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#1000 by Danny Dot on 13 Sep, 2007 16:22
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mkirk - 12/9/2007 11:39 AM
Quotehmh33 - 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
Very true. Normally the diameter of the HAC is just a function of altitude. This is done because at higher altitude the shuttle is flying a higher true airspeed and needs more room to turn. The HAC is a cone.
There is also a provision that if the shuttle gets very low energy, the HAC will also shrink. As far as I know this has happened twice in the history of the shuttle.
Danny Deger -
#1001 by GLS on 13 Sep, 2007 19:02
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Danny Dot - 13/9/2007 5:22 PM
There is also a provision that if the shuttle gets very low energy, the HAC will also shrink. As far as I know this has happened twice in the history of the shuttle.
That's the MEP (Minimum Entry Point), and I think the A/L interface goes down to 5Kft in those cases...
Do you know which 2 missions had the MEP??? STS 37??? -
#1002 by mkirk on 13 Sep, 2007 22:30
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GLS - 13/9/2007 2:02 PM
QuoteDanny Dot - 13/9/2007 5:22 PM
There is also a provision that if the shuttle gets very low energy, the HAC will also shrink. As far as I know this has happened twice in the history of the shuttle.
That's the MEP (Minimum Entry Point), and I think the A/L interface goes down to 5Kft in those cases...
Do you know which 2 missions had the MEP??? STS 37???
NO! I am afraid you are confusing a couple of different concepts. So let me attempt to make this as clear as mud.
Space Shuttle Landing 201
Prerequisites: Space Shuttle Landing 101*
(*Please note Shuttle Landing 101 is not offered during the Fall, Spring, or Summer Semesters, so plan accordingly when attempting to satisfy this requirement.)
Nominally the HAC is an imaginary, upside down cone, located about 7 nautical miles away from the runway and tangent to the runway centerline (i.e. the approach path). This establishes a Nominal Entry Point (NEP) at 12,000 feet and 7 miles out.
As Danny indicated, the HAC is an energy management tool that is intended to allow the orbiter to “roll out” on final at the appropriate altitude, distance, and airspeed.
Getting to the original question, about HAC size and diameter, the size is a direct function of altitude which means as the altitude decrease the diameter decreases. If you were to look at the actual ground track of the orbiter as it flies around the HAC it would appear as a spiral if it were drawn on the ground.
The angle formed between the points where the orbiter enters the HAC and the point where it is lined up with the runway (NEP or MEP) is known as the HAC Turn Angle (HTA). A large HTA, let’s say over 300 degrees, will result in a larger diameter HAC to allow the orbiter to bleed off the extra altitude and airspeed.
In a situation where the orbiter is low on energy when it arrives at and flies around the HAC the crew has the option of selecting a Minimum Entry Point (MEP) HAC which basically moves the HAC closer to the runway (about 4 nautical miles instead of the nominal 7).
Do not confuse NEP and MEP with the Nominal and Close-In aim points. Aim points are located on the ground in front of the runway and help establish the intended glide slope. The Nominal aim point is located about 7500 feet in front of the runway while the Close-In aim point is located 6000 feet in front of the runway. Normally the orbiter is targeted to touchdown on the runway at 2500 feet past the runway threshold at either 195 or 205 knots (depending on weight), if the orbiter is predicted to touchdown at less than 1000 feet from the threshold then the crew will select the Close-In aimpoint.
All of these changes (MEP vs NEP, Nominal vs Close-In, etc) are made by typing in specific item entries to the SPEC 50 Horizontal Situation Display of the General Purpose Computers. (see the attached photo below Item Entry 7 and 8 respectively)
As for the HAC shrink that Danny Dot referred too, that is not the same thing as MEP. HAC shrink would occur if the orbiter was very low in altitude during the HAC phase. The orbiter’s guidance system would shrink the diameter of the HAC in an effort to decrease the total range the orbiter still has to fly to get to the runway. The magnitude of this shrink adjustment is dependent on the HTA remaining and the amount of altitude error (i.e. how low the orbiter is compared to the planned trajectory). If the orbiter has an HTA of less than 90 degrees then the shrink function is inhibited.
Here are some "ROUGH" numbers for the approximate altitude of the orbiter at a given point on the HAC (HTA) - Nominally.
360 - 50,000 feet
270 - 38,000
180 - 28,000
90 - 20,000
0 - 12,000
Mark Kirkman
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#1003 by GLS on 14 Sep, 2007 00:20
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Hi mkirk, thanks for that piece of bible!!!!
I wasn't even thinking about the aim points (I was still in the HAC).... I didn't knew of the *automatic* HAC shrinkage... is that what happened on the 2 missions instead of MEP???
Also, one small question (small answer will do), on MEP the HAC is closer to the runway but also lower, right?! Those altitudes you give above would be lower and A/L I/F would also be lower, to keep the OGS at 18-20 degrees, right?!
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#1004 by mkirk on 14 Sep, 2007 02:06
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GLS - 13/9/2007 7:20 PM
Hi mkirk, thanks for that piece of bible!!!!
I wasn't even thinking about the aim points (I was still in the HAC).... I didn't knew of the *automatic* HAC shrinkage... is that what happened on the 2 missions instead of MEP???
Also, one small question (small answer will do), on MEP the HAC is closer to the runway but also lower, right?! Those altitudes you give above would be lower and A/L I/F would also be lower, to keep the OGS at 18-20 degrees, right?!
I new that you were not confusing NEP/MEP with the aimpoints, I was just making a general statement for people not to confuse them because they often do.
I don't know what happened on the missions Danny is talking about, I will have to research that.
I have attached a diagram that should show you how the entry points line up. In this case with both the NEP and MEP using the same aimpoint (nominal 7500 feet) the glide slope geometry is the same and a 12,000 foot nominal entry point (at t nautical miles) lines up with a 6000 foot minimum entry point (at 4 nautical miles).
Mark Kirkman
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#1005 by GLS on 14 Sep, 2007 13:26
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Ok, thanks!!! I thought it was at 5Kft but it's at 6Kft, thanks!
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#1006 by APG85 on 16 Sep, 2007 12:15
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On STS-5, why did Joe Allen sit on the mid-deck for launch? With only four crew members on board there should have been room for him upstairs. Just curious. Thanks.
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#1007 by psloss on 16 Sep, 2007 12:50
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APG85 - 16/9/2007 8:15 AM
Columbia still had the ejection seats for the CDR and PLT at that point.
On STS-5, why did Joe Allen sit on the mid-deck for launch? With only four crew members on board there should have been room for him upstairs. Just curious. Thanks.
FYI, this was already asked:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=6156&start=166#M115293
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#1008 by APG85 on 16 Sep, 2007 13:37
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psloss - 16/9/2007 8:50 AM
QuoteAPG85 - 16/9/2007 8:15 AM
Columbia still had the ejection seats for the CDR and PLT at that point.
On STS-5, why did Joe Allen sit on the mid-deck for launch? With only four crew members on board there should have been room for him upstairs. Just curious. Thanks.
FYI, this was already asked:
http://forum.nasaspaceflight.com/forums/thread-view.asp?tid=6156&start=166#M115293
Are ejection seats that much bigger? I read that prvious post but it doesn't go into detail. If you can get three upstairs with ejection seats, why not four? I'm assuming there was only room for a seat between the CDR and PLT (aft)... Thanks. -
#1009 by Jim on 16 Sep, 2007 14:09
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The ejection seats were a lot bigger
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#1010 by APG85 on 16 Sep, 2007 14:12
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Jim - 16/9/2007 10:09 AM
The ejection seats were a lot bigger
What became of those two seats - museum, storage, junk? -
#1011 by brahmanknight on 17 Sep, 2007 12:09
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Any idea how much weight the post Columbia tile repair kits are?
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#1012 by MKremer on 17 Sep, 2007 14:27
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APG85 - 16/9/2007 9:12 AM
What became of those two seats - museum, storage, junk?
Or maybe just refurb'd and put back into the vendor's spares inventory.
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#1013 by Danny Dot on 17 Sep, 2007 15:12
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mkirk - 13/9/2007 9:06 PM
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I don't know what happened on the missions Danny is talking about, I will have to research that.
Mark Kirkman
One was a flight that the shuttle was poorly flown while the crew was transitioning from PLT flying to CDR flying while on the HAC (the shuttle went to almost wings level for a few seconds). After this flight, it became an unwritten rule to not let the PLT fly part of the HAC. The crew made up the energy and had a normal landing. I am pretty sure STS-37 had HAC shrink also.
Danny -
#1014 by kimmern123 on 17 Sep, 2007 15:30
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Danny Dot - 17/9/2007 5:12 PM
Quotemkirk - 13/9/2007 9:06 PM
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I don't know what happened on the missions Danny is talking about, I will have to research that.
Mark Kirkman
One was a flight that the shuttle was poorly flown while the crew was transitioning from PLT flying to CDR flying while on the HAC (the shuttle went to almost wings level for a few seconds). After this flight, it became an unwritten rule to not let the PLT fly part of the HAC. The crew made up the energy and had a normal landing. I am pretty sure STS-37 had HAC shrink also.
Danny
But don't they let the PLTs fly part of the HAC now? I'm Mark Kelly on STS-121 and Chris Ferguson on STS-115 flew part of the HAC. I'm not sure about the flights after those though. -
#1015 by nathan.moeller on 17 Sep, 2007 16:11
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kimmern123 - 17/9/2007 10:30 AM
QuoteDanny Dot - 17/9/2007 5:12 PM
Quotemkirk - 13/9/2007 9:06 PM
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I don't know what happened on the missions Danny is talking about, I will have to research that.
Mark Kirkman
One was a flight that the shuttle was poorly flown while the crew was transitioning from PLT flying to CDR flying while on the HAC (the shuttle went to almost wings level for a few seconds). After this flight, it became an unwritten rule to not let the PLT fly part of the HAC. The crew made up the energy and had a normal landing. I am pretty sure STS-37 had HAC shrink also.
Danny
But don't they let the PLTs fly part of the HAC now? I'm Mark Kelly on STS-121 and Chris Ferguson on STS-115 flew part of the HAC. I'm not sure about the flights after those though.
There are plenty of times that CDRs will let the PLTs fly part of the HAC nowadays. All depends on the CDRs choice. Pam Melroy got a shot on STS-112, Lockhart on STS-113, Jim Kelly on STS-114, Mark Kelly, Chris Ferguson, etc. Not sure about any flights after that. It's not uncommon anymore.
As for the orbiter that went wide around that HAC, I'm not sure which mission it was, but it was an Edwards landing. If it had been a KSC landing, the orbiter would have been in the swamp. -
#1016 by Danny Dot on 17 Sep, 2007 18:25
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nathan.moeller - 17/9/2007 11:11 AM
Quotekimmern123 - 17/9/2007 10:30 AM
QuoteDanny Dot - 17/9/2007 5:12 PM
Quotemkirk - 13/9/2007 9:06 PM
snip
I don't know what happened on the missions Danny is talking about, I will have to research that.
Mark Kirkman
One was a flight that the shuttle was poorly flown while the crew was transitioning from PLT flying to CDR flying while on the HAC (the shuttle went to almost wings level for a few seconds). After this flight, it became an unwritten rule to not let the PLT fly part of the HAC. The crew made up the energy and had a normal landing. I am pretty sure STS-37 had HAC shrink also.
Danny
But don't they let the PLTs fly part of the HAC now? I'm Mark Kelly on STS-121 and Chris Ferguson on STS-115 flew part of the HAC. I'm not sure about the flights after those though.
There are plenty of times that CDRs will let the PLTs fly part of the HAC nowadays. All depends on the CDRs choice. Pam Melroy got a shot on STS-112, Lockhart on STS-113, Jim Kelly on STS-114, Mark Kelly, Chris Ferguson, etc. Not sure about any flights after that. It's not uncommon anymore.
As for the orbiter that went wide around that HAC, I'm not sure which mission it was, but it was an Edwards landing. If it had been a KSC landing, the orbiter would have been in the swamp.
I got out of training entry in 1995. Things might have changed. Keep in mind the PLT can fly part of the profile from Mach .95 to HAC intercept without flying any of the HAC itself. I know this is so common as to be standard proceedure.
I think transitioning while on the HAC would be OK as long at they have been trained to make sure the orbiter does not roll out of its needed bank angle during the transition. -
#1017 by mkirk on 18 Sep, 2007 01:02
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Okay it looks like I am a little late to the party with regard to this PLT stick time topic but here is what I can offer.
The General preference is to allow the PLT to fly during the period from .95 MACH to HAC intercept. If you listen to the air to ground comm when MCC reads up the DEL PAD you will hear something called "Delta T MACH < 1 to HAC". Some times this number is just a few seconds other times it will be a minute or more (For 118 I believe it was just over a minute).
Anyway as long as this interval is long enogh for the handover then that is when it will occur. Once the orbiter is stabilized and subsonic the CDR will hand over to the PLT for a portion of the interval and will retake control prior to rolling onto the HAC (i.e. CDR will execute the roll)
If there is not enough real time prior to HAC intercept then the handover will be on the HAC (assuming nominal conditions and a big enough HAC turn angle). Once the CDR has the orbiter stabilized he/she will hand over to the PLT for few seconds. Typically the CDR should have control back befor reaching the 90 degree point.
Mark Kirkman -
#1018 by C5C6 on 18 Sep, 2007 13:59
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just wanted to ask......in cases like STS-120, you can see that the payload bay is not completely occupied........why don't they occupy it with a smaller payload for the iss, or an independent sattelite/experiment......looks like they were wasting payload capability......
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#1019 by Lee Jay on 18 Sep, 2007 14:06
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C5C6 - 18/9/2007 7:59 AM
just wanted to ask......in cases like STS-120, you can see that the payload bay is not completely occupied........why don't they occupy it with a smaller payload for the iss, or an independent sattelite/experiment......looks like they were wasting payload capability......
Empty space does not mean available lift performance. ISS is in a tough orbit to get to and most of the remaining flights are very performance limited.
