Saturn V staging?....

[astronaut23]

I've noticed on the Saturn V launch videos that I've watched that at staging and even after there is is still a heck of a whole lot of stuff coming out the back end of the rocket?

What is that? I thought the engines shut completly down at staging and then explosive bolts fired along with separation motors to separate the 1st stage from the 2nd before the 2nd stage engines are fired up to get accelerating again.

Even after staging you can see a whole lot of stuff still coming out of the 1st stage while you can't really even see the 2nd stage engines burning in the same way as the first. What is this?

[trebor]

Good question,
I imagine it may well be unburned fuel or oxygen venting...

[banjomd]

Oct 6, 2010 16:02:56 GMT -4 astronaut23 said:

... you can't really even see the 2nd stage engines burning in the same way as the first....

Saturn V first stage, F1 engines used kerosene fuel propellant;
second stage, J2 engines fuel was liquid hydrogen.

[astronaut23]

Yeah, I guess your right. Like you can see the SRB's really burning on the shuttle but the main engines once they get up to full power are pretty hard to see the plumes even at ground level.

You can see the 3 white circles on the back of the shuttle stack even when its high up in the flight but we got a lot better tracking cams now than they did in the Saturn V days.

I'd love to have a modern HD video of a Saturn V launch. ;D


I knew the upper stages burned hydrogen while the first stage was kerosene. But what is the stuff that continues to come out of the engines after shutdown? The engines keep venting fuel after shutdown or what?

[echnaton]

Oct 7, 2010 7:19:12 GMT -4 astronaut23 said:

But what is the stuff that continues to come out of the engines after shutdown? The engines keep venting fuel after shutdown or what?

Yes they vent the unburned propellants.

[ka9q]

Remember also that this is happening at an extremely high altitude. The jettisoned stages, which are following a ballistic trajectory, continue to climb even higher for a while before falling toward the ocean. The atmospheric pressure up there is almost zero, so liquid propellants like hydrogen and oxygen will vaporize quite vigorously as they stream out of the engines. If it's daytime, the ultraviolet in unfiltered sunlight will start to ionize them as well. All this certainly helps to enhance their visibility.

[banjomd]

Anybody know how ullage was achieved for the SIVB TLI burn?
(BobB's Apollo pages states that there were 2 ullage motors and they were jettisoned after the first ullage.)

[Tanalia]

Two solid-propellant ullage motors were used for the first burn, and then jettisoned. Ullage for subsequent burns was provided by the two Auxiliary Propulsion System (APS) modules (essentially overgrown RCS units, fueled by nitrogen tetroxide/hydrazine)

[banjomd]

Thanks, Tanalia.

[ka9q]

Oct 24, 2010 4:08:10 GMT -4 Tanalia said:

Two solid-propellant ullage motors were used for the first burn, and then jettisoned. Ullage for subsequent burns was provided by the two Auxiliary Propulsion System (APS) modules (essentially overgrown RCS units, fueled by nitrogen tetroxide/hydrazine)

Close -- the fuel was MMH, monomethyl hydrazine, CH3HN2H2. The thrusters were pressure fed with helium.

I don't think straight hydrazine (N2H4) is used in bipropellant engines as its ability to spontaneously decompose (as with a catalyst bed in a monopropellant thruster) is actually a drawback -- it can spontaneously decompose in storage. MMH has a much lower freezing point (-52C vs +1C for hydrazine) and is more stable.

From what I've been able to gather, nearly all maneuvering thrusters on US manned spacecraft use MMH/N2O4 while the larger hypergolic engines like the SPS and the LM ascent and descent engines burn Aerozine-50, a 50-50 mix of UDMH and straight hydrazine. I'm not sure why AZ-50 is preferred in bigger engines, but it's probably more dense.