12 September 2013

Space reentry vehicles, part 6

Part 6: Control


After atmospheric reentry the heat shields on the bottom of space reentry capsules have a characteristic burn pattern that starts at a point off centre and emanates to the edge. 

Gemini heat shield after reentry

The central point of the burn pattern is the stagnation point where the air isn't moving parallel with respect to the surface but hits it at right angles.  The temperature and pressure is at a maximum at the stagnation point such as the following diagram depicts:

Diagram from here

Because the Gemini, Mercury and Apollo capsules were to come in at an angle and fly to some extent in the sense of having aerodynamic lift -- a sort of flying wing -- the stagnation point isn't in the middle of the heat shield but is off to one side.

Not only was the angle of the reentry capsule meant to be consistent in terms of yaw and pitch but in terms of roll as well; in terms of rotating about the axis of the capsule. How was this orientation maintained?  

Imagine how difficult it would be to maintain the capsule in this orientation with the huge aerodynamic forces pulling on the craft strongly every which way during reentry.

The primitive 1960s onboard reentry control system had to counter forces never before encountered. 
The angle about the long axis of the reentry craft had to be consistent through reentry for the burn pattern above to be achieved.

When Felix Baumgartner made his record-breaking jump 14 October 2012 from an altitude of 39 km he started spinning uncontrollably and almost lost consciousness.  The reaction control system on the reentry capsules would have had to deal with even more severe forces.

There was a reentry control system run by a very simple computer attached to small thrusters on the side of the craft.  These thrusters were placed not on the leading face (which would be hot) but in turbulent after section where their operation would be unreliable at best.
Thrusters placed in the turbulence of the aft section would be ineffectual.  (NASA calls this aft section the forward section).
At subsonic speeds the thrusters would be in an eddy-current rich, turbulent aft section interacting with air that could have little bearing on controlling the reentry craft or stabilising it in flight. 

At supersonic speeds the problem is even worse.  At the reentry speed of Mach 33 the supersonic shock wave can't turn the corner quick enough and becomes detached from the surface at the sides of the craft. 

Maximum angle air can turn corner for a given Mach
Above a certain angle the supersonic shock wave can't turn the corner and separates from the craft.  This would render the control rockets useless.
The primitive guidance computer didn't seem to make a difference anyway.  For example in the Gemini 4 reentry the computer failed.  The result on the craft?  Not much.  The capsules were basically drop and forget.  Without the computer the Gemini 4 reentry was just the same as any other:
A computer malfunction on the 48th revolution made the planned computer-controlled reentry impossible.
The onboard computers were more like stop watches that controlled when to descend to earth. 

Mercury had small 1 and 24 pound thrusters

There is no record of an astronaut practicing flying a reentry vehicle.  And how could a human possibly cope with and counter several series of superimposing oscillations and rotations if they were called to do so? They were presumably called to do so in Gemini 4 when the computer failed...but actually they didn't have to do anything.

There's no actual account of a reentry capsule being flown by the astronauts during any of the reentries; no record of the need for the pilots to intervene and correct the angle of the reentry capsule.  

In design and in practice space reentry vehicles were drop and forget vehicles.  From the Vostok to the Apollo the occupants were just passengers.  Buzz Aldrin said they were "just passengers".

All space reentry vehicles of the capsule type are not stable in flight like a B2 flying wing.  A B2 has wings, ailerons, an aerodynamic shape and modern computers to help fly it. And this is in a basically aerodynamic shape. The reentry capsules weren't aerodynamic and they didn't have computers of the necessary power or speed to cope.

How could a simple 1960s computer cope with several series of superimposing slow and rapid oscillations and rotations that were never encountered before and never truly modelled in wind tunnels or computers? There's no way Apollo's computer, that was more primitive than a calculator, was up to the task.

The capsules would have tipped over and rotated out of control exposing all the wrong surface to the huge heat of reentry -- surfaces that weren't meant to be exposed such as the sides or the top.


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