Path: utzoo!utgpu!water!watmath!clyde!att!uniq!rjnoe From: rjnoe@uniq.UUCP (Roger J. Noe) Newsgroups: sci.space.shuttle Subject: Re: Orbiter/SRB separation Message-ID: <478@uniq.UUCP> Date: 25 May 88 18:20:08 GMT References: <1869@bigtex.uucp> <4706@hplabsb.UUCP> <1934@ssc-vax.UUCP> <4712@hplabsb.UUCP> Sender: news@uniq.UUCP Organization: Uniq Digital Technologies, Batavia, IL Lines: 134 > In article <1934@ssc-vax.UUCP>, eder@ssc-vax.UUCP (Dani Eder) writes: > > 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. So far this looks pretty accurate. At launch the OV is something like 165000 lbs. with no cargo. The LO2 in the ET is about 1.33e6 lbs., the LH2 is 220000 lbs. and the ET itself is another 78000 lbs. for a total of 1.63e6 lbs. in the full ET. That does bring the ET+OV weight to about 1.8e6 lbs. at liftoff. > > The push > > from the SSMEs represents about 15% of the total thrust of the > > Shuttle early in the flight. I think comparing the sea-level thrust values (11.88e6 N per SRB, and a nominal 1.668e6 N per SSME) and assuming the SSMEs are running at 104% of nominal, you get more like 18% of the total thrust shortly after liftoff as being due to the three SSMEs, with the remainder due to the two SRBs. Good enough for government work. (For those of you who haven't yet learned metric, those thrust values are 2.67e6 lbs. per SRB and nominal 375000 lbs. per SSME, at sea level.) > > 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. This sounds right. Since the gross weight of an SRB at launch is 1.287e6 lbs. and the whole stack is accelerating at about 0.48g as the stack clears the tower, each SRB exerts a force of about 767000 lbs. (3.4e6 N) on the ET. The OV, with about half the allowable cargo, weighs almost 200000 lbs. and exerts a force of around 877000 lbs. (3.9e6 N) on the ET given the 0.48g upward acceleration value. Until the SRB thrust decreases greatly (and the SSME thrust increases; vacuum thrust per SSME is almost 100000 lbs. greater than their sea-level thrust) the ET and SRBs are inseparable, barring an accident. In article <4712@hplabsb.UUCP>, dsmith@hplabsb.UUCP (David Smith) writes: > 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. I feel that a certain Monty Python/Holy Grail quote about swallows and coconuts is about to become relevant. :-) > 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 AW&ST said this? My figures (which I admit are somewhat old, but at least they're ballpark correct) are 181000 lbs. for an inert SRB. The amount of reactant looks about right; I show a gross liftoff weight of 1.287e6 lbs. But can that thrust be right? I've got 2.67e6 lbs. sea-level thrust per SRB. > 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. I get more like 2.08 thrust/weight per SRB at launch and 5.85 for the OV. No matter; all three propulsive components are exerting forces on the ET. > 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. This is the danger of believing your numbers when your intuition tells you otherwise. The excess of thrust over weight is more like 1.38e6 lbs. per SRB, 975000 lbs. for the OV with some cargo and 104% SSMEs, at liftoff. Yes, the OV does exert a greater force on the ET than does either of the SRBs, but *not* more than both of the SRBs! It has to be almost equally shared. My figures show about 36% of the net force on the ET coming from the OV with 32% from each of the SRBs. Of course, it's a dynamic relationship and I'm certain it changes considerably as the stack gains altitude. > 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. The total weight of the STS at liftoff is more like 4.4e6 lbs. and the total thrust is around 6.5e6 lbs. for a ratio of 1.48, which gives the initial acceleration of +0.48g I mentioned earlier. This improves very substantially as the launch progresses; it's nearer to 2.5 thrust/weight (or better) as you get near SRB separation, and the OV+ET combination gets up to 4.0 as you near MECO. Now back to the original question: > > 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? The fact that the OV is exerting a net force on the ET is not the problem. Consider what would happen if you tried separating the orbiter vehicle from the rest of the system while everything's running. First you have to shut down the SSME's and disconnect the OV from the ET propellant lines. In that time, all the thrust is from the SRB's and both the ET and the OV are dead weight. If about one fourth the ET fuel is gone, the ET weighs a total of about 1.24e6 lbs. The OV still weighs about 200000 lbs. and the SRBs may be down to about 460000 lbs. each. So the total weight of the stack is down to about 2.36e6 lbs. with a thrust of around 6.2e6 lbs. for a thrust to weight ratio of about 2.63. Cutting off the SSMEs loses around 1.2e6 lbs. thrust for a ratio of 2.12, a loss of about 0.5g. If this cutoff takes 0.1 second, that's a change of 5g/sec. Now what happens when you detach the OV from the ET? The OV suddenly loses another 2.12g of acceleration! If this separation takes 0.1 second, that's 21g/sec! The ET+SRBs combination would gain about 0.19g in this same time from the dropped mass. I think we've already seen what happens when you subject the STS to such rates of change in acceleration while it's flying: the ET disintegrates, the SRBs fly off, and the OV becomes something you stuff down an abandoned missile silo. -- Roger Noe ihnp4!att!uniq!rjnoe Fox Valley Software ihnp4!nwuxf!rjnoe Uniq Digital Technologies +1 312 510 2105 Batavia, Illinois 60510 41:50:56 N. 88:18:35 W.