Re; Gemini 8 reentry

On Thu, 21 Aug 2003 10:23:05 -0400, Jan Philips
wrote:

It is well-known that Gemini 8 fired their retros to get control of
the spacecraft, and then reentered. Was there only one firing that
got control of the ship, or was there a second one that actually
brought it down? If there were two, approximately how much time
elapsed in between?

http://science.ksc.nasa.gov/history/...mini-viii.html

" The crew regained control of their spacecraft by using the reentry
control system, which prompted an early landing in a secondary
landing
area in the Pacific. "


Three different rocket systems were involved in the Gemini 8 incident:

Take a look at this diagram for reference:

http://www.hq.nasa.gov/office/pao/Hi...ms/gemini4.gif

1. The Orbit Attitude Control Thrusters on the Adapter Section
malfunctioned and put Gemini 8 in a spin. This system were shutdown in
an effort to regain control of the spacecraft.

2. The Reentry Attitude Control System (in the nose of the Gemini
8,usually only used during reentry) was turned on to regain control of
Gemini 8. Once this system was activated, the flight rules said they
must land as soon as possible.

3. The Retrograde Rockets (retro rockets) were fired to bring them out
of orbit and start their return to earth.

The episode is described at this URL:

"On the Shoulders of Titans: A History of Project Gemini"

http://www.hq.nasa.gov/office/pao/Hi...203/ch13-6.htm

and this one:

Encyclopedia Astronautica

http://www.astronautix.com/flights/gemini8.htm

They docked with the Agena at 6:33 into the flight. The problem
happened 27-minutes later. They landed at 10:23 MET (mission elapsed
time). Retro-fire happened about 30 - 45 minutes prior to landing.


--
Rusty Barton - Antelope, California

On 21 Aug 2003 08:00:42 -0700, (Rusty B) wrote:

They docked with the Agena at 6:33 into the flight. The problem
happened 27-minutes later. They landed at 10:23 MET (mission elapsed
time). Retro-fire happened about 30 - 45 minutes prior to landing.

Thanks for all of the information. So they made nearly 2 more orbits
before firing retros.

(Rusty B) writes:


1. The Orbit Attitude Control Thrusters on the Adapter Section
malfunctioned and put Gemini 8 in a spin. This system were shutdown in
an effort to regain control of the spacecraft.

Was the malfunction in the control system, or a leak in the electric
valve in the thruster? If the latter, how did they shut it down;
were there other valves in series?

--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

In article , David Lesher writes
(Rusty B) writes:


1. The Orbit Attitude Control Thrusters on the Adapter Section
malfunctioned and put Gemini 8 in a spin. This system were shutdown in
an effort to regain control of the spacecraft.

Was the malfunction in the control system, or a leak in the electric
valve in the thruster? If the latter, how did they shut it down;
were there other valves in series?


Details for nerds (I love this sort of stuff):

It was an electrical short in a control wire leading to a particular
thruster. Apparently the system was designed so that when the control
wire shorted to the capsule ground, the thruster fired. This made it
vulnerable to a wire insulation fault. After Gemini 8, the system was
redesigned so that power was only applied to the thruster valve when the
thruster was commanded to fire, that way such an insulation fault would
result in a thruster that was stuck *off*, a much more benign condition.

--
Jonathan Griffitts
AnyWare Engineering Boulder, CO, USA

In article ,
says...

snip

2. The Reentry Attitude Control System (in the nose of the Gemini
8,usually only used during reentry) was turned on to regain control of
Gemini 8. Once this system was activated, the flight rules said they
must land as soon as possible.

I just wanted to point out that there was a little softness in that "as
soon as possible." As Henry pointed out, once the valve seals were
broken on the Re-entry Control System, the system had the potential of
losing fuel and degrading. You wanted to land within hours, not days, of
activating the Re-entry Control System.

However, on the two really long-duration Gemini flights, Geminis V and
VII, the OAMS (the primary reaction control system) lost thrusters and
suffered from fuel line clogs, such that the spacecraft had degraded
attitude control by the end of the flight. On both flights, Houston gave
the crew orders to activate the RCS a rev or two prior to the planned
retrofire event, to make sure they got the capsule correctly oriented for
the retrosequence. Those RCS activations were actually somewhat in
violation of the flight rules, but it was obviously more important for
the crews to have a good control mode when lining up for retrofire than
to slavishly follow a mission rule that made little difference as you
neared the scheduled end of the flight.

--

Do not meddle in the affairs of dragons, for | Doug Van Dorn
thou art crunchy and taste good with ketchup |

Doug... wrote in message
...
In article ,
says...

snip

2. The Reentry Attitude Control System (in the nose of the Gemini
8,usually only used during reentry) was turned on to regain control of
Gemini 8. Once this system was activated, the flight rules said they
must land as soon as possible.

I just wanted to point out that there was a little softness in that "as
soon as possible." As Henry pointed out, once the valve seals were
broken on the Re-entry Control System, the system had the potential of
losing fuel and degrading. You wanted to land within hours, not days, of
activating the Re-entry Control System.

I heard the reason is that the thrusters used peroxide with a catalyst, and
that once used, the catalyst would start to break down.

In article ,
Doug... wrote:
I just wanted to point out that there was a little softness in that "as
soon as possible." As Henry pointed out, once the valve seals were
broken on the Re-entry Control System, the system had the potential of
losing fuel and degrading...

Especially since N2O4 systems -- that was the oxidizer in the Gemini RCS
systems -- are notoriously prone to valve leaks. (N2O4 plus moisture
gives nitric acid, nitric acid plus metal plumbing gives metal nitrates,
and metal nitrates tend to be carried downstream and crystallize in random
places, like valve seats.)

You wanted to land within hours, not days, of
activating the Re-entry Control System.

It's worth noting something that Mary has pointed out in the past: there
are two different flavors of "as soon as possible", two different classes
of in-flight emergencies.

One involves imminent loss of control or life support. In that case, you
put the thing on the ground somewhere, right away, anywhere it looks like
you can probably land it. What happens after that is a concern for later.
If the reentry RCS suddenly springs a big leak, you drop everything else,
line up in the right attitude, jettison the adapter, and fire the retros,
no matter where you are or where the recovery ships are. If it's going to
bring you down in Siberia, well, that's unfortunate, but it's better than
being a corpse in orbit.

The other is when you've got slowly-developing trouble, or you're on your
last emergency fallback. You want to get down soon, before things get
worse or something else goes wrong, but there's no requirement to do it
instantly. In that case, you've got some leeway for picking the right
time and place, following preferred procedures, and generally doing things
to improve the odds of a safe and smooth recovery. This obviously needs
care and a good assessment of the health of the vehicle, so that immediate
trouble doesn't sneak up on you while you're making preparations.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

In article ,
Bob Martin wrote:
...As Henry pointed out, once the valve seals were
broken on the Re-entry Control System, the system had the potential of
losing fuel and degrading...

I heard the reason is that the thrusters used peroxide with a catalyst, and
that once used, the catalyst would start to break down.

Sorry, no peroxide on Gemini. Only Mercury used peroxide.
--
MOST launched 1015 EDT 30 June, separated 1046, | Henry Spencer
first ground-station pass 1651, all nominal! |

Jonathan Griffitts writes:

Was the malfunction in the control system, or a leak in the electric
valve in the thruster? If the latter, how did they shut it down;
were there other valves in series?


Details for nerds (I love this sort of stuff):

It was an electrical short in a control wire leading to a particular
thruster.

So the irony is, if they had stayed docked, they could have completed
more of the mission. (Previous posts mentioned the Agena's own
manuvering system....) Hindsight is oh so clear.

Followup: How did the Gemini control it? Did the docking probe engage
some 100 pin connector? (ISTM the LM mating involved a manual plug
connection...) Or was there some short-range RF system? Did it
connect to the Gemini stick{s}?





--
A host is a host from coast to
& no one will talk to a host that's close........[v].(301) 56-LINUX
Unless the host (that isn't close).........................pob 1433
is busy, hung or dead....................................20915-1433

"Chris Jones" wrote in message
...

It's worth noting that Gemini 8 wasn't on its last fallback, as there
were two independent rings of RCS thrusters, and they had only activated
one to stop the roll. But that meant there could be no backup RCS
thruster ring, and the flight rules said that was cause for landing
"with all due haste" (or whatever you call the second flavor of ASAP).

Actually they had activated both RCS rings, but stopped using one ring to
conserve fuel once they were able to re-gain control (from the Gemini VIII
Mission Report).
--
John Fongheiser
President, Historic Space Systems
http://www.space1.com
Exhibits that Launch Imaginations (tm)