiple stages in order to take advantage of reducing the mass remaining to be accelerated with each successive stage burnout.

Payload fairing or compartment - The structure used to protect the payload from the environment before liftoff and during ascent through the atmosphere. On an expendable vehicle, the base of the fairing is typically the weakest structural part of vehicle.

Payload penalty - The amount of payload weight reduction resulting from the addition of weight to different parts of the launch vehicle. For example, one additional pound of weight added to the last stage of a multi-stage launch vehicle reduces the payload performance by one pound. However, weight added to lower stages of a vehicle have less of an impact on the payload, e. g., every 50 pounds of extra mass added to the first stage of a three-stage vehicle results in only one pound of payload performance loss.

Propellant mass fraction - The ratio of the mass of the propellant to the mass of the total vehicle at lift-off. Today's technology has limited this to about 83% to 88% for expendable stages. Any single-stage-to-orbit vehicle would require this fraction to be at least 89% to 90% because of the limitations on propellant energy.

Regenerative cooling - The practice of pumping liquid propellant (usually fuel) between the walls of exhaust nozzles and, sometimes, combustion chambers, before entering the chamber itself. This technique keeps the nozzle cooler, allowing higher temperatures in the exhaust gases leading to higher specific impulse values.

RLV- Reusable launch vehicle. Relative to ELVs, if RLVs become available they will have the advantage of:

+ Amortization of vehicle costs over many flights
+ Very high reliability due to redundant systems and health monitoring devices
+ Recovery and reuse and possible return of payloads

Specific impulse (Isp) - An expression for relative impulse. Although there are at least two separate definitions of this parameter, the most common one is the ratio of total impulse during an engine burn to the propellant weight used for the maneuver. In other words, it is usually interpreted as the amount of impulse available per unit of propellant weight.

Wind Shear - Rapid change in wind direction with height. This is particularly important for a launch vehicle rising through the atmosphere, since it places increased structural loads on the vehicle. The on-board guidance and control system must compensate for this loading during flight. It is also the primary source for lateral load on the vehicle.

Related Links
Launchspace
Rocket Science News at Space-Travel.Com

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