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#980
by
SiameseCat
on 08 Sep, 2007 13:31
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Here's something interesting... for a shuttle with no payload but a full fuel load, attached to an empty tank, the SSMEs must run at about 56% to keep acceleration under 3g. I'm using an orbiter (fully fueled) mass of 94596 kg, an SLWT mass of 26068 kg and an SSME thrust (at 104%) of 2174290.7 N. Are my numbers wrong, or are the SSMEs really unable to keep acceleration below 3g for a depleted tank and a light payload?
Edit: After doing a bit more math, if the shuttle has a payload of about 22300kg, the SSMEs can maintain 3g acceleration at 67% thrust. For missions with lighter payloads, is ballast added to maintain this weight?
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#981
by
pippin
on 08 Sep, 2007 22:52
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Early engine shutdown (of one engine)?
How is it to be done for LON missions?
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#982
by
Jim
on 09 Sep, 2007 04:32
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Falacy is that all the ET propellants are burned. They aren't, there is more left in the ET when there is a lighter payload
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#983
by
SiameseCat
on 09 Sep, 2007 05:19
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Jim - 9/9/2007 12:32 AM
Falacy is that all the ET propellants are burned. They aren't, there is more left in the ET when there is a lighter payload
So what happens in a STS-93 situation, where a minor fuel leak causes a low-level cutoff?
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#984
by
MKremer
on 09 Sep, 2007 06:49
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OMS-1 burn if needed for underspeed, otherwise adjusted OMS-2 burn to compensate and circularize.
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#985
by
SiameseCat
on 09 Sep, 2007 12:43
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I was referring to 3g throttling (see my post above), not compensating for any underspeed at MECO.
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#986
by
mkirk
on 09 Sep, 2007 16:15
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SiameseCat - 9/9/2007 12:19 AM
Jim - 9/9/2007 12:32 AM
Falacy is that all the ET propellants are burned. They aren't, there is more left in the ET when there is a lighter payload
So what happens in a STS-93 situation, where a minor fuel leak causes a low-level cutoff?
I am not sure what you are asking with regard to the “Low Level Cut”, can you expand your question a little?
A Low Level Cut is a bad thing and at no time do you ever want to run a tank dry. In the case of 93 it was obviously not planned or expected. Although fuel (LH2) was leaking, the resultant shift in mixture caused the LO2 to run out – fortunately it happened right at the planned MECO when the engines had already throttled back and were in the process of shutting down. An underspeed of around 15-16 feet per second occurred, this was not big enough to necessitate an OMS-1 burn and the OMS-2 and subsequent burns allowed mission to continue.
Jim is right about the ET fuel loading/consumption. The ET is always filled to 100% for both LO2 and LH2. During the flight design process it is the LO2 that determines if the intended mission is achievable. So for every shuttle ascent there should always be LH2 remaining in the ET and some LO2 (with the remaining LO2 being referred to as the performance margin).
Many variables constrain the ascent; G-loading, MECO targets (velocity, range, altitude, theta), weight, cg (center of gravity) location, OMS/RCS delta V available, ascent performance margin, trajectory shape, abort capability/targets, and ET impact point are just some that readily come to mind.
The actual structural limit on ascent is about 3.5 Gs with 3 G throttling being imposed to protect that limit with a little bit of safety margin. Certified minimum power for the SSMEs is 67% (used to be 65%) so if a flight designer sees that 3 G throttling at 67% does not ensure the vehicle remains at or below 3 Gs then one of the other variables needs to be changed or the mission is not going to be flown. I don’t think the program has ever flow with ballast that was only intended for this purpose. If a violation of the G limit was the only constraint to the intended flight profile then it makes more sense to re-manifest payload items until you are within the desired weight.
Ballast (usually in the form of extra OMS/RCS propellant) is flown in order to manage CG limits.
Mark Kirkman
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#987
by
kneecaps
on 09 Sep, 2007 17:26
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What is the Major New Flight Support Software Item in the STS-120 PCRB documents? It referred to as MCR 23448 (ECO Sensor System Instrumentation Installation)?
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#988
by
mkirk
on 09 Sep, 2007 17:50
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kneecaps - 9/9/2007 12:26 PM
What is the Major New Flight Support Software Item in the STS-120 PCRB documents? It referred to as MCR 23448 (ECO Sensor System Instrumentation Installation)?
You are going to have to be more specific about which document you are referring to.
However, this is the first flight of the OI-32 software upgrade and I think the item you listed is just referencing the same ECO mod that was flown on 118. I would have to know the context in order to be sure.
Mark Kirkman
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#989
by
SiameseCat
on 09 Sep, 2007 19:27
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mkirk - 9/9/2007 12:15 PM
SiameseCat - 9/9/2007 12:19 AM
Jim - 9/9/2007 12:32 AM
Falacy is that all the ET propellants are burned. They aren't, there is more left in the ET when there is a lighter payload
So what happens in a STS-93 situation, where a minor fuel leak causes a low-level cutoff?
I am not sure what you are asking with regard to the “Low Level Cut”, can you expand your question a little?
My question was referring to the ability of a lightweight shuttle to keep acceleration below 3g before MECO. I referred to a low level cutoff to create a situation where the ET was as close to empty as possible.
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#990
by
Jim
on 09 Sep, 2007 20:31
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The mission would be designed to prevent it
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#991
by
kneecaps
on 09 Sep, 2007 22:30
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mkirk - 9/9/2007 6:50 PM
kneecaps - 9/9/2007 12:26 PM
What is the Major New Flight Support Software Item in the STS-120 PCRB documents? It referred to as MCR 23448 (ECO Sensor System Instrumentation Installation)?
You are going to have to be more specific about which document you are referring to.
However, this is the first flight of the OI-32 software upgrade and I think the item you listed is just referencing the same ECO mod that was flown on 118. I would have to know the context in order to be sure.
Mark Kirkman
Thanks Mark! The document is the STS-120 PRCB Program Freeze Point dated 07/24/7.
Page 66 of 119.
ECO sensor mod? I may must have missed this? I'm not sure if the document is referring to a software or hardware change.
Pete
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#992
by
psloss
on 09 Sep, 2007 23:28
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kneecaps - 9/9/2007 6:30 PM
The document is the STS-120 PRCB Program Freeze Point dated 07/24/7.
Page 66 of 119.
ECO sensor mod? I may must have missed this? I'm not sure if the document is referring to a software or hardware change.
Note that it says "First OV-103 implementation of..." If you go back and look at the STS-118 documentation, you'll see MCR 23448 there, as Mark already noted.
(And I believe this change was also made to OV-104 and backed out prior to STS-117.)
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#993
by
thomasafb
on 11 Sep, 2007 10:40
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Hi all,
i am looking for a detailed floorplan of the flight deck. So far, i only found a drawing without scale and even in the SCOM there are only floorplans of the mid deck and the panel overview for the flight deck. I know that there is not much floor up there since it is rather crammed, especially with the MS seats, but i'd like to have the dimensions nevertheless.
Any help would be very much appreciated.
Thomas
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#994
by
SpaceNutz SA
on 12 Sep, 2007 06:41
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Does the diameter of the HAC change from flight to flight / landing site to landing site or does it have a constant diameter for all flights? If it changes what are the factors which determine it's diameter?
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#995
by
GLS
on 12 Sep, 2007 12:46
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I think it's fixed at about 18000 ft diameter... not sure but I think if you fly the MEP (Minimum Entry Point) (low energy case), the HAC has a smaller diameter and is lower....
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#996
by
hmh33
on 12 Sep, 2007 16:17
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I thought it changed not just from flight to flight but actually during the entry process itself, adjusting depending on how much energy the orbiter is predicted to have when it reaches the HAC. The way I understand it, the HAC is basically a controlled way to dump the remaining energy in order to land at a safe speed. So if the orbiter is low on energy the HAC will become smaller, and vice versa.
I'm not sure about any of this though, would be good to hear from someone who is?
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#997
by
mkirk
on 12 Sep, 2007 16:39
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hmh33 - 12/9/2007 11:17 AM
I thought it changed not just from flight to flight but actually during the entry process itself, adjusting depending on how much energy the orbiter is predicted to have when it reaches the HAC. The way I understand it, the HAC is basically a controlled way to dump the remaining energy in order to land at a safe speed. So if the orbiter is low on energy the HAC will become smaller, and vice versa.
I'm not sure about any of this though, would be good to hear from someone who is?
I don’t know how detailed you want to get but since it is lunch time I will start with the quick and simple and will follow up if that isn’t enough.
For starters, and there are other variables involved, the HAC size is actually altitude dependent. Think of it as an upside down cone rather than a cylinder, as you decrease altitude the diameter will decrease.
Mark Kirkman
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#998
by
brahmanknight
on 12 Sep, 2007 16:48
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That's a great explanation, mkirk, but if you have more detailed explanation, please post when you get time.
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#999
by
SpaceNutz SA
on 12 Sep, 2007 18:09
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Thanks Mark - that answers another question - ie: HAC = Heading Alignment Circle/Cylinder/Cone?? I've heard all three terms used but following your explanation I guess Cone it is. Please post more when you can.
