July 5, 2013

Launch Systems

We move from our discussion of rockets in general to launch vehicles, rockets specifically designed to lift a spacecraft from earth's surface into orbit.  Unlike spacecraft propulsion systems launch vehicles must produce a great deal of thrust over a very short time.  Launch typically takes about ten minutes, but during that time the rocket must produce a total delta-v of 8,000m/s.  Not only must the launch vehicle carry enough fuel to produce that delta-v (remember the ideal rocket equation from the last lesson) it must produce that velocity in a very short time.  This takes a lot of thrust.  One of the measures of a vehicle's effectiveness is its thrust-to-weight ratio, the thrust force it can produce divided by its weight force.  If this isn't greater than one then it's impossible for the rocket to even lift off the ground.  While chemical rockets may not produce very high specific impulse they do produce very high thrust, and are the only rockets used for launch.
As the launch vehicle passes through the atmosphere it produces dynamic pressure effects.  To cope with this it is necessary to be able to throttle the engine, increasing and decreasing the thrust as the spacecraft moves through the atmosphere.  The vehicle's thrust profile is tailored to the atmospheric conditions during launch and the anticipated pressure load on it.
Atmospheric effects also produce the need to be able to steer the rocket.  Thrust-vector-control is a method of steering the rocket by controlling the direction of thrust.  Nozzles are put on gimbals, which allows them to move over a range of angles.  Vanes can also be used to direct thrust by changing the shape of the nozzle itself.  Vanes also alter the shape of the nozzle, affecting its efficiency.  Nozzles allow direction over a larger range of angles, but are heavier and more complicated.
Launch vehicles typically have multiple engines.  In addition to redundancy this provides another method of steering.  Individual throttling of engines can direct the vehicle by providing an uneven force, more force on one side causes a moment which pivots the spacecraft.  Another method of accomplishing the same task is cold gas injection.  Cold gas, either unused propellant or from the atmosphere, is injected into the exhaust stream before it reaches the throat.  This has the effect of cooling the exhaust, even though more mass is leaving the nozzle it has less velocity and so less thrust.
Steering rockets are small thrusters placed on the skin of the vehicle.  Like the maneuvering thrusters of a spacecraft, steering rockets produce relatively small thrust and are placed so the launch vehicle pivots.
In the atmosphere aerodynamic steering methods, like ailerons and rudders, can also be used for control.  Fins are put on the launch vehicle to provide stability.  Flaps on the fins produce aerodynamic forces which can cause the rocket to tilt and turn, just as an airplane's flaps do.
Steering isn't the only problem that exists during launch.  As we mentioned before an ideally expanded nozzle, one which produces maximum efficiency, releases exhaust at the same pressure as its surroundings.  But the atmosphere gets thinner as the vehicle climbs, so the pressure goes down.  Conventional bell nozzles are ideally expanded only at one point during ascent.  Before this they are over-expanded and after it under-expanded.  It has been found that the best compromise is to design for a point two-thirds of the way from engine ignition to engine shutdown.  However for the entire trip through the atmosphere this is a long range with wasted fuel.
A solution to this is staging, a technique where one rocket lifts another.  Each stage of the vehicle is a self contained rocket, with all the tanks, engines, and support hardware.  This means that each stage can be designed for the altitude range it passes through.  Because each stage also releases the now empty fuel tanks and support hardware from the last stage it carries less weight.  Stage two needs to lift less than stage one did, stage three less than stage two, and so on.
Staging improves efficiency and reduces the weight that must be lifted into space.  However staging also increases complexity and reduces reliability.  Since each stage is self-contained it increases the amount of support hardware.  Each additional stage provides less of a benefit than the last.  There's a functional limit of three to four stages, beyond that the extra weight from more stages is more than the fuel saved by staging.
Boosters are additional rockets attached to the launch vehicle to provide extra thrust.  Boosters provide some of the benefit of staging in that they can be discarded when no longer needed.  The advantage of boosters is that they give extra thrust without the need for extra engines on the main vehicle.  For vehicles that use them, booster separation is the first stage.
At present launch requires some form of staging.  Too much fuel is required, which requires more tank mass, which requires more fuel to lift, and so on.  Staging and boosters reduce the fuel mass necessary to make space flight possible.  The expense of recovering or rebuilding stages and integrating the vehicle, however, has inspired many to wonder if a vehicle could reach orbit and return without staging.  Single stage to orbit (SSTO) vehicles are, as yet, theoretical only.  The X-33 program was thought by many to beginning of a true SSTO vehicle.  Its lightweight composite structure and aerospike nozzle were thought to make staging unnecessary.  The program, however, was cancelled in 2001 due to issues with the composite fuel tank.
Other proposals for SSTO vehicles have been made over the years.  Combined cycle engines are hypothetical engines which act as jets in the atmosphere (saving unnecessary oxidizer weight) and rockets at high altitude.  Scramjets are thought to be able to accelerate a vehicle to hypersonic speeds, and it could then coast into orbit.  Instead of an air vehicle some propose that the same technique could be accomplished on the ground, using a railgun to accelerate a vehicle enough to coast into orbit.  Even more exotic is the idea of a space-elevator, a massive tether reaching from the surface into orbit allowing cheap lifting of goods.  Since none of these have achieved fruition, and many experts consider some of them to be impractical if not impossible to ever achieve, we'll confine the rest of our lesson to conventional chemical rocket lifting vehicles.
source: wikipedia
At the top of the launch vehicle is the payload fairing, which sits at the head of the final stage.  This empty part of the vehicle houses and protects the spacecraft inside during launch.  It limits vibration and acoustic loads reaching the spacecraft and insulates it.  When the aerodynamic force is acceptable (usually an altitude of about 100km) the fairing opens and releases the spacecraft.  When the fairing opens the launch vehicle gives the spacecraft the final push into orbit.
When a launch vehicle takes off it is initially facing straight up (perpendicular to the surface of the earth).  Shortly after clearing the platform it pitches very slightly to its initial kick angle.  This small change for perfectly vertical causes it to begin moving away from its platform horizontally.  A small pitch angle causes the vehicle to move more and more horizontally, in turn increasing the pitch angle.  The initial kick pushes the vehicle into a gravity turn, where thrust is always pointing in the same line as the vehicle's motion, also known as a zero-lift or zero angle of attack trajectory.  A gravity turn trajectory reduces the structure mass needed by reducing the number of loads the vehicle is subjected to.
Ignition happens at time zero of a mission.  Everything leading up to ignition happens before zero, or negative seconds of the mission.  "T-minus 10" means ten seconds before ignition.  After ignition the numbers are positive.  Initial kick happens at about t-plus 15 seconds.  Gravity turn portion of ascent lasts until the vacuum phase.  At this point the vehicle has left the thickest part of the atmosphere and its motion is dominated by orbital, rather than atmospheric, effects.  During this portion of ascent the vehicle maneuvers to enter its desired injection orbit.  Injection is the final phase of launch, when the spacecraft enters orbit and control transfers from launch operations to mission operations.  Here the launch vehicle is discarded and provides the spacecraft its final push into orbit.

Many launch systems are available, each system provides a vehicle and launch site.  Launch site must be carefully chosen.  Although vehicles only launch from specific locations, there are other concerns behind when and where a spacecraft is launched.  Launch sites are chosen for safety.  In the even the vehicle must abort its launch it must do so over a location where there is no risk posed by falling debris.  Most launch sites are chosen so that following initial kick the launch vehicle moves over water, although most Russian launches take place at Baikonur, Kazakhstan which is a remote desert area.
If necessary the spacecraft can be lifted into a parking orbit, a temporary stable orbit before its own propulsion system moves it into its final orbit.  It is easier and less expensive if the launch vehicle can lift the spacecraft directly into its destination orbit.  This means that the launch site must find itself under the desired orbit.  Launch windows are periods of time when the launch site and the desired orbit line up.  They usually last a few minutes or hours.
A launch window occurs when the launch site's sidereal time coincides with that of the orbital plane.  Because of the motion of the earth around the sun a normal, 24 hour day, isn't exactly how long the earth takes to rotate.  Sidereal time is based on a star, rather than the sun, to compensate for this.  A sidereal day is about 3:56 shorter than a solar day.  Local sidereal time is given as an angle between the launch site and a fixed star, called the vernal equinox.  Orbital plane is just the flat, two-dimensional shape that the mission orbit traces out.  So when the orbital plane's sidereal time (which doesn't change) is the same as the launch site's (which does) we have a launch window.  This usually happens twice each sidereal day, because its orbit takes the spacecraft over both sides of the planet.

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