Path: utzoo!attcan!uunet!lll-winken!lll-tis!ames!umd5!purdue!decwrl!hplabs!hplabsb!dsmith
From: dsmith@hplabsb.UUCP (David Smith)
Newsgroups: sci.space.shuttle
Subject: Re: Orbiter/SRB separation
Summary: orbiter-et, not srb-et
Message-ID: <4712@hplabsb.UUCP>
Date: 18 May 88 19:52:07 GMT
References: <48048@ti-csl.CSNET>, <1869@bigtex.uucp> <4706@hplabsb.UUCP> <1934@ssc-vax.UUCP>
Organization: Hewlett-Packard Laboratories
Lines: 61

In article <1934@ssc-vax.UUCP>, eder@ssc-vax.UUCP (Dani Eder) writes:
> In article <4706@hplabsb.UUCP>, dsmith@hplabsb.UUCP (David Smith) writes:
> > True, but since the orbiter is pushing forward on the tank, and not
> > vice-versa, might it be feasible to throttle down the SSME's to the
> > point that the loads are manageable?
> > 
> 
> I believe you are misinterpreting what is pushing on whom in the
> Shuttle stack.  The majority of the mass of the Shuttle core,
> which is the Orbiter plus External Tank, is the oxygen in the ET.
> This comprises about 1.4 million pounds of the total 1.8 million
> pounds in the core at liftoff.
> 
> ((description of SRB-ET connection omitted))
>                                                   The push
> from the SSMEs represents about 15% of the total thrust of the
> Shuttle early in the flight.

I'm not impressed.  See below.

>                                  The push from the SSMEs is transferred
> into the skin of the hydrogen tank through the connecting
> struts in the aft portion of the Orbiter.
> 
> With the present design of the ball and socket joint, there is
> no way to separate the SRBs as long as they are firing, they
> simply apply too much force (3 million pounds each) to separate
> the joint.

You apparently gathered that I was talking about separating SRBs from ET.
By referring to "the orbiter ... pushing forward on the tank", I was
addressing the detachment of the orbiter from the ET/SRB combination
with the SRB's still firing.

Thank you for the weight of the core (orbiter+ET).  Now let's look at
weights of the SRB's.  According to AW&ST, each SRB weighs 1.82 million
pounds empty, and is loaded with 1.11 million pounds of propellant (note 1).
Each SRB produces 3.3 million pounds of thrust at liftoff, "throttles" down
to 2.4 million for max-Q, and back up to 2.7 million afterward (note 2).
That's a thrust/weight ratio of 1.13 for the SRBs at liftoff, or 1.48 near
burnout, vs. 5.22 for the orbiter (if I wildly guess its weight at 250,000
pounds).  Clearly, the orbiter is pushing forward on the tank.

At liftoff, each SRB's excess of thrust over weight comes to 370,000
pounds, or 740,000 pounds for the pair.  The orbiter's excess of thrust
over weight is 1,050,000 pounds using the 250,000 pound weight estimate.
Acceleration of SRB's and orbiter reduce the amount of this available
to accelerate the ET, so as to more heavily penalize the SRB's.  Therefore,
I conclude, >>>the SSMEs apply more thrust to the ET than do the SRBs,<<<
notwithstanding the fact that they produce only around 15% of the total
thrust.

Notes:
1. That is a truly lousy mass ratio compared with liquid propellant systems.

2. Add the quoted component weights up, and you get 7.66 million pounds.
    Add the thrusts, you get 7.905 million.  The resulting thrust/weight
    of 1.03 sounds small: perhaps due to rounding somewhere, or relieved
    by the propellant burned in the mains before liftoff.

		    David Smith
		    HP Labs