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#1140
by
Jorge
on 17 Oct, 2007 21:12
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Danny Dot - 17/10/2007 2:40 PM
swhitt - 17/10/2007 12:56 PM
snip
Are all NCx burns RCS burns?
snip
If I recall correctly the break over to use the OMS is about 4 feet/second on the burn amount.
Close. Multiaxis RCS is used for less than 4 fps, +X RCS between 4 and 6 fps, and OMS for greater than 6 fps.
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#1141
by
Jorge
on 18 Oct, 2007 01:07
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tnphysics - 17/10/2007 8:00 PM
Why?
Why what?
There are thousands of questions answered in this thread. You should quote the post you're replying to.
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#1142
by
tnphysics
on 18 Oct, 2007 01:09
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Jorge - 17/10/2007 5:12 PM
Danny Dot - 17/10/2007 2:40 PM
swhitt - 17/10/2007 12:56 PM
snip
Are all NCx burns RCS burns?
snip
If I recall correctly the break over to use the OMS is about 4 feet/second on the burn amount.
Close. Multiaxis RCS is used for less than 4 fps, +X RCS between 4 and 6 fps, and OMS for greater than 6 fps.
Why?
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#1143
by
tnphysics
on 18 Oct, 2007 01:11
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What happens if the throttle down fails?
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#1144
by
Jorge
on 18 Oct, 2007 01:33
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tnphysics - 17/10/2007 8:09 PM
Jorge - 17/10/2007 5:12 PM
Danny Dot - 17/10/2007 2:40 PM
swhitt - 17/10/2007 12:56 PM
snip
Are all NCx burns RCS burns?
snip
If I recall correctly the break over to use the OMS is about 4 feet/second on the burn amount.
Close. Multiaxis RCS is used for less than 4 fps, +X RCS between 4 and 6 fps, and OMS for greater than 6 fps.
Why?
OMS is generally more propellant-efficient than RCS (Isp is higher), but OMS guidance runs open-loop for the last six seconds of a burn to avoid large guidance transients. For very short burns, this can result in large residuals that must be trimmed out with the RCS. Below 6 fps it is more efficient to just do the whole burn with the RCS. OMS engine lifetime is rated in number of starts so this also reduces unnecessary starts.
FRCS propellant is typically more limited than ARCS, so it is desirable to limit the amount of FRCS propellant used during an RCS burn. Above a certain point, it is more efficient to maneuver to +X RCS attitude, even though the maneuver itself uses some FRCS peopellant. MCC uses the criteria of 2 fps FRCS propellant to decide between multiaxis and +X, but the crew has no easy way to determine the FRCS contribution to a burn, so it is simpler to just draw the line at a total delta-V of 4 fps.
These are all guidelines and can be overridden by MCC depending on propellant margins and/or system malfunctions.
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#1145
by
Jorge
on 18 Oct, 2007 01:42
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tnphysics - 17/10/2007 8:11 PM
What happens if the throttle down fails?
The shuttle will accelerate too quickly, resulting in max-Q above load limits, possibly resulting in structural damage or loss of vehicle. If all three SSMEs fail to throttle down, Flight Rules require the crew to take manual throttle control. If one SSME fails to throttle down, no action is required. If two SSMEs fail to throttle down during second stage 3-g throttling, the crew is required to manually shut down one of the stuck engines, but is not required to take action during first stage.
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#1146
by
tnphysics
on 18 Oct, 2007 02:08
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Would it be more than 1.4 times max nominal load?
This is the FOS.
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#1147
by
Jorge
on 18 Oct, 2007 02:16
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tnphysics - 17/10/2007 9:08 PM
Would it be more than 1.4 times max nominal load?
This is the FOS.
Depends on how many engines don't throttle down. The flight rules I cited should give you a clue.
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#1148
by
swhitt
on 18 Oct, 2007 03:19
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Jorge - 17/10/2007 2:12 PM
Close. Multiaxis RCS is used for less than 4 fps, +X RCS between 4 and 6 fps, and OMS for greater than 6 fps.
All of this is excellent information. Thank you very much.
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#1149
by
usn_skwerl
on 18 Oct, 2007 08:15
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what is the orbiter's nominal angle relative to horizon at SRB SEP? (i know it's inverted)
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#1150
by
kimmern123
on 18 Oct, 2007 10:09
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I'm not an expert, but I think the angle is about 18 degrees relative to the horizon. I'm sure there are many other people here that know more than I do.
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#1151
by
mkirk
on 18 Oct, 2007 14:19
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usn_skwerl - 18/10/2007 3:15 AM
what is the orbiter's nominal angle relative to horizon at SRB SEP? (i know it's inverted)
Kimmern123 is correct, the pitch or theta is about 18-20 degrees. It varies from flight to flight by a degree or so. I should point out that pitch angle relative to the horizon is not as important as angle of attack (or alpha) - which is the angle between the wing chord line and the ralative wind. Proper anlge or attack is critical to ensuring a clean seperation.
A good place to find the approximate pitch angle for a given flight would be in the Ascent Checklist for the specific mission. It would be listed on the ADI Cue Card (page 2-28 ish) , under the second column (theta) after the line labled staging (this would be an approximation of the theta). These checklists are available here on Chris' site and usually available on the public NASA site.
Mark Kirkman
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#1152
by
SiameseCat
on 19 Oct, 2007 02:00
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One of the methods for targeting OMS burns is called PEG4, where C1, C2, HT and thetaT are entered, and these values specify the target orbit (I think). Does anyone know what these inputs actually mean ?
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#1153
by
mkirk
on 19 Oct, 2007 14:40
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SiameseCat - 18/10/2007 9:00 PM
One of the methods for targeting OMS burns is called PEG4, where C1, C2, HT and thetaT are entered, and these values specify the target orbit (I think). Does anyone know what these inputs actually mean ?
In the PEG 4 (powered explicit guidance) targeting scheme C1 and C2 are the slope and intercept that define the relationship between the horizontal and vertical components of the burn velocity.
V(vertical ) = C2 V(horizontal) + C1
HT is the height above the earth in nautical miles.
Theta T is the angular location of the target in degrees. For OPS 1 burns (such as OMS 1 an OMS2) this is the downrange angle from the launch site to the radius target vector. For OPS3 burns such as the deorbit burn it is the angle from the point of ignition (or TIG) to Entry interface.
I might have some notes or documents that explain this better, if I do I will post them here.
Mark Kirkman
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#1154
by
dawei
on 19 Oct, 2007 15:20
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What is the "lifetime" of an OMS engine, measured in engine starts?
How many OMS engines are in the current inventory?
Are the OMS engines integral to the OMS pods or can an engine be changed out without removing/changing the pod?
Thanks in advance.
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#1155
by
C5C6
on 19 Oct, 2007 15:45
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while ETs are delivered to KSC by barge, how would it have been delivered to VAFB???
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#1156
by
DaveS
on 19 Oct, 2007 15:48
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C5C6 - 19/10/2007 5:45 PM
while ETs are delivered to KSC by barge, how would it have been delivered to VAFB???
Barge.
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#1157
by
C5C6
on 19 Oct, 2007 15:57
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DaveS - 19/10/2007 12:48 PM
C5C6 - 19/10/2007 5:45 PM
while ETs are delivered to KSC by barge, how would it have been delivered to VAFB???
Barge.
then it should have gone through the panama canal??? that would have taken weeks!!!
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#1158
by
DaveS
on 19 Oct, 2007 17:02
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C5C6 - 19/10/2007 5:57 PM
DaveS - 19/10/2007 12:48 PM
C5C6 - 19/10/2007 5:45 PM
while ETs are delivered to KSC by barge, how would it have been delivered to VAFB???
Barge.
then it should have gone through the panama canal??? that would have taken weeks!!!
Bingo. That is the route that it would have taken.
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#1159
by
Namechange User
on 19 Oct, 2007 17:17
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dawei - 19/10/2007 10:20 AM
What is the "lifetime" of an OMS engine, measured in engine starts?
How many OMS engines are in the current inventory?
Are the OMS engines integral to the OMS pods or can an engine be changed out without removing/changing the pod?
Thanks in advance.
The OMS engines are quite robust and we do a maintenance cycle on the during the orbiter OMDP. The are removed from the pod and sent to White Sands for this. There is no limit on starts.
We have enough engines for the entire fleet with approximately 4 spares. The engines are obviously integrated onto the pod but they are there own seperate LRU (Line Replaceable Unit). Yes we can remove and change an engine if necessary without removing the pod. We have done this several times in the past in the OPF.