Armchair analysis of Delta performance shortfall

lou_at_cadence.com
Date: 12/29/04 

Date: 29 Dec 2004 00:38:00 -0800

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.

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.

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.

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.
My guess, worth exactly what you paid for it,

   Lou Scheffer

Relevant Pages

  • Re: XR250R Fixed
    ... There isn't a lot of throttle fuss, it just takes a lot of kicks before it ... Since it takes two full turns of the crankshaft to get the engine to ... fuel air mixture into the intake tract. ... kick with all his might at the kick starter lever. ...
    (rec.motorcycles.tech)
  • Re: 1986 Virago 1100 Starting Problems
    ... > idle mixture screw and got nowhere. ... You've probably opened it too far, but the engine does NOT rely on ... The engine won't get enough fuel to start if the ... throttle butterflies are too far open. ...
    (rec.motorcycles.tech)
  • Re: adding water to fuel
    ... you then throttle an SI engine? ... the throttle pedal does not control the flow of fuel. ... can reduce pumping losses to something equivalent to those of a diesel. ...
    (rec.crafts.metalworking)
  • Re: adding water to fuel
    ... you then throttle an SI engine? ... the throttle pedal does not control the flow of fuel. ... can reduce pumping losses to something equivalent to those of a diesel. ...
    (rec.crafts.metalworking)
  • Re: Armchair analysis of Delta performance shortfall
    ... > How can the Delta performance shortfall be explained? ... > shut off while fuel still remained. ... Flight software not taking into proper account flight profile differences ... > same engine has always throttled down OK about 40 seconds from the end ...
    (sci.space.policy)
Quantcast