Re: Armchair analysis of Delta performance shortfall

From: w9gb (onw9mapsgb_at_no.ar.spam.rl.net)
Date: 12/30/04 

Date: Thu, 30 Dec 2004 03:51:41 GMT

<lou@cadence.com> wrote in message
news:1104309480.271242.89060@z14g2000cwz.googlegroups.com...
> How can the Delta performance shortfall be explained?
>
> First, the rockets fired for too short of a time. Either (a) they did
> not start with a full load, or (b) they ate it too fast, or (c) they
> shut off while fuel still remained.

Actually there is another perspective, which appeared with the Ariene 5
first launch.
Flight software not taking into proper account flight profile differences
between Medium / Heavy

> But (a) is unlikely since the same CBC have flown before, so they know
> the exact capacity. (b) seems unlikely since they adjust the thrust by
> measuring the fuel flow, I'd assume, and since the Delta 4 medium with
> the same tanks and engines fired for the expected amount of time.
> Also, if the engines were using extra fuel they would have
> proportionally higher thrust while firing, leading to little or no net
> loss of performance.
>
> We are left with (c), they shut down with fuel remaining. But why?
> Watching the replay, for both the outer boosters and the core,
> everything is normal until they try to throttle back. When the
> throttle back time comes, they shut off completely instead.

It will be interesting to see if the flight software had this contingency
covered and that sufficient allowances were fully developed to maximize the
situational parameters actually experienced on this flight.

> The side boosters are supposed to run for 10 sec at reduced thrust; the
> core booster for 16 seconds. If cutoff is due to running out of fuel,
> then the side boosters are missing about 10sec times 58% thrust = 5.8
> seconds of (full throttle) fuel. The core stage is missing 16sec times
> 58% = 9.3 seconds of (full throttle) fuel. This would be quite some
> coincidence if it was fuel starvation, which is further evidence the
> fuel amount is not the problem.
>
> On the other hand, the center engine throttled down OK at about 1
> minute on the heavy flight. Also, on the regular delta 4 flights, the
> same engine has always throttled down OK about 40 seconds from the end
> of the first stage firing.
>
> So what's different about the Heavy? Two things stand out; the
> acceleration is less, since the second stage is heavier, and the amount
> of fuel in the tanks is less. Both of these lead to less pressure at
> the pump inlets.

Good points and the flight software must take these into account.

> So my guess is that when they reduced thrust on the side engines, the
> combination of less fuel in the tank, low acceleration, and maybe
> structural rebound from the removal of full thrust, caused the pressure
> at one or both engines to get too low. This caused at least one engine
> to go out (on the video it looks like the left one goes out first) and
> hence the other to get shut off. The center engine was unaffected at
> that point (and when it throttled down earlier) because it still had
> plenty of fuel in the tank.
>
> Then the same thing then happened when the center engine tried to
> throttle. Compared to a Delta 4 medium flight, there was less fuel in
> the tank (16 sec vs 40 sec), and less acceleration (heavier second
> stage and payload). The combination caused less pressure at the inlet,
> so this engine too ate a bubble or cavitated or whatever it does when
> it gets too little pressure, and went out. The rest, as they say, is
> sci.space.history.

This highlights a difference (and disadvantage in this case) of the 3 core
configuration versus the Saturn V or even the Space Shuttle clustered engine
approach. (I can't remember the final Saturn I-C configuration). You lose
an engine, you still have all of the fuel available for the remaining
engines -- more options (burn longer on remaining engines).

If this was a manned mission, then this would have been an abort scenario.

gb

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