May 22, 2009

STS125, You're cleared to hold...

An old song from the 70's echoes in my brain ...

 "Eight miles high and when you touch down
You’ll find that it’s stranger than known"

Fast forward to 2009 and make that Eighty Miles High and today I'm thinking about Space Shuttle Mission 125 and comparing it's current situation stuck in orbit, to our more familiar experience of an airliner "orbiting" around a holding pattern***.

Size of a holding pattern 
    - Airliner: 10 nautical miles long, 6 nm wide

    - Space Shuttle: Circumference of the earth plus orbital height: 22,000 nm

Speed in the hold
   - Airliner: best speed for fuel economy - typically 200 to 250 knots

   - Shuttle: 15,000 kts. - fuel consumption is not an issue

Altitude of the holding pattern
   - Airliner: as high as practicable for fuel economy - typically 10,000 to 35,000 feet

   - Shuttle: typical orbital heights: 980,000 to 1,700,000 feet - fuel is not an issue

Time to fly one "orbit"
   - Airliner: 10 minutes

   - Shuttle: 90 minutes

Limitations on holding time
   - Airliner: Minimum Fuel on board must not go below that needed to divert to the alternate, plus 30' reserve.  A typical maximum holding time is about 1 hour, but usually much less.

   - Shuttle: Limited by Life Support systems. The crew of STS-125 have enough supplies to remain in space until Monday.

Alternate airports:
   - Airliner: Alternates must have suitable weather forecasts and ground support facilities.  They are typically located within an hour of flight time but closer is much better.
   - Shuttle: Weather and facility requirements are similar but of course the scale of operations differs.  One unique aspect is that an airliner can divert in any compass direction but the Shuttle cannot.  An alternate landing site must coincide with the existing orbital path.  The shuttle cannot change its ground track until after re-entering the atmosphere.

As a side note - when an airliner takes off in poor weather, a specific Departure Alternate must be designated in case of the need for a quick return to landing.  Similarly, the Space Shuttle has designated emergency diversion airports planned during the launch in case it is unable to continue to orbit.

Time to descend:
   - Airliner:  Typically, from the Top of Descent to touchdown takes 30 minutes.

   - Shuttle:  The De-Orbit maneuvering occurs about 1 hour before landing, but the spacecraft begins transitioning to an aircraft at about 400,000 feet, 35 minutes before landing.

Descent profile
   - Airliner:  As a rule of thumb take the altitude to lose and drop the last three zeros (i.e. div. by 1,000).  Multiply this number by 3 and add 10.  Make adjustments for known wind, and weight of the aircraft (At a heavier landing weight an airliner takes more miles to descend than when it is empty.  In descent the forward speed is maintained by gravity.  A relatively heavier aircraft maintains the proper speed with a gentler slope.)  At busy airports deviations from the ideal profile are often required by ATC.  The pilot initiates the descent by reducing thrust and pitching the aircraft slightly downwards (either manually or via the automation).

   - Shuttle:  Somewhere on the other side of the world, at a point in time and space calculated by means we can't approximate as rules of thumb the De-Orbit Burn is initiated.  This requires the Shuttle to face backwards and fire the re-entry engine for about 20 seconds shaving about 150 kts off its orbital speed.   Then the craft has to be flipped over and prepared to meet the atmosphere in a flying attitude.  As far as I know this is all hand-flown by the pilots.  This reshaped orbit will now intersect the atmosphere just short of the landing airport.  ATC clearances are not currently an issue.

Approach, Landing and Touchdown
   - Airliner: The descent can be interrupted at any time by adding power and levelling off.  Adjustments to the descent angle can be tweaked by adding drag via speedbrakes, or adding thrust.  Deceleration to 250 knots is usually required by law below 10,000 feet, and slower still nearing the airport.  Airliners like to be about 30 miles of flight path from touchdown when they reach 10,000 feet and 250 knots.  Wing flaps must be extended within a small window of speeds as the aircraft decelerates to the approach target.  Typically airliners fly a stabilized approach of 150 knots for the last 1,000 feet of descent along a 3 degree glide slope.  The landing gear and final flap configuration should all be set by this point. Various automatic aids and even automatic landing capability is installed in most modern airliners.  Still the vast majority of landings are flown manually to one degree or another by the pilot.

Touchdown speed is just a few knots slower than the stabilized approach speed.  If needed, the airliner can quickly be reconfigured into a "Go Around" mode by applying take-off thrust and aiming the nose skyward again.  Otherwise the touchdown should occur in the first 1,000 feet or so of a runway that is at least 6,000 feet long.  Then wheel brakes, aerodynamic speed brakes and engine reverse thrusters are available to complete the deceleration.

   - Shuttle: At some point soon after the de-orbit burn the shuttle is headed for an unavoidable encounter with the earth - no exceptions, no delays.  At about 400,000 feet and Mach 25 the Space Shuttle becomes a rapidly descending glider.  Someone has described it as not so much gliding as falling with style.  As speed reduces to about mach 3 it becomes possible to extend the air data probes and not have them destroyed by the heat of friction.  The pilots can now add airspeed to their list of measured flight parameters.  Speed brakes and steeply banked turns are used to dissipate excess energy and align with the runway.

An initial flare begins at 10,000 feet where plummeting is converted to plummeting gently and airspeed begins to reduce towards a more survivable touchdown level.  A shuttle typically passes through 10,000 feet 8 nms from landing and continues down a 20 degree glideslop.  At 3,000 feet it continues flaring and decelerating to reach 190 kts at the runway threshold.  The landing gear is extended during the final flare just before reaching the runway.  The time to deal with landing gear malfunctions is zero - or close enough.  The option to go-around does not exist.  Various automatic aids and even automatic landing capability is installed in the shuttle.  Still the vast majority of landings are flown manually to one degree or another by the pilot.

After touchdown, usually about 2,500 to 3,000 feet into a 15,000 foot runway, a drogue chute aerodynamic speed brakes and wheel brakes are used to dissipate the remaining kinetic energy.

Ground holds and airport congestion are not currently an issue for the Space Shuttle.

The holding numbers I'm giving are gross generalizations.  To see just how detailed holding airspace and procedures get when they are officially calculated, see this paper.  

As for numbers related to Space Shuttles, I'm using gross generalizations gleaned from the NASA internet site.  These units are all given in Nautical Miles - multiply by 2 for an approximate conversion to Kilometers.

Here's a more detailed explanation of Space Shuttle re-entry.

Congratulations to the crew of STS125 and happy landing whenever it occurs!


Sarah said...

I've been following this mission closely, Hubble lover that I am.

Good luck to the crew for a happy landing and finish to a wonderful flight.

PS.. As an instrument student, I was amused by this approach plate. Probably one of the few remaining MLS approaches in the US.


Aluwings said...

Cool - thanks for finding that plate. It shows specifically the points the astronaut pilots are referring to in the video (i.e. "ON at the 90, etc)...

Peter said...

I was involved with an experiment aboard STS77 in 1996 and got to experience the landing at KSC in the bleachers. It is truly amazing how the shuttle passes overhead (just prior to the long CW turn shown in that plate) over the speed of sound (nice double sonic boom), does a long sweeping right turn, and drops like a stone to the runway. The video doesn't do the feat justice.

Capt. Schmoe said...

I witnessed a STS landing at Edwards AFB once. That thing descended like a brick, landed like a feather. I agree with Peter, the vid can't do it justice. If you want to see one land, do it soon. Not too many missions left.