Saturn V 1st stage separation

Waspie_Dwarf · #1 (**permalink**) 21-August-2006, 12:02 AM

I wonder if someone could help me with a question that has always puzzled me regarding the 1st stage separation of the Saturn V. Why was the interstage separated after the SIC?

It seems to me that not separating the interstage and the SIC as a single unit adds extra complexity and also requires additional pyrotechnics. Also it seems from the film clips that I have seen that the J2s are burning when the interstage separates from the SII. The additional weight must have required a small amount of additional fuel. I assume there was a very good reason for this sequence of events and would be extremely grateful if someone would enlighten me.

Thank you in anticipation.

Hamlet · #2 (**permalink**) 21-August-2006, 12:39 AM

Quote:

Originally Posted by Waspie_Dwarf

I wonder if someone could help me with a question that has always puzzled me regarding the 1st stage separation of the Saturn V. Why was the interstage separated after the SIC?

It seems to me that not separating the interstage and the SIC as a single unit adds extra complexity and also requires additional pyrotechnics. Also it seems from the film clips that I have seen that the J2s are burning when the interstage separates from the SII. The additional weight must have required a small amount of additional fuel. I assume there was a very good reason for this sequence of events and would be extremely grateful if someone would enlighten me.

Thank you in anticipation.

The interstage had only about 1 meter of clearance from the J-2 engines on the second stage and the separation needed to be carefully controlled so as not to hit them. I don't think a simultaneous S-IC/interstage separation could ensure this. The J-2's on the second stage were indeed burning. There needed to be positive acceleration to ensure the S-II and interstage separated cleanly.

I'm sure others will have more details, but I think this is the gist of it.

Hamlet · #3 (**permalink**) 21-August-2006, 01:45 AM

After looking at some reference material I realized I had forgot one other, very important, reason that the interstage didn't separate with the S-IC. The interstage contained 4 ullage rockets that were used to help settle the propellants in the S-II tanks before ignition. These rockets provided brief, positive acceleration to the stack to ensure that there was propellant at the intake ports of the turbopumps. Without this settling there was a possibility that the turbopumps would cavitate and possibly destroy themselves.

AGN Fuel · #4 (**permalink**) 21-August-2006, 03:28 AM

Quote:

Originally Posted by Hamlet

After looking at some reference material I realized I had forgot one other, very important, reason that the interstage didn't separate with the S-IC. The interstage contained 4 ullage rockets that were used to help settle the propellants in the S-II tanks before ignition. These rockets provided brief, positive acceleration to the stack to ensure that there was propellant at the intake ports of the turbopumps. Without this settling there was a possibility that the turbopumps would cavitate and possibly destroy themselves.

Yep, that's my understanding also. Interestingly, this was scrapped for Apollo 15. From here:

Quote:

[The S-IC/S-II staging sequence for AS-510 differed markedly from earlier missions. On previous flights, the interstage, or skirt, a 4.9-metre tall ring matching the 10-metre diameter of the S-IC and S-II that it sits between, carried solid-fuel rockets which fired shortly after the first stage separated to settle the S-II propellants in their tanks. AS-501 and AS-502 had eight of these ullage rockets, while AS-503 to AS-509 had four. They were deleted from the Apollo 15 launch vehicle, along with four of the eight retro rockets built into the conical engine fairings around the base of the S-IC, in order to save weight and increase payload. Ullage is a brewers' term for the portion of a barrel occupied by air, not liquor. The separation did not quite go according to plan. After the F-1 engines were shut down, the thrust they generated during the tail-off period was greater than expected. The engines don't instantly stop thrusting when they receive their cut-off command. After a quick drop to about 2% thrust, they take over four seconds to decay to zero. As the engines expired, the acceleration imparted to the, now separate, empty and therefore light S-IC stage was above the predicted value. Despite deliberately coasting for longer than usual between separation and S-II ignition, the distance between the two stages was less than engineers had planned and the blast of hot gases from five J-2s against the top of the empty stage disabled a telemetry package with which the S-IC was to be monitored until its impact with the Atlantic Ocean.]

[Staging of the S-IC and S-II is technically described as a "dual plane separation," as the vehicle is cut across two geometrical planes. The first plane is between the skirt and the S-IC, with the S-II engines starting 1.1 seconds later. The second plane separation, when the second stage loses the skirt, occurs at 3 minutes, 10.7 seconds; 30.0 seconds after the S-IC separation. This time allows the S-II's attitude to stabilise because if either part of the launch vehicle were to be yawing or pitching excessively, there would be a danger of the engine bells striking the S-IC and skirt as the two great metal cylinders coast along before ignition of the S-II. The skirt provides clearance above the first stage's LOX tank for the five J-2 engines of the S-II stage.]

Hey, Waspie Dwarf! Welcome to the BAUT - haven't seen you since 'Unexplained Mysteries'! Hope you have been keeping well.

Jason Thompson · #5 (**permalink**) 21-August-2006, 01:53 PM

After looking at some reference material I realized I had forgot one other, very important, reason that the interstage didn't separate with the S-IC. The interstage contained 4 ullage rockets that were used to help settle the propellants in the S-II tanks before ignition.

Whilst this is true, I think it is a case of putting the cart before the horse. The first priority was to figure a way of avoiding impact between the J-2 engines of the S-II stage and the top of the S-IC stage. For that reason the interstage separated separately. Once that two-plane separation manoeuvre was settled on it made sense to put the ullage rockets on the part that would be dropped after S-II ignition. Had they not needed the two-plane separation I suspect the ullage rockets would have been located on the S-II just as the ullage rockets for the S-IVB were on the stage itself and not on the second stage interstage, which separated as part of the S-II. Two-plane separation was not needed in this case because there was greater clearance and the S-IVB could not generate off-centre thrust to an extent that might cause impact of its engine with the departing stage.

You are correct about the extra weight of the interstage though. It was significant, and a failure of the interstage to separate was a scenario that required an abort.

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