Updated Wednesday, 21-Jan-2004 10:00:09 EST
(National Space Transportation System --NSTS)
Concept of reusable space transportation system originated in the 1960's
Development of shuttle system begun in 1970's
Major components contracted to various companies
Delays in development of thermal protection system (heat tiles) and the turbo-pumps on main engines
First orbiter -- Enterprise (named by popular demand of Star Trek fans)
- IBM -- computers
- Morton Thiokol -- solid rocket boosters (SRB's)
- Rockwell -- orbiter vehicle (OV)
- Martin-Marietta -- external tank (ET)
- General Electric -- kitchen and toilet
First complete system launched April 12, 1981 -- Columbia orbiter
List of orbiters:
- Not flight-qualified
- Test vehicle for landing checkout only
- Now at Dulles Airport near Washington, DC
- Endeavour (replaced Challenger)
- Challenger (destroyed during launch, Jan. 28, 1986 -- all crew were lost)
- Columbia (destroyed on re-entry, Feb. 1, 2003 -- all crew were lost)
- thermal sensor data (PowerPoint slides from NASA)
For more details on the history of the shuttle's
development, see The Space Shuttle Decision: : NASA's Search
for a Reusable Space Vehicle, by T.A. Heppenheimer, NASA SP-4221 (in the
U.S. Documents section of the Paterno Library).
Major Subsystems (textbook covers most of these -- notes here describe some unusual aspects). An excellent source of detailed information is the NASA News Media Reference Manual (it's originally dated 1988, but the pages are all constantly updated as modifications are made by NASA).
Launch Preparation and Sequence (Mission Profile)
Stack assembled on mobile launch pad in Vehicle Assembly Bldg. (VAB)
Transported to launch pad 39A or 39B
Liquid H2-O2 tanks filled (requires 4.5 hrs) -- must allow boil-off of gases
- The Launch Assembly (Stack)
- Orbiter (crew, payloads, main engines)
- ET (liquid hydrogen and liquid oxygen for main engines in orbiter)
- SRBs (reusable solid chemical engines)
- Thermal protection system (heat tiles)
- Previous s/c (Mercury, Gemini, Apollo) employed ablative heat shields. During atmospheric re-entry,, a layer of glass-phenolic material chars as it reaches high temperatures, and the hot particles are sheared away by the high-velocity air flow -- this is the ablation process. The hot particles carry the heat away from the s/c. Major disadvantages are weight of the shield and non-reusability (since a new shield cannot be easily bonded to the s/c).
- Shuttle orbiters use a system of 30,000 tiles made of a silica compound that does not ablate, but does rapidly radiate heat away from the orbiter. These tiles can be repaired in space. Major disadvantages are fragility (tiles easily damaged before launch and by orbital debris -- lots of tile damage due to debris since anti-satellite tests in mid-80's) and complexity (many people needed to manually attach tiles to orbiter in a tedious and time-consuming process, and to inspect them all before launch).
- Computers -- five processors to monitor status of the orbiter and automatically control engines, life support, etc.
Launch is nominally at time T
T-6.60 sec -- 1st main engine ignites
T-6.48 sec -- 2nd main engine ignites
T-6.36 sec -- 3rd main engine ignites
- O2 vented to atmosphere
- H2 collected and re-liquified in ground storage tank
T-0 sec -- SRBs ignited
- These engines can be shut down if necessary (if computers detect a problem -- this has happened twice in the shuttle's history). Specifically, the engines must all reach 90% of full thrust by T-3 sec, or else a shut-down is commanded by the computers.
- Thrust from these engines causes the orbiter to rock towards the ET and then bounce back, creating a "twang" sound in the structure
- Thrust from main engines = 5 million Newtons (1.125 million pounds)
T+0.3 sec -- Liftoff from pad
- 8 large bolts hold the SRBs to the launch pad. Until these bolts (containing explosives) are detonated, the stack cannot lift off
- Thrust from SRBs = 29 million Newtons (6.6 million pounds)
- So total thrust at liftoff = 34 million N. (7.725 million pounds). Entire stack (orbiter, external tank, and SRBs) weighs 19.57 million N. (4.4 million pounds)
- Just after the SRBs ignite, control shifts to Johnson Space Center (JSC) in Houston
- Stack rotates about vertical axis to get proper orientation and then begins an upward curved path with orbiter on the bottom. This creates the sensation of positive acceleration into their seats for the astronauts, which is necessary to prevent disorientation and loss of consciousness.
Red (wide) arrow shows force (centripetal force) needed to keep orbiter moving along curved path. This force is transmitted through the structure to the astronaut (to keep him/her also moving along the curved ascent path). To the astronaut, this feels like acceleration down into the seat, indicated by the black (thin) arrow.
- T+26 to 60 sec -- maximum dynamic pressure (max q)
- dynamic pressure (q) depends on shuttle's speed and air density
q = r v2/2
where r = air density and v = shuttle velocity
- since r is decreasing with altitude and v is increasing with altitude, there is a point where q reaches a maximum value -- there the Shuttle experiences large forces on its structure.
- To help reduce these forces, the Shuttle reduces its speed slightly before reaching the max-q point (this is accomplished by using the engine throttle to reduce the thrust). A short time later, when the air density has fallen considerably, the Shuttle commander increases speed again (by opening the throttle to increase the main engines' thrust).
- Average acceleration during some time interval t is
aavg = (Sf - Si)/t
where Sf is the final speed (at the end of the interval), Si is the initial speed (at beginning of the interval).
- Because Earth is rotating, the initial speed at launch is not zero, but the speed of that point on Earth's surface due to the rotation
- Launch sites closer to the equator (point A in figure below) have higher speeds because they are farther from Earth's spin axis. A launch site very near the North or South Pole (point B in figure) would have very small speeds.
Kourou, French Guiana
Kennedy Space Center
- Example: Rocket launched from equator is moving 0.894 km/s at 1 minute after launch. What is its average acceleration?
aavg = (0.894 - 0.463 km/s)/60 sec. = 0.0072 km/s2 = 7.2 m/s2
Note: average acceleration is different from instantaneous acceleration (ainst =F/m), because the acceleration will change as the rocket's mass decreases.
- T+2 min 4 sec -- SRB separation
- SRB's continue burning for a short time to carry them along desired path, then parachutes deploy to lower them into ocean. They are retrieved and refueled for a later shuttle mission
- T+8 min 50 sec -- ET separation
- ET is too large and moving too fast for parachute return. It burns up over the Indian Ocean
- T+12 min -- orbiter has reached typical orbiting altitude and fires OMS engines (orbital maneuvering system) to achieve correct speed and direction
- For some missions, orbiter may continue to ascend using several short OMS engine burns for another 30-40 minutes. Maximum altitude is about 600 miles.
- OMS engines use propellant combination MMH and NTO (monomethyl hydrazine and nitrogen tetroxide (two very dangerous chemicals!), which are hypergolic -- they ignite on contact, without the need for an ignition spark.
- Orbiter also uses 44 smaller thrusters located in the nose and aft sections to make very fine adjustments in speed -- important for rendezvous with a satellite or space station.
- Once on orbit, orbiter must open cargo bay doors immediately to allow radiator panels (inside the doors) to radiate excess heat into space. This excess heat comes from the equipment and astronauts. If doors cannot open, orbiter must return to Earth within just a few minutes, otherwise overheating will occur very shortly.
- Re-entry generates high temperatures due to atmospheric friction. Maximum temp at 40 miles altitude; max speed 15,000 mph. For13 minutes no communications possible because air around orbiter is ionized by high temperatures -- this creates an impenetrable barrier to radio signals.
- Return to Earth -- orbiter can land either at Kennedy Space Center in Florida or at Edwards Air Force Base in California (and then be carried on the back of a Boeing 747 to Kennedy Space Center). Preferred landing site (for cost) is Kennedy Space Center, but astronauts don't like the narrow runway there, with surrounding, alligator-infested swamps.
Copyright © 1998, Robert G. Melton
Updated Wednesday, 21-Jan-2004 10:00:09 EST