Probably a simple question about SRBs

[Jens]

I've been wondering, but the SRBs on the shuttle are not throttleable(?), so how can they make sure that they will burn out at the same time? I would imagine that if one the SRBs stopped burning before the other, it would create problems for the stability of the vehicle.

[astromark]

Advanced electronics and the fact that a great deal of information gathering sensors are working to ensure a clean uncoupling of the solid fuel boosters. just moments before they are empty. They fall away from the fuel tank at the same moment because they are triggered together. The shuttle and its tank continue accelerating toward space for a considerable time after those SRB are gone. The on board inertial guidance system does not stop correcting flight paths until orbital velocity is established and on track attained. She be a very complex fire cracker.

[joema]

I've been wondering, but the SRBs on the shuttle are not throttleable(?), so how can they make sure that they will burn out at the same time? I would imagine that if one the SRBs stopped burning before the other, it would create problems for the stability of the vehicle.

The SRBs actually are throttled up and down, according to a pre-planned schedule. The varying thrust comes from the internal construction, and can't be changed. When the "throttle back", and "throttle up" calls are made, it is the SRBs -- NOT the SSMEs which are providing most of the varying thrust. See attached graph.

E.g, SRBs thrust decreases from a peak thrust of about 6.2 million lbf at T+20 sec, down to 4.2 million lbf at T+50 sec. Afterward the throttle up call, SRB thrust again increases.

Unlike the SRBs, the SSMEs can be dynamically throttled. However they typically follow a fixed schedule -- in practice no different than the SRBs.

Left/right SRB thrust mismatch is a critical issue, so steps are taken during manufacturing and testing to ensure they are very similar. E.g, propellant from the same batch is used for left/right SRBs, a small propellant sample from each SRB segment is burned to verify the characteristics, each SRB is X-rayed, etc. Left/right SRB thrust is usually very similar.

However the vehicle can accommodate a substantial amount of left/right SRB thrust mismatch. This is because the SRB and SSME nozzles gimble to compensate for this.

The end of the SRB burn is called the "thrust tail-off region", and sometimes thrust variation happens there. STS-107 (the Columbia disaster) had a lot of thrust imbalance during this region, but it was unrelated to the later problem. It briefly peaked at 140,000 pounds left/right thrust differential, or nearly 5% of total max SRB thrust. See attached graph.

[cjl]

I've been wondering, but the SRBs on the shuttle are not throttleable(?), so how can they make sure that they will burn out at the same time? I would imagine that if one the SRBs stopped burning before the other, it would create problems for the stability of the vehicle.

The solid fuel grains are made to be exactly the same shape in both boosters, and the chemistry is kept as consistent as possible throughout the boosters. Therefore, they both behave almost exactly the same - it is quite predictable.

[mugaliens]

WANTED:

Schroedinger's Cat

Dead And Alive

Sorry, but both of them went through that wall. Over there.

You're right about the boosters, though.

[mugaliens]

She be a very complex fire cracker.

Bang. As always, astromark, you're always on the spot.

[mugaliens]

The SRBs actually are throttled up and down, according to a pre-planned schedule. The varying thrust comes from the internal construction, and can't be changed. When the "throttle back", and "throttle up" calls are made, it is the SRBs -- NOT the SSMEs which are providing most of the varying thrust. See attached graph.

E.g, SRBs thrust decreases from a peak thrust of about 6.2 million lbf at T+20 sec, down to 4.2 million lbf at T+50 sec. Afterward the throttle up call, SRB thrust again increases.

Unlike the SRBs, the SSMEs can be dynamically throttled. However they typically follow a fixed schedule -- in practice no different than the SRBs.

Left/right SRB thrust mismatch is a critical issue, so steps are taken during manufacturing and testing to ensure they are very similar. E.g, propellant from the same batch is used for left/right SRBs, a small propellant sample from each SRB segment is burned to verify the characteristics, each SRB is X-rayed, etc. Left/right SRB thrust is usually very similar.

However the vehicle can accommodate a substantial amount of left/right SRB thrust mismatch. This is because the SRB and SSME nozzles gimble to compensate for this.

The end of the SRB burn is called the "thrust tail-off region", and sometimes thrust variation happens there. STS-107 (the Columbia disaster) had a lot of thrust imbalance during this region, but it was unrelated to the later problem. It briefly peaked at 140,000 pounds left/right thrust differential, or nearly 5% of total max SRB thrust. See attached graph.

You, too, Joema.

Take care, all.