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Space Shuttle Technical Questions  
User currently offlineLnglive1011yyz From Canada, joined Oct 2003, 1608 posts, RR: 15
Posted (5 years 10 months 3 weeks 5 days 16 hours ago) and read 5076 times:

All:

With the wealth of knowledge out there, I'm hoping I might be able to get a good discussion going here on some technical questions on the Space Shuttle vehicle.

I was fortunate enough back in May to witness a Shuttle launch FROM Kennedy Space Centre (on the Causeway), which was the efforts of many years of hoping and wishing, and attempting to see it live, which was a dream come true, and I've had a few questions that I am curious about.

1) SSME thrust rating (%'s) --> It's my understanding that the initial SSME block I engines were designed with the 109% thrust rating in mind. My question is, was this ever achieved in a flight-mode (i.e, any engine shutdowns during flight) or even for a testing mode????

2) Secondarily to the above question, I thought I read that the SSME's were, at one point, the subject of possibly reaching up to 120% of rated thrust. What were the purposes of having the engines perform at such a high thrust level? (I am aware that Vandenburg flights required a 109% thrust rating, and as such, would not be able to sustain flights until the SSME's were up to snuff later in the program)

3) IF a SSME performs a RSLS abort on the pad, and it's determined that the RSLS abort was caused by a sensor outside of the engine itself, would that necessitate a change of engine before the next launch attempt?

-- In addition, do they refurb the SSME's after each flight, or are they flight-capable again after being run?

4) On the LC 39 pads, there was an old Apollo era slide evacuation system that would evacuate the crew to a position deep under the pad. I'm curious as to whether or not this infrastructure is still in place, and whether or not it can be used in the event the Shuttle crew cannot use the slide-baskets that are in place.

I think that's it for now.. I'm hoping we can get a discussion going!

Thanks to all!

Mike/1011yyz


Pack your bags, we're going on a sympathy trip!
13 replies: All unread, jump to last
 
User currently offlineThorny From , joined Dec 1969, posts, RR:
Reply 1, posted (5 years 10 months 3 weeks 5 days 16 hours ago) and read 5060 times:



Quoting Lnglive1011yyz (Thread starter):
1) SSME thrust rating (%'s) --> It's my understanding that the initial SSME block I engines were designed with the 109% thrust rating in mind. My question is, was this ever achieved in a flight-mode (i.e, any engine shutdowns during flight) or even for a testing mode????

Testing, I am fairly sure, yes. In flight, no. The only in-flight shut downs (STS-51F and STS-93) happened too late in ascent to require ramping up the remaining engines that high.

Quoting Lnglive1011yyz (Thread starter):
2) Secondarily to the above question, I thought I read that the SSME's were, at one point, the subject of possibly reaching up to 120% of rated thrust. What were the purposes of having the engines perform at such a high thrust level? (I am aware that Vandenburg flights required a 109% thrust rating, and as such, would not be able to sustain flights until the SSME's were up to snuff later in the program)

120% was fantasy with anything close to the existing design. 106% and 109% were being developed for Shuttle-Centaur (Galileo and Ulysses in 1986) and Vandenberg launches. It was abandoned after Challenger and thereafter is an emergency option (early abort) only.

Quoting Lnglive1011yyz (Thread starter):

-- In addition, do they refurb the SSME's after each flight, or are they flight-capable again after being run?

Rocketdyne says yes. NASA still doesn't do it. Rocketdyne (now P&W/R) says the engines can go three flights without teardown, but NASA doesn't take the chance.


User currently offlineRwessel From United States of America, joined Jan 2007, 2353 posts, RR: 2
Reply 2, posted (5 years 10 months 3 weeks 3 days 16 hours ago) and read 4865 times:
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Quoting Lnglive1011yyz (Thread starter):
2) Secondarily to the above question, I thought I read that the SSME's were, at one point, the subject of possibly reaching up to 120% of rated thrust. What were the purposes of having the engines perform at such a high thrust level?

Quite simply to increase the payload to orbit (although it never happened). Even increasing the available thrust a few percent at launch will significantly increase acceleration (velocity basis), in effect saving more fuel for when the vehicle is further off the ground. The effect decreases as the mass of the vehicle decreases due to fuel burn, and as the need to fight one G worth of gravity diminishes as your horizontal velocity increases.

Think of it this was: at launch the vehicle needs to generate one G worth of thrust just to hover. Only the thrust in excess of that actually accelerates the vehicle. As somewhat overdramatic example, if your 100lb rocket generates 101lbs of thrust, its net (velocity) acceleration will be .01G. Increase the thrust to 105lbs, and you quintuple the useful acceleration (to .05G), for a mere 4% increase in fuel flow. As I mentioned, the effect diminishes as fuel burns off. By the time the vehicle mass is down to 50lbs (and assuming your still accelerating vertically), you're only increasing acceleration by 8% (1.1G vs 1.02G) for the same 4% increase in fuel flow.

Since the most fuel per unit of acceleration is burned at liftoff, small improvements there can significantly improve the capabilities of your launcher. Witness the very wide use of relatively small strap-ons.


User currently offlineThorny From , joined Dec 1969, posts, RR:
Reply 3, posted (5 years 10 months 3 weeks 2 days 22 hours ago) and read 4784 times:

Quoting Rwessel (Reply 2):
Quite simply to increase the payload to orbit (although it never happened).

Which is traced back to the Shuttle Orbiters themselves being heavier than planned. Today's orbiter was born from the 150K Orbiter concept, for a vehicle weighing 150,000 lbs, sans engines, or 168,000 lbs. with three Main Engines. But the three later "mature design" Orbiters (Discovery, Atlantis and Endeavour) are in the vicinity of 172,000 lbs. Challenger was a little heavier at around 178,000 lbs, and Columbia heavier still at over 185,000 lbs. with three SSMEs.

Heavier Orbiter = Less Payload

But NASA and the Air Force had been planning missions based on the 150K orbiter and its 65,000 lbs. design payload capacity, particularly the deep space probes Galileo and ISPM (later Ulysses) planned to be launched by a Shuttle using the heavy LOX/LH2 Centaur upper stage, and the polar orbit spy satellite flights out of Vandenberg (which gets no payload benefit from Earth's rotational velocity.)

Thus was born the idea of increasing Main Engine performance. The SSMEs had already proven a little more capable than their initial design, and were already operating 104% of original design performance and had bought back some of the payload lost to the heavier airframe. Improvements to 106% and 109% were incremental, originally intended for abort capability, but soon considered for operational use with Shuttle-Centaur and Vandenberg. The proposed 120% improvement would have required a massive redesign and was much farther down the road.

Ultimately, NASA gave up on major SSME changes (as they would again for the Ares program in 2006) because the engine is too complex (although they did make many safety improvements post-Challenger and again in the 1990s with the Block IA and Block II Main Engine projects.) Instead, they proposed an improved Solid Rocket Booster, the Advanced Solid Rocket Motor. That was eventually cancelled circa 1993 as too expensive and was replaced by the Super Lightweight (External) Tank, which did buy back about 7,000 lbs. of payload capacity. (This proved most useful when overweight Columbia launched the Chandra X-Ray Observatory in 1999, the heaviest payload ever launched by the Space Shuttle.)

[Edited 2008-11-01 12:04:12]

User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6451 posts, RR: 54
Reply 4, posted (5 years 10 months 2 weeks 6 days 20 hours ago) and read 4513 times:



Quoting Lnglive1011yyz (Thread starter):
It's my understanding that the initial SSME block I engines were designed with the 109% thrust rating in mind. My question is, was this ever achieved in a flight-mode (i.e, any engine shutdowns during flight) or even for a testing mode????



Quoting Lnglive1011yyz (Thread starter):
I thought I read that the SSME's were, at one point, the subject of possibly reaching up to 120% of rated thrust.

Please don't put too much into those percentage numbers. It all depends upon how 100% is defined.

The main engines are throttled during the launch. (The strap on boosters have a similar variable thrust profile). If I am well informed, then the 100% is defined as take off thrust. But at that time the engines have to fight against the one bar ambient atmopheric pressure.

Laster, as speed and drag builds up, the engines are throttled down somewhat to preserve fuel in the high drag. When above most of the atmosphere, then drag becomes insignificant and the engines work better in the near vacuum of space. Consequently 105 - 110% coresponds better to max performance. Later again, when the shuttle has lost much of its weight (burned much of the fuel), then the engines are throttled back once more to keep G forces on the crew within fairly comfortable limits. (And within Shuttle structural limits).

But surely NASA has varied those engine performance profiles over the years. They have monitored how well the engines survived each launch and adjusted max payload to what they considered "safe performance". And since it's expensive stuff, and there are humans on board, then they always played in a somewhat conservative way.

To my knowledge there has never been a really serious main engine malfunction.

Our car engine has a very well defined max performance when the pedal is at the bottom. Not so with turbine- or rocket engines. With turbine and rocket engines you can in principle just inject more fuel until the engine melts or explodes.

Consequently you design an engine, test it in a laboratory environment, then decide upon a "safety margin", and finally adjust the fuel injection system in accordance with your best data. Then you have a max thrust.

Finally, on re-useable engines like on the Shuttle you may fine tune the fuel injection in accordance with operational experience.

On turbine engines the whole issue is in priciple a lot simpler. Only a minor fraction of the oxygen is burned in the combustion chambers. The hot flame is mixed with the cold air before it hits the turbine disk. The more fuel you pump in, the higher thrust and the higher the exhaust gas temperature (EGT). There is wast experience about how various turbine blade designs survive various EGTs over time. So you probably first decide upon a time span between engine overhaul, then you automatically get a max EGT, and you adjust the fuel injection for max trust accordingly.

In fact you often have two turbine engine variants which are technically completely identical, but with different max thrust and different time between engine overhaul. The more powerful engine has a shorter on wing time. Only difference is a single figure in the engine control software.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineThorny From , joined Dec 1969, posts, RR:
Reply 5, posted (5 years 10 months 2 weeks 6 days 19 hours ago) and read 4501 times:



Quoting Prebennorholm (Reply 4):
If I am well informed, then the 100% is defined as take off thrust. But at that time the engines have to fight against the one bar ambient atmopheric pressure.

No. The engines do produce more thrust in vacuum that at sea level, but this is not reflected in the throttle settings.

Quoting Prebennorholm (Reply 4):
To my knowledge there has never been a really serious main engine malfunction.

Two in-flight incidents:

STS-51F Challenger (July 1985)
Faulty sensor shuts down one Main Engine at T+6 mins. Quick-thinking ground controller ordered the Challenger pilot to disable auto-shutdown of the other two engines only seconds before another faulty sensor would have shut down a second engine and forced a Transoceanic Abort Landing (TAL.)

STS-93 Columbia (July 1999)
Engine LOX cap "plug" breaks off during ignition and Columbia loses a small amount of LOX throughout ascent, resulting in low-LOX ECO sensor triggering shutdown about one second earlier than planned and a slightly lower orbit than planned.


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6451 posts, RR: 54
Reply 6, posted (5 years 10 months 2 weeks 6 days 17 hours ago) and read 4474 times:



Quoting Thorny (Reply 5):
The engines do produce more thrust in vacuum that at sea level, but this is not reflected in the throttle settings.

I assume that what you are telling is that when the Shuttle main engines are operating at, say, 109%, then it is not just telling that it is operating more efficiently in vacuum, but also that it is operating with a higher fuel burn rate than at 100% at sea level.

Also thanks for the information about two incidents. It's a question about taste whether they shall be called "major malfunctions". In any case, with over a hundred launches - that's well over 300 operational engine burns - those two incidents tell us about incredably reliable engines when taking the complexity into acount. That's very well done. But it hasn't been cheap either.

Dear Thorny, you sure have studied the Shuttle in detail!!! Tnx for sharing. You were probably also glued to the TV back in 1981 when John Young ran down the ladder after landing Columbia STS-1 at Edwards and ran to the nose of Columbia to see how she had survived re-entry before he had time to shake hands with his ground crew.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineOroka From Canada, joined Dec 2006, 913 posts, RR: 0
Reply 7, posted (5 years 10 months 2 weeks 6 days 15 hours ago) and read 4445 times:



Quoting Prebennorholm (Reply 6):
I assume that what you are telling is that when the Shuttle main engines are operating at, say, 109%, then it is not just telling that it is operating more efficiently in vacuum, but also that it is operating with a higher fuel burn rate than at 100% at sea level.

The extra % in throttling is from upgrades to the SRBs. The original SRBs could do 100%, but the new boosters had more power, so they do 107% (IIRC) of the original boosters thrust. IIRC there was another upgrade to the boosters that didnt happen that would have pushed the boosters up to 110%.


User currently offlineThorny From , joined Dec 1969, posts, RR:
Reply 8, posted (5 years 10 months 2 weeks 6 days 14 hours ago) and read 4441 times:



Quoting Prebennorholm (Reply 6):
I assume that what you are telling is that when the Shuttle main engines are operating at, say, 109%, then it is not just telling that it is operating more efficiently in vacuum, but also that it is operating with a higher fuel burn rate than at 100% at sea level.

Basically, the throttle settings are turbopump settings/capabilities, not the amount of thrust coming out the nozzle (which varies with atmospheric pressure.) 100% at sea level is going to be less thrust than 100% in vacuum, but the throttles are still set to 100%.

Quoting Prebennorholm (Reply 6):
You were probably also glued to the TV back in 1981 when John Young ran down the ladder after landing Columbia STS-1 at Edwards and ran to the nose of Columbia to see how she had survived re-entry before he had time to shake hands with his ground crew.

Sure was! And I was on the NASA Causeway East at Kennedy Space Center for the launch!
Got up at 3am both on Friday, April 10 (for the scrubbed launch attempt) and again on Sunday, April 12 for the actual launch. I got very little sleep that weekend!

Quoting Oroka (Reply 7):

The extra % in throttling is from upgrades to the SRBs. The original SRBs could do 100%, but the new boosters had more power, so they do 107% (IIRC) of the original boosters thrust. IIRC there was another upgrade to the boosters that didnt happen that would have pushed the boosters up to 110%.

No, sorry, none of this is correct.

The "new boosters", introduced after the Challenger Accident, are fundamentally the same boosters as those before... same thrust, same amount of propellant. What changed was the field joint design (the "capture feature" was added) and other safety improvements. The original SRBs were modified to the new safety standards. Since the modified boosters were a little heavier, they actually are slightly less powerful than the earlier models.

The only other new SRB program was the Advanced Solid Rocket Motor, canceled in 1993. ASRM would have increased thrust, and added about 8,000 lbs. of payload capacity to the Shuttle, but more importantly would have eliminated the need to throttle the Main Engines at Max Q, which was then and still is now considered one of the highest risk operations of the launch.


User currently offlineDfwRevolution From United States of America, joined Jan 2010, 977 posts, RR: 51
Reply 9, posted (5 years 10 months 2 weeks 6 days 10 hours ago) and read 4407 times:



Quoting Thorny (Reply 8):
ASRM would have increased thrust, and added about 8,000 lbs. of payload capacity to the Shuttle, but more importantly would have eliminated the need to throttle the Main Engines at Max Q, which was then and still is now considered one of the highest risk operations of the launch.

Can you expand on this MaxQ point Thorny? I would have figured that a more powerful SRB would require that the SSME throttle down more dramatically to limit aerodynamic forces on the vehicle at Max Q.

What is particularly risky about that point of the launch, just the fact that the SSME turbopumps are twitchy and the vehicle still has relativly low glide energy if an abort were required?


User currently offlineThorny From , joined Dec 1969, posts, RR:
Reply 10, posted (5 years 10 months 2 weeks 6 days 3 hours ago) and read 4345 times:



Quoting DfwRevolution (Reply 9):
Can you expand on this MaxQ point Thorny? I would have figured that a more powerful SRB would require that the SSME throttle down more dramatically to limit aerodynamic forces on the vehicle at Max Q.

The existing SRBs already have their propellant "shaped" to change the amount of thrust they provide at different points in the ascent. The SRBs burn from a central vertical core outward, and the shape of that core (simply circular at some points, many-sided-star-shaped at others) changes how much of the propellant is burning at one time. The ASRM would have taken the technique considerably further and had a shape that the 40% SSME throttle-down from T+20 to T+30 seconds or so would not have been necessary. The ASRM would also have featured one less field joint than the current SRB. ASRM was about equal parts performance improvement and safety improvement.

Quoting DfwRevolution (Reply 9):
What is particularly risky about that point of the launch, just the fact that the SSME turbopumps are twitchy and the vehicle still has relativly low glide energy if an abort were required?

The danger that the SSMEs might not throttle back up, which would force a contingency abort (bailout.) Don't forget that the SSME was the first large throttleable rocket engine, and they're still a fickle beast. The upgraded engines introduced piece-by-piece in the 1990s went a long way toward improving the SSME safety (which is probably why there have been no RSLS T-0 aborts since the engines were updated.)


User currently offlinePrebennorholm From Denmark, joined Mar 2000, 6451 posts, RR: 54
Reply 11, posted (5 years 10 months 2 weeks 5 days 22 hours ago) and read 4291 times:



Quoting Thorny (Reply 8):
Sure was! And I was on the NASA Causeway East at Kennedy Space Center for the launch!
Got up at 3am both on Friday, April 10 (for the scrubbed launch attempt) and again on Sunday, April 12 for the actual launch. I got very little sleep that weekend!

I'm envious.

That's what comes from living five thousand miles away.



Always keep your number of landings equal to your number of take-offs, Preben Norholm
User currently offlineLnglive1011yyz From Canada, joined Oct 2003, 1608 posts, RR: 15
Reply 12, posted (5 years 10 months 2 weeks 1 day 13 hours ago) and read 4045 times:



Quoting Thorny (Reply 10):
The danger that the SSMEs might not throttle back up, which would force a contingency abort (bailout.) Don't forget that the SSME was the first large throttleable rocket engine, and they're still a fickle beast. The upgraded engines introduced piece-by-piece in the 1990s went a long way toward improving the SSME safety (which is probably why there have been no RSLS T-0 aborts since the engines were updated.)

I've also found, just by looking at history, and looking at lots of stuff in the book "Space Shuttle, The History of the National Space Transportation System (VERY detailled book) that since the Challenger accident, and the Block IIA upgrades, the whole launch system has been upgraded and more reliable since then.

Most of the aborts or launch scrubs have been from inclement weather, or actually been scrubbed prior to launch DAY.

Quoting Prebennorholm (Reply 11):

I'm envious.

That's what comes from living five thousand miles away.

Prebennorholm, make the trek. I fulfilled a lifelong dream earlier this year by being at the same spot Thorny was when he saw the STS-1 launch, and I can tell you, it's worth the risk (tickets are non refundable or reusable once you're on the bus to the launch site), and it's worth the trek. There is NOTHING on this planet that compares with a launch of the shuttle.

I'm urgently attempting to arrange my 2009 vacation schedule around a few launches, so I can try to make it there.

1011yyz



Pack your bags, we're going on a sympathy trip!
User currently offlineChecksixx From United States of America, joined Mar 2005, 1095 posts, RR: 0
Reply 13, posted (5 years 10 months 1 week 1 day 11 hours ago) and read 3781 times:

As far as the point of launch goes...when the boosters are lit off, the SSME's are at 104% of rated thrust. Shortly after, before passing through maxQ, they are throttled down...and when the call 'go at throttle up' comes, they have already been throttled back up to 104% and that is a comfirmation call.

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