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#160
by
sdsds
on 06 Oct, 2012 19:47
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A payload closer to the M4,2 limits would not have made it.
It's probably fair to say a payload
at the M4,2 limit would not have made it. But do we know yet if the second stage burned to depletion? The achieved orbit is so good it looks like maybe it had propellant to spare, in which case a payload slightly closer to the M4,2 limit might also have been delivered successfully.
But the main point is that the USAF knew the mass of their payload and knew the capability of the launcher and
didn't try to squeeze more payload mass onto the flight. They paid for margin, and yes "luckily" this anomaly was of the type where that margin could be applied to make up the shortfall.
[P.S.: If I were a launch service provider this is definitely the kind of customer I would want to have!]
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#161
by
edkyle99
on 07 Oct, 2012 00:19
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It's probably fair to say a payload at the M4,2 limit would not have made it. But do we know yet if the second stage burned to depletion?
No, the second stage did not burn to depletion. It reached the planned orbit, and it has been tracked in a proper disposal orbit that could only have been reached through the addition of some subsequent delta-v.
Assuming that the problem did not affect specific impulse substantially, a few percent (5-6%?) lower thrust, causing a few percent longer burns, would likely have only required a few more meters per second delta-v. The biggest problem, if it is a problem, would be the late arrival of the payload to its orbit, placing it behind its planned insertion point - something that would not be a big deal to subsequently adjust.
The RL10 expander cycle engine has, or once had, a thrust control valve - even for fixed thrust setups - that was used to handle start up thrust transients. An issue with that thingy is a possibility, I think.
- Ed Kyle
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#162
by
Chris Bergin
on 07 Oct, 2012 08:17
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#163
by
avollhar
on 07 Oct, 2012 11:47
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It's probably fair to say a payload at the M4,2 limit would not have made it.
For the Ariane 5 I am pretty sure that any 'missing' payload mass is topped off with balance weights. I know how stupid this sounds but redoing the math for lighter payload is much more work than bolting some iron weights.Just think about the moved centre of gravity when having a lighter payload AND fueling less...
By adding weight you homogenize your ascent profile.. And that is where your internal IMU can sense any performance shortfalls and compensate by burning longer. Apparently that happened with the RL10-2.
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#164
by
ugordan
on 07 Oct, 2012 12:00
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Just think about the moved centre of gravity when having a lighter payload AND fueling less...
That doesn't make much sense to me. Why would you want to offload propellant if your payload is lighter? Why not carry more propellant instead of iron weights so you increase propellant margins?
By adding weight you homogenize your ascent profile.. And that is where your internal IMU can sense any performance shortfalls and compensate by burning longer. Apparently that happened with the RL10-2.
IMU senses the delta V achieved. It doesn't need a homogenous ascent profile to do be able to compensate for a shortfall, it has its altitude/velocity targets and will work toward that goal. Depending on how adaptive the guidance is, it may miss the precise perigee/apogee target in severe underburn/overburn conditions, but AFAIK that would happen even in an "standard" ascent profile with thrust profiles deviating from expected.
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#165
by
avollhar
on 07 Oct, 2012 15:30
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I was as surprised as you.. but people working at CSG confirmed it to me.
Think it through: less payload means less weight. Your center of gravity shifts. This might(will) influence your flight control algorithm.
Second: less mass means more acceleration. While your IMU can/should compensate for it (as it did for the Delta IV) being a launch provider I would hesitate to generate a new thrust profile for each payload mass. Just imagine the required certification (and hence cost) for each new flight profile..
It is certainly not reasonable to use dead balance weights, but it is the most cost efficient solution. Having an IMU for fine tuning SECO to enhance injection precision is still priceless.. as it can compensate for quirks as just seen.
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#166
by
Targeteer
on 07 Oct, 2012 15:59
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I'm going with lucky. A payload closer to the M4,2 limits would not have made it. It's not like hardware is assigned thinking, "There might be a problem with this engine, so let's put it on a high-margin mission."
But, fortune smiled and that's how it came out. If it's a fleet problem, it manifested itself on a mission where it didn't matter. The investigation should make the fleet more robust.
So the M4,2 had extra capacity, would the M4 have had enough--assuming a nominal RL-10 burn?
This question may require the exact details of the problem but what have happened if this was the NROL-15 flight? From the discussions so far, NRO dodged a bullet and is breathing a huge sigh of relief...
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#167
by
Star One
on 07 Oct, 2012 19:53
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I'm going with lucky. A payload closer to the M4,2 limits would not have made it. It's not like hardware is assigned thinking, "There might be a problem with this engine, so let's put it on a high-margin mission."
But, fortune smiled and that's how it came out. If it's a fleet problem, it manifested itself on a mission where it didn't matter. The investigation should make the fleet more robust.
So the M4,2 had extra capacity, would the M4 have had enough--assuming a nominal RL-10 burn?
This question may require the exact details of the problem but what have happened if this was the NROL-15 flight? From the discussions so far, NRO dodged a bullet and is breathing a huge sigh of relief...
But how close was NROL - 15 really to the edge of the capacity of the uprated DIVH though?
As we know very little about this payload we don't know if in that flight there wasn't also a calculated margin of safety should anything like this happen which would still have meant it was delivered on orbit correctly.
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#168
by
robertross
on 08 Oct, 2012 00:29
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#169
by
ugordan
on 08 Oct, 2012 01:31
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What an artist. Amazing images
Indeed. He always leaves me wanting higher resolution versions.
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#170
by
Antares
on 08 Oct, 2012 02:39
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Wait, avhollar, you're saying Ariane doesn't reanalyze trajectory for each mission?
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#171
by
avollhar
on 08 Oct, 2012 05:20
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Their ascent profile is remarkably uniform. Always 36000x250km, 5 deg inclination, argument of perigee around 176 (or was it 178) deg.They will certainly update upper wind data though.
Do you recall how tiny the center-of-gravity allowed region for the space shuttle was? Given that, bolting some weights to the payload adapter is much more reasonable than ending up with 20% fuel left at your destination orbit.
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#172
by
Antares
on 08 Oct, 2012 14:53
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I doubt ATV is launched into 36000x250 or an environmental satellite.
Just because Shuttle was a finicky system doesn't mean others are. I'd rather not ballast and be able to fly through problems like Delta IV or Falcon 9 did, or develop an algorithm to use that extra propellant, or have a disposal burn that gets the stage out of the operational orbit.
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#173
by
Jim
on 08 Oct, 2012 16:42
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Do you recall how tiny the center-of-gravity allowed region for the space shuttle was? Given that, bolting some weights to the payload adapter is much more reasonable than ending up with 20% fuel left at your destination orbit.
Totally unrelated. Shuttle CG limits were for landing not ascent. Shuttle ascent was just as adaptive as Delta or Atlas.
And no, adding ballast is not more reasonable.
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#174
by
Jim
on 08 Oct, 2012 17:01
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A payload closer to the M4,2 limits would not have made it.
It's probably fair to say a payload at the M4,2 limit would not have made it. But do we know yet if the second stage burned to depletion? The achieved orbit is so good it looks like maybe it had propellant to spare, in which case a payload slightly closer to the M4,2 limit might also have been delivered successfully.
But the main point is that the USAF knew the mass of their payload and knew the capability of the launcher and didn't try to squeeze more payload mass onto the flight. They paid for margin, and yes "luckily" this anomaly was of the type where that margin could be applied to make up the shortfall.
[P.S.: If I were a launch service provider this is definitely the kind of customer I would want to have!]
Using Gunter's info.
Atlas V 401 is to be used to deliver the next GPS. Its GTO performance is 4950 kg. Delta IV M+ (4,2) capability to GTO is 5845 kg. A DIV M was not used since it does not have the capability to meet the mission requirements (it will with an RS-68A)
So that should give an indication of the margin that this mission had.
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#175
by
Lurker Steve
on 08 Oct, 2012 17:39
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A payload closer to the M4,2 limits would not have made it.
It's probably fair to say a payload at the M4,2 limit would not have made it. But do we know yet if the second stage burned to depletion? The achieved orbit is so good it looks like maybe it had propellant to spare, in which case a payload slightly closer to the M4,2 limit might also have been delivered successfully.
But the main point is that the USAF knew the mass of their payload and knew the capability of the launcher and didn't try to squeeze more payload mass onto the flight. They paid for margin, and yes "luckily" this anomaly was of the type where that margin could be applied to make up the shortfall.
[P.S.: If I were a launch service provider this is definitely the kind of customer I would want to have!]
Using Gunter's info.
Atlas V 401 is to be used to deliver the next GPS. Its GTO performance is 4950 kg. Delta IV M+ (4,2) capability to GTO is 5845 kg. A DIV M was not used since it does not have the capability to meet the mission requirements (it will with an RS-68A)
So that should give an indication of the margin that this mission had.
I assume the Atlas 401 costs the DOD less than a Delta IV M+. Any insight why they chose the more expensive launcher for this one ?
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#176
by
kevin-rf
on 08 Oct, 2012 17:58
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Availability? Atlas and Delta are booked solid.
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#177
by
Star One
on 08 Oct, 2012 19:03
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Availability? Atlas and Delta are booked solid.
But performance wise wasn't it overkill for its payload and costly overkill at that?
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#178
by
Xplor
on 09 Oct, 2012 11:00
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Wait, avhollar, you're saying Ariane doesn't reanalyze trajectory for each mission?
Their ascent profile is remarkably uniform. Always 36000x250km, 5 deg inclination, argument of perigee around 176 (or was it 178) deg.They will certainly update upper wind data though.
EELV's do not have this restriction. EELV's fly to every orbit that customers have dreamed up uses for satellites, LEO, MEO, GTO, GSO, Earth escape and many more. Just look at the past EELV launches.
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#179
by
pippin
on 09 Oct, 2012 11:06
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It's not a restriction. Ariane 5 has been flying all kinds of profiles, too.
It's a simplification that probably helps with cost and reliability of delivering comsats to GTO which is Ariane 5's main business. Something EELVs don't have to bother about since they are not competitive there anyway...
Just kidding. I could bet, EELVs are doing similar things for common flight profiles.
