Why do boosters need aft skirts?

[12oh2alarm]

I was watching this episode of "Inside KSC", where a booster aft skirt was transported.

The Shuttle's SRBs had them as well.

It looks massive and heavy. Which made me wonder why it is required. Aerodynamics comes to mind, but is there more to it?

[Allan F]

It increases the efficiency of the engine. A lot of the thrust is created in the engine "bell" - of which the aft skirt is a big part.

[Allan F]

Also, the entire mass of the Space Shuttle stack - orbiter, ET, SRBs, rested on those skirts.

[ka9q]

It increases the efficiency of the engine. A lot of the thrust is created in the engine "bell" - of which the aft skirt is a big part.

I think the aft skirt is a different structure from the engine bell (nozzle extension).

[ApolloGnomon]

The aft skirt covers and protects the nozzle, nozzle vectoring hardware et cetera from both pre launch weather and fauna damage and in-launch air pressure issues. The nozzle bell is inside of the skirt, with the gap between occupied by equipment, insulation and some kind of curtain/gasket/trim stuff.

http://georgesrockets.com/grp/scale/ShuttleData/SRB.html
Modeler data, a good source for certain odd facts about hardware.

[Peter B]

I was watching this episode of "Inside KSC", where a booster aft skirt was transported.

The Shuttle's SRBs had them as well.

It looks massive and heavy. Which made me wonder why it is required. Aerodynamics comes to mind, but is there more to it?

I have to say it didn't look that heavy to me. It was placed on a simple trailer which looked custom-built, presumably to allow easy maneuvering of the object within the VAB. Note, in particular, the trailer has only four wheels, which are about the same size as the truck's, and the tyres seem to be carrying the load lightly. That suggests to me a weight in the region of one or two tons. Is that heavy? I don't know - I'm not a rocket engineer! :-)

I wouldn't be surprised if it could, in theory, have been placed on the back of a flat bed truck (like pictures I've seen of the Mercury capsule back in the 1950s and 1960s). But I imagine they wouldn't want to be maneuvering a flat-bed truck around inside the VAB, which I assume is why the trailer is shown being pulled by a smaller tractor inside the building. Having that trailer pulled around by a prime mover truck on the open road seems like over-engineering, but it presumably wouldn't be that much more expensive than a smaller truck.

[ka9q]

I suspect the aft skirt also holds some separation rockets.

[Glom]

I have a relates question. Why did the S-IC interstage skirt not separate from the S-II at staging but seperated a few seconds later while the S-II interstage skirt separated from the S-IVB at staging and remained attached to the S-II.

[bknight]

IIRC there was a close tolerance between the engine nozzles of the SII and the interstage.  I suspect that the designers didn't want any moment caused by the departing S1C to have the nozzles hit by the departure.  Waiting a brief time period would allow any oscillations to dampen and reduce the chance of nozzle hits.  The SIV-B only had one nozzle and had more relative clearance.

[Glom]

IIRC there was a close tolerance between the engine nozzles of the SII and the interstage.  I suspect that the designers didn't want any moment caused by the departing S1C to have the nozzles hit by the departure.  Waiting a brief time period would allow any oscillations to dampen and reduce the chance of nozzle hits.  The SIV-B only had one nozzle and had more relative clearance.

There is sense in what you say. I therefore shan't accuse the whole thing of being a hoax.