Apollo 13 re-entry.
Roger Kane
> My research into Apollo 13 has not yet answered this question,
>
> Why did the re-entry take longer than expected? This is puzzling considering
> that the Apollo 13 splashdown was one of the most accurate in the program.
I'm no expert, but they said that they were under weight, maby that lack
of weight caused the CM to reach terminal velocity earler, causing them to
slow down and take longer.
What do you think?
Brendan
Henry Spencer
>
> Why did the re-entry take longer than expected? This is puzzling considering
> that the Apollo 13 splashdown was one of the most accurate in the program.
Don't take the movie too literally. The length of the blackout period
wasn't that predictable; there was considerable variation. Apollo 13's
was unusually long but not remarkably so.
--
Space will not be opened by always | Henry Spencer
leaving it to another generation. --Bill Gaubatz | he...@zoo.toronto.edu
E59Edsel
flight dynamics of Apollo 13's trajectory, the CM reentered the atmosphere
at a slightly more shallow angle than usual. This resulted in a longer
ionization period as the capsule burned through more atmosphere and took
longer to complete the entry. Nothing major, though; the reentry corridor
wasn't wide, but significant variations in the trajectory would be
measured in only tenths and hundredths of a degree...
Thomas J. Frieling
>> My research into Apollo 13 has not yet answered this question,
>>
>> Why did the re-entry take longer than expected? This is puzzling considering
>> that the Apollo 13 splashdown was one of the most accurate in the program.
>Don't take the movie too literally. The length of the blackout period
>wasn't that predictable; there was considerable variation. Apollo 13's
>was unusually long but not remarkably so.
While it's true that the lenght of blackouts was not as predictable as the
movie implies, nevertheless, 13's blackout was indeed longer than usual.
It was long enough that it caused some very real worry in Mission Control. See
Cooper's Thirteen: The Flight That Failed (recently reprinted as Thirteen:
The Apollo Flight that Failed):
"After three minutes of balckout, Kranz put through a call to Deiterich to
find out how much longer they had to wait. Deiterich said it should be over in
another thirty seconds. At the end of thirty seconds, there was still no word
from the astronauts, and Deiterich began to get concerned. Thirty seconds
later, the astronauts still hadn't reported in, and Deiterich was alarmed.
Blackouts didn't always end on time, but this one was already excessively
long. Kranz asked the Network Officer whether the spacecraft's radio beacon
had been acquired. It hadn't been. Another thirty seconds went by, and Kranz
asked Kerwin to put in a call to the spacecraft. "Odyssey, Houston. Standing
by, " Kerwin said. There was no answer. Everyone was beginning to despair.
Then five seconds later, Swigert called in--"OK, Joe." He sounded exhilarated
and relieved. He was a minute and forty five seconds late--one of the longest
dellays in any Apollo blackout. Kerwin told Swigert he read him. None of the
flight controllers said a word." Page 196. (new reprint edition)
The movie added the countdown clock device to focus the dramatic tension for
the plot's climax (to my knowledge there really was no such blackout
countdown clock) but the blackout really was very long and it did concern
Mission Control--mainly because of the recent eyeball evidence of the violence
of the explosion reported by the crew and worries that it cracked the
heatshield.
Rick Cooper
>
> > My research into Apollo 13 has not yet answered this question,
> >
> > Why did the re-entry take longer than expected? This is puzzling considering
> > that the Apollo 13 splashdown was one of the most accurate in the program.
>
> Don't take the movie too literally. The length of the blackout period
> wasn't that predictable; there was considerable variation. Apollo 13's
> was unusually long but not remarkably so.
Yes, of course dramatic license was taken in the movie, but the black
really
did last 4.5 minutes, and the previous best was about 3 minutes. The
incident
was not manufactured for the movie.
> --
> Space will not be opened by always | Henry Spencer
> leaving it to another generation. --Bill Gaubatz | he...@zoo.toronto.edu
--
ss8...@den.mmc.com - Rick Cooper, Lockheed Martin
-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-=-
"If you don't want to go around in circles, you have to
be somewhat eccentric." - Equations of Orbital Motion
Pat
Henry Spencer <he...@zoo.toronto.edu> wrote:
>> My research into Apollo 13 has not yet answered this question,
>>
>> Why did the re-entry take longer than expected? This is puzzling considering
>> that the Apollo 13 splashdown was one of the most accurate in the program.
>
>Don't take the movie too literally. The length of the blackout period
>wasn't that predictable; there was considerable variation. Apollo 13's
>was unusually long but not remarkably so.
nevertheless, the time to breakout was much greater then predicted.
I asked kranz about this, the A-13 return speed was higher then any
previous mission and they picked an angle of re-entry that was
on the high hard side. of the corridor. Because they did not have
a handle on the constant shallowing problem, the Guidance officer
had the final correction burn put them deep in the corner of the
window to offset the later determined steam venting issue.
The blackout prediction did not properly account for either of these
issues and it was very tense during those moments for the JSC team.
When Kranz listened to the tapes of the audio loops 25 years after
the fact it made that veteran marine cry remembering the tension.
Henry cooper and Lovell do a decent job of describing the moment
but in the PBS Special "To the edge and beyond", you can see
kranz unwinding from the events.
The problem with the blackout was the team was very uncertain
about the condition of1) the heat shields, 2) the attitude
control thrusters, 3) the guidance platform and 4) the chutes.
All that time of the return mission boiled down to a ten minute
final flight profile, that was a classic dramatic moment and
it made an excellent movie.
pat
--
One mans desperate mundane existence is anothers technicolor - Tik
Julian Fitzherbert
Given that there is only a finite amount of thruster fuel on board
how stable is the spacecraft once in the re-entry attitude? Are
the thrusters firing continuosly to hold the orientation? I know
that the Apollo guidance computer flew a sort of "skip" trajectory
in order to hit the target landing site so the fuel usage must have
been significant. How much fuel did they have on board and how long
would it last? Also the Apollo heat shield was thicker on one side than
the other which implies a certain assymmetry. Was the CofG offset by the
mass of the equipment bay (guidance system and batteries)?
Thanks for any answers folks.
Bill Moore
>
> > My research into Apollo 13 has not yet answered this question,
> >
> > Why did the re-entry take longer than expected? This is puzzling considering
> > that the Apollo 13 splashdown was one of the most accurate in the program.
>
> Don't take the movie too literally. The length of the blackout period
> wasn't that predictable; there was considerable variation. Apollo 13's
> was unusually long but not remarkably so.
Everything I've read shows that the reentry blackout was long only from a voice
communications point-of-view; controllers in the MOCR were getting telemetry from
Odyssey within seconds of the predicted time.
Some technical parameters of the Apollo 13 entry:
Entry Interface to Earth Atmosphere occurred 142hrs 40min 45.7 sec MET at an altitude of
65.83 mi at latitude 28.23 S longitude 173.44 E with a velocity of 36,210.6 fps with an
entry angle of -6.269 deg to horizontal at a heading angle of 77.210 deg. (Source:
MSC-07565, Apollo Lunar Trajectory Notes, NASA 72-FM-263).
--
================================
Bill Moore wmo...@iu.net
Melbourne, FL
Andy Haber
From the book "Spaceflight Dynamics" by William E. Wiesel, p.p. 230-232
8.4 "DOUBLE-DIP" REENTRY
A more sophisticated version of the skip reentry was used by the American
Apollo and Soviet Zond capsules during their return from the moon. The
Apollo capsule had its center of mass offset from its centerline, causing
it to fly with a slight angle of attack, thus producing lift. The first
phase of the reentry was flown with the lift vector up and was a skip
trajectory as discussed in the last section. However, the Apollo capsule
would have left the atmosphere still above escape speed if the skip was
flown to completion. With only a few hours of life support available once
the service module was discarded, this would have been disastrous. The
solution is to roll the vehicle over so the lift vector points down, thus
using the lift to help hold the vehicle in the atmosphere while it
completes its deceleration.
... The flip should be on the ascending portion of the skip trajectory,
since we have no desire to dive into the ground. ...
[ Several paragraphs of equations relating height above the earth, distance
downrange, velocity and angle to horizontal deleted ]
... The typical rollover maneuver, then, is performed at a point very
close to the edge of disaster. ...
[ More equations deleted ]
Figure 8.7 shows several rollover trajectories for an entry angle of
-10 degrees ... The entry altitude was 70 km, and the rollover points
are all indistinguishable on this scale.
[ The figure shows 4 cases. All start at 70 km altitude, 0 km downrange.
Each desends to 35 km altitude at approx 300 km downrange. Then each
climbs to a rollover point at approx 40 km altitude and 350 km downrange.
The first then climbs to 50 km altitude and 860 km downrange then drops
to 35 km altitude and 1,370 km downrange.
The second climbs to 70 km altitude and 1,900 km downrange then drops
to 35 km altitude and 3,360 km downrange.
The third climbs to 85 km altitude and 5,000 km downrange then drops
to 35 km altitude and 10,000 km downrange.
The fourth leaves the atmosphere headed to infinity.]
The trajectory with a maximum altitude of about 85 km ... covers almost
10,000 km of distance before it begins its final reentry. The long
time spent at high altitude produces a relatively benign aerodynamic
heating environment, since the air density is low. This keeps the
heating rate within reason while stetching the reentry time to its
maximum extent.
Near the end of the inverted trajectory the speed of the vehicle will
be low enough that it is no longer possible to neglect gravity in the
equations of motion. This too has advantages, and so it is possible to
use lift to produce crossrange by rolling the capsule, or to bring the
vehicle down on a precise landing point. Since the Apollo capsule could
not change its effective angle of attack, it always produced lift. So,
once the landing point was correctly targeted, it was necessary to
place the capsule in a slow roll about the velocity vector to average
out the now undesired lift.
*** End Excerpt ***
My interpretation of the "edge of disaster" statement is that if the
roll is done too late, the vehicle will leave the atmosphere never to
return, if the roll is done too early, the vehicle will stay in the
atmosphere, but the aerodynamic heating will be fierce and one's goose
may be cooked, plus the vehicle may come down thousands of kilometers
off target.
One of the mission control display screens in the movie Apollo 13
shown as they approach reentry looks much like a version of Wiesel's
Figure 8.7
Back to the original questions, I think the capsule is designed to
be aerodynamically stable so no, one would not have to burn fuel
continuously to hold at the correct angle. The fuel is needed to
do the required rolling, and yes, the CofG is offset.
Henry Spencer
>A general question about Apollo re-entry (or even any vehicle).
>Given that there is only a finite amount of thruster fuel on board
>how stable is the spacecraft once in the re-entry attitude? Are
>the thrusters firing continuosly to hold the orientation?
In general, no. The spacecraft is balanced to be stable at the desired
angle to the airflow. (Holding a different angle by thruster firings
would indeed be very expensive in fuel.) Typically this angle is not zero,
because you want a bit of lift for trajectory control. You can also use
lift to give a gentler reentry, by "lofting" the trajectory, holding it
in the thinner high-altitude air as long as possible. Even a blunt body,
reentering at an angle, generates enough lift for this sort of thing.
The thrusters are there mostly to roll the spacecraft around its direction
of motion, so that the lift vector can be pointed up, down, or sideways as
required. If you don't happen to want lift at the moment, a slow
continuous roll is done, so that the average effect of the lift is zero.
(A spacecraft which wants a purely-ballistic reentry, with no lift, will
also use a slow roll, to cancel out any accidental lift arising from minor
errors of balance.)
--
Americans proved to be more bureaucratic | Henry Spencer
than I ever thought. --Valery Ryumin, RKK Energia | he...@zoo.toronto.edu
Pat
There must be regions though where the vehicle undergoes some degree
of instability. I've seen several references to forcibly
spinning the spacecraft to a high rate in order to hold a re-entry
attitude prior to hitting the atmosphere.
Was it Soyuz-3? where they spun the vehicle as a compensation for
a failed attitude control system?
Also in Coopers book on A-13, there was a serious discussion at houston
over pre-spinning the vehicle because of serious concerns over
the thrusters being frozen.
Maybe the vehicle has trouble during certain transition points.