Why does external tank on Space shuttle fall back to earth?

[Rally]

How come if the shuttle and the external tank are going the same speed and the shuttle's engines are off at external tank release does the external tank fall back to earth and not stay in orbit?

 

[avbug]

When the external tank is jettisoned, the orbital maneuvering system is ignited. The OMS is used to maneuver the shuttle to it's orbit altitude and track.

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/sts-oms.html#sts-oms

The orbital maneuvering system provides the thrust for orbit insertion, orbit circularization, orbit transfer, rendezvous, deorbit, abort to orbit and abort once around and can provide up to 1,000 pounds of propellant to the aftreaction control system. The OMS is housed in two independent pods located on each side of the orbiter's aft fuselage. The pods also house the aftRCS and are referred to as the OMS/RCS pods. Each pod contains one OMS engine and the hardware needed to pressurize, store and distribute the propellants to perform the velocity maneuvers. The two pods provide redundancy for the OMS. The vehicle velocity required for orbital adjustments is approximately 2 feet per second for each nautical mile of altitude change.

The ascent profile of a mission determines if one or two OMS thrusting periods are used and the interactions of the RCS. After main engine cutoff, the RCS thrusters in the forward and aft RCS pods are used to provide attitude hold until external tank separation. At ET separation, the RCS provides a minus (negative) Z translation maneuver of about minus 4 feet per second to maneuver the orbiter away from the ET. Upon completion of the translation, the RCS provides orbiter attitude hold until time to maneuver to the OMS-1 thrusting attitude. The targeting data for the OMS-1 thrusting period is selected before launch; however, the target data in the onboard general-purpose computers can be modified by the flight crew via the cathode ray tube keyboard, if necessary, before the OMS thrusting period.

During the first OMS thrusting period, both OMS engines are used to raise the orbiter to a predetermined elliptical orbit. During the thrusting period, vehicle attitude is maintained by gimbaling (swiveling) the OMS engines. The RCS will not normally come into operation during an OMS thrusting period. If, during an OMS thrusting period, the OMS gimbal rate or gimbal limits are exceeded, RCS attitude control is required. If only one OMS engine is used during an OMS thrusting period, RCS roll control is required.
During the OMS-1 thrusting period, the liquid oxygen and liquid hydrogen trapped in the main propulsion system ducts are dumped. The liquid oxygen is dumped out through the space shuttle main engines' combustion chambers and the liquid hydrogen is dumped through the starboard (right) side T-0 umbilical overboard fill and drain. This velocity was precomputed in conjunction with the OMS-1 thrusting period.

Upon completion of the OMS-1 thrusting period, the RCS is used to null any residual velocities, if required. The flight crew uses the rotational hand controller and/or translational hand controller to command the applicable RCS thrusters to null the residual velocities. The RCS then provides attitude hold until time to maneuver to the OMS-2 thrusting attitude.

If the ascent profile for a mission uses a single OMS thrusting maneuver, it is referred to as direct insertion. In a direct-insertion ascent profile, the OMS-1 thrusting period after main engine cutoff is eliminated and is replaced with a 5-feet- per-second RCS translation maneuver to facilitate the main propulsion system dump. The RCS provides attitude hold after the translation maneuver. The OMS-2 thrusting period is then used to achieve orbit insertion. The direct-insertion ascent profile allows the MPS to provide more energy to orbit insertion and permits easier use of onboard software.

 

[Hugh Johnson]

Gravity?

 

[mzaharis]

Short version:

When the Space Shuttle Main Engines cut off and the tank is separated, the stack is going approximately 200 mph (if memory serves) too slow for orbit. This is quite intentional, so the tank debris will land in the Indian Ocean. As mentioned in Avbug's posts, the Orbital Maneuvering System (a couple of of 6,000lb engines) adds the additional velocity and circularize the orbit during the course of one or two two burns.

Beyond that, Avbug's link is about as definitive as it gets.

Schemes have been proposed to use the tanks as the basic structure for a space station. This would involve keeping the tank attached until the orbit is circularized, taking it with the orbiter.

 

[bocefus]

Speed at MECO is around 17,500 mph

 

[Sig]

So you have a space shuttle on a vertical treadmill...

 

[mzaharis]

Speed at MECO is around 17,500 mph

Correct (roughly). At first, I didn't understand what you were getting at, but then I realized that you may have misunderstood my post (my apologies if I phrased it in a manner that led to it being understood). I said that the shuttle 200 MPH too slow to reach orbit at MECO, not that it's going 200 mph at MECO. Basically, MECO (Main engine cutoff) is at approximately 17,400 mph (dependent on orbit). The OMS engines bring the orbiter speed up to 17,600 mph (again, depending on intended orbit).

 

[minitour]

I didn't think the tank actually made it back...doesn't it burn up?

-mini

 

[Bjammin]

Yes it burns. Wonder if anyone gets to see it burn?

 

[A Squared]

Yes it burns. Wonder if anyone gets to see it burn?

If an external tank burns in the outer atmosphere but there's noone there to see it burn, does it actually make a flame?