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Due to the space shuttle orbiter’s high weight (potentially up to 110 tonnes [121 short tons] in certain abort scenarios, although nominally around 85 tonnes [94 short tons]) and abysmal subsonic aerodynamics, it landed at very high speeds (nominally 195 to 205 KIAS1, corresponding to as much as 215 KGS with a ten-knot tailwind allowance2). As a result, a gear-up landing would have posed a considerable risk of voiding the warranties on the vehicle’s occupants:

NOTE

Bailout is preferred over a gear-up landing attempt. [Space shuttle FCOM, page 6.9-2 (page 900 of the PDF of the manual); bolding in original.]

Despite the danger posed by a gear-up landing, the procedures for a normal shuttle landing did not call for extending the landing gear until on short final, approximately 300 feet AGL and a mere 20 or so seconds before ground impact; this would leave the crew with no good options in the event of a landing-gear-extension failure, forcing them to attempt a gear-up landing which would most likely destroy the orbiter and result in serious or fatal injuries to at least some of its crew (like we saw when we were trying to ditch the orbiter in the sea).

The safe deployment envelope for the shuttle’s landing gear opened much earlier than this; the gear were rated for safe deployment at up to 312 KIAS, and the shuttle, during a nominal approach, was already below this airspeed by the time it descended through 15 kft AMSL,3 and did not exceed it at any point thereafter. If the gear were commanded down at 15 kft instead of 300 feet, this would leave time, in the event of a deployment failure, to turn towards a sparsely-populated area, pull out to 185-195 KIAS (the recommended bailout airspeed), engage the autopilot, and (for at least some of the crew) abandon ship (rather than pushing over into the normal 300-KIAS dive down the outer MLS glideslope in preparation for landing).

Deploying the gear at 15 kft, rather than when 300 feet above the rabbit, would likely have required the outer glideslope to be steepened slightly to allow the orbiter to maintain the 300-knot target speed with the added drag of deployed landing gear,4 but this would have been an easy and simple adjustment for the MLS providing the glideslopes (unlike with the more common ILS, where such an adjustment would require complicated reconfiguration of the glideslope antennae, possibly including physically moving the antenna array).

So why was the shuttle’s landing gear held in until it was far too late for any sort of meaningful remedial action in the event that the gear failed to deploy when commanded to do so?


(Not a dupe of this question; theirs asks whether, mine asks why.)


1: The shuttle FCOM uses KEAS (Knots Equivalent Airspeed) rather than KIAS (Knots Indicated Airspeed), but the two should normally be equal (assuming that one’s flight instrumentation is properly maintained and calibrated and that one’s pitot-static system is functioning properly).

2: The maximum allowable groundspeed at main gear touchdown was 225 knots, beyond which the orbiter’s tyres could not be guaranteed to remain intact.

3: It’s quite possible that the shuttle orbiter never reached or exceeded 312 KIAS at any point during a nominal reentry, approach, and landing (and not all that surprising if so, given that this was only 21 knots below the shuttle’s VMO of 333 KIAS, a margin easily wiped out by moderate horizontal windshear); however, I can’t confirm this above 15 kft, as this is the first entry in the FCOM’s nominal-reentry-approach-and-landing timeline for which an airspeed value (rather than a mach number) is listed.

4: The portions of entry and approach prior to MLS acquisition (S-turning, air-data incorporation, transonic flight, transition to manual control, and most of the nominal-energy procedure turn5) would not be affected (except for being moved slightly closer to the runway to compensate for the steeper outer glideslope), as these took place before the vehicle descended through 15 kft; the inner glideslope would likewise have been unaffected, as even a 300-foot gear deployment would still see the orbiter flying down all but the very beginning of the inner glideslope with gear locked down (unless, of course, it failed to deploy). The minimum-energy procedure turn, performed closer to the runway than the nominal procedure turn and at a lower altitude, would likely have to be moved closer to the runway and flown at a higher descent rate to compensate for the higher drag from flying the procedure turn with gear deployed, but, again, ‘twould be but a simple tweak for the MLS.

5: Referred to by NASA as the “Heading Alignment Cone” (HAC), which is a fancy way of saying “procedure turn”.

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    $\begingroup$ 15kft doesn't leave you enough time for a full bailout. Remember there is no ejection seat so it's quite slow for the crew to climb out one by one and there are quite a few of them. Even if you start at 30k you would be left with 10k when everyone is out. $\endgroup$ – user3528438 Jun 7 '20 at 21:14
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    $\begingroup$ @quiet flyer: When people using units of measurement other than feet manage to build their own space shuttles, they will be entitled to use those units. $\endgroup$ – jamesqf Jun 8 '20 at 16:16
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    $\begingroup$ @jamesqf -- personally I'm kind of a fan of "octometers". Pronounced like "thermometers". $\endgroup$ – quiet flyer Jun 8 '20 at 16:32
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    $\begingroup$ @jamesqf good one. Remind me again, who was the first person to orbit earth in space? I keep forgetting. $\endgroup$ – Jpe61 Jun 8 '20 at 18:01
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    $\begingroup$ Oh goodness. My nation is yet to produce a manned spacecraft, let alone a shuttle, what unit are we allowed to use? Fathoms? 😂 $\endgroup$ – Jpe61 Jun 9 '20 at 18:24
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What good options would it add?

Remember that the Shuttle was a glider, and a spectacularly bad one at that. You say "committed to landing" as if there was any point from re-entry onwards where the Shuttle was not fully committed to landing.

On a regular aircraft you can hit the throttle for a go-around. Even on a regular glider you can stooge around in lift to buy time to think and perhaps time to figure out what's gone wrong. You couldn't do that on the Space Shuttle though. There's a reason it was nicknamed "The Flying Brick" - once you had started re-entry, you were on a one-way sled ride to the ground. Whether the gear extended or not, there were no other options - you were going to "land" with or without gear!

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    $\begingroup$ They could bail out, given enough warning. $\endgroup$ – Organic Marble Jun 8 '20 at 14:19
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    $\begingroup$ "Aahm Kennedy space center, STS-000 we have an unsafe gear indication, we're gonna go around and work our checklist." "STS-000 go around, maintain this frequency, advice when ready for another approach, Kennedy space center." $\endgroup$ – Jpe61 Jun 8 '20 at 18:41
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Inflight Crew Escape System

At an altitude of about 9,150 meters (30,000 feet), astronauts would pull a handle that turns on the depressurization valve in the crew compartment bulkhead. This equalizes the cabin pressure and outside air before the side hatch is released.

At 7,620 meters (25,000 feet), the hatch is jettisoned and two telescoping sections of the aluminum and steel escape pole deploy through the hatch. Each crew member hooks a Kevlar® strap onto the pole, jumps out the hatch opening, slides down the 3.1-meter (10-foot) pole and goes into a freefall until the parachute opens to ease the journey to the ground. It takes approximately 90 seconds for a crew of eight to bail out of the Space Shuttle, and by that time, the vehicle is at 3,050 meters (10,000 feet) altitude.

So basically 15kft isn't high enough for everyone to bailout. Then you are faced with a few alternatives:

  1. Still deploy the landing gear and evaluate at 15kft, and before the orbiter hits ground, hopefully but unlikely all of the crew get out and fewer have a chance to deploy parachute. It'll be extremely hard to explain this half-way solution to the American public.

  2. Or, modify the design and procedure so that landing gears are deployed higher up so that they have sufficient time to fully bailout. This comes with all the problems with it. First of all, designing the landing gear to be deployed this high and fast, and the airplane that flies with it, wouldn't be easy. Also, the problem of inspecting the landing gear this high and fast is hard as well (it's pretty high, fast and faraway). Safe to say, the more you think about it, the more complicated it gets, while the project is already overly complicated.

  3. Designing a landing gear that won't fail to deploy. This may sound impossible but since other things that may and do fail have lowered the bar considerably, making a landing gear that is relatively the least likely to fail isn't that hard at all.

For deployment of the landing gear, the uplock hook for each gear is activated by the flight crew initiating a gear-down command. The uplock hook is hydraulically unlocked by hydraulic system 1 pressure applied to release it from the roller on the strut to allow the gear, assisted by springs and hydraulic actuators, to rotate down and aft. Mechanical linkage released by each gear actuates the respective doors to the open position. The landing gear reach the full-down and extended position within 10 seconds and are locked in the down position by spring-loaded downlock bungees. If hydraulic system 1 pressure is not available to release the uplock hook, a pyrotechnic initiator at each landing gear uplock hook automatically releases the uplock hook on each gear one second after the flight crew has commanded gear down.

If the orbiter gets to the point that the pyro+spring mechanism fails, it would have suffered sufficient structural damage that it wouldn't have made it anywhere near the runway(e.g. Columbia).

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    $\begingroup$ In (1) did you mean "evaluate", and it seems you a word. $\endgroup$ – chrylis -cautiouslyoptimistic- Jun 8 '20 at 7:22
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    $\begingroup$ +1 for using NASA-required units. $\endgroup$ – Criggie Jun 8 '20 at 7:23
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    $\begingroup$ Also, we've gotten pretty good at landing gear. Tens of thousands of flights per day for decades, and there've only been a relatively small number of landing gear failures in the history of aviation, most of which still managed to land safely (for the passengers at least, if not the plane.) $\endgroup$ – Darrel Hoffman Jun 8 '20 at 14:47
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    $\begingroup$ Designing reliable, redundand and robust landing gears is not rocket science. $\endgroup$ – Jpe61 Jun 8 '20 at 18:30

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