MOL discussion

[Jim]

The hatch through the heat shield was the chosen method for the crew to move from the capsule to the lab.  This raises a question in my mind.  How was the tunnel to the lab connected to the capsule and how did it provide an airtight seal?

[simonbp]

IIRC, the heat shield hatch swung inside the Gemini (between the crew seats), and was connected by a narrow tunnel to the main pressurized compartment.

[Jim]

IIRC, the heat shield hatch swung inside the Gemini (between the crew seats), and was connected by a narrow tunnel to the main pressurized compartment.

So how does the tunnel interface with the hatch and account the heatshield.  Did the tunnel butt up against the heatshield with a compliant material that could handle the interior air pressure?

[e of pi]

IIRC, the heat shield hatch swung inside the Gemini (between the crew seats), and was connected by a narrow tunnel to the main pressurized compartment.

So how does the tunnel interface with the hatch and account the heatshield.  Did the tunnel butt up against the heatshield with a compliant material that could handle the interior air pressure?

Just a butt connection like that seems like it could be made to work but it does seem like it could be problematic. Perhaps there was a actual docking mechanism that was exposed once the heat shield hatch swung aside?

[tankmodeler]

No, there couldn't be a separate mechanism because you'd have to have a way to disengage it to provide a smooth heat shield surface for a proper re-entry. I've always suspected that it was to be a compliant seal. Tolerance control of the mating surfaces would have been a real b*tch, for sure, but if there is a perfect sealing door at the base of the tunnel, the approach could be to permit the capsule & tunnel to bleed down to near vacuum due to an imperfect seal. This would likely have been an acceptable ops concept during the period. It likely wouldn't fly today, but it might have then.

You could get away, perhaps, with an inflated collar with an RTV interface seal between the collar and the heat shield, the RTV actually being bonded to both sides of the interface. At capsule separation you pop the separation plane and either the forces involved break the RTV seal or there is a  small line charge in the inflated collar leaving a rubber collar bonded to the heat shield. Cut it close enough to the heat shield surface and you won't disturb the airflow enough to destabilise the capsule during the start of re-entry. As re-entry proceeds the collar and RTV will all boil/burn away rather quickly and the capsule re-enters normally.

[hoku]

Figure 5.1-5 of the "GEMINI SPACECRAFT STUDY FOR MORL FERRY MISSIONS" from 1963 hints that a "large pressure bulkhead" would have encompassed the entire heat shield base: http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19750069218_1975069218.pdf
(Don't know, though, if this was the final design).

Here is a view of the modified heat shield of the Gemini 2/Gemini B spacecraft (suborbital flight with the modified heat shield in 1966):
http://upload.wikimedia.org/wikipedia/en/e/ea/Gemini2xrear.jpg

[Blackstar]

So here's an intriguing question: how would they have brought back to Earth the big roll of exposed film they took?

[Danderman]

The whole concept seems like a mess.

I suspect that had The Powers That Be made a more rationale decision in 1963, it would have been to significantly redesign Gemini to allow crew transfer via the nose, separate Gemini launches from MOL, and thus allow MOL to be launched on Titan IIIC (obviating the need for Titan IIIM), and probably would resulted in MOL launches in 1967.

In technical programs, the initial constraints and requirements dictate success or failure of the program, if in doubt, see "Mike Griffin" "Ares".

[Blackstar]

You're assuming that the requirement for a Titan IIIM was the long pole in the tent for MOL.  I don't think that's the case.  MOL was incredibly complex, and had little real justification.

[Danderman]

You're assuming that the requirement for a Titan IIIM was the long pole in the tent for MOL.  I don't think that's the case.  MOL was incredibly complex, and had little real justification.

AFAIK, Titan IIIM was a long pole, with flight tests not even started as late as 1969. Without the requirement for development of SLC-6 to accommodate Titan IIIM, along with development of Titan IIIM itself, the program would have attained flight status might earlier. Of course, without Titan IIIM, a different flavor of Gemini would have been required, but IMHO, early modifications to Gemini would have been cheaper over the program lifetime than the path actually chosen, which was later and smaller modifications.

[mike robel]

I am away from my Spacecraft Films disc, but the hatch was pushed "out" from the heatshield and stored in the tunnel.  There was a distinct lip on the hatch that overlapped the heatshield.  The storage compartment had some positive latch mechanism that prevented the hatch from floating up. 

When I last launched the video, I did not notice how the join was made air tight, perhaps someone else has the video at hand.

The astronauts tested their ability to go through the tunnel both in and out of suits, with film cartridges, and towing or pushing a suited/unsuited incapacatated astronauts.

[Proponent]

AFAIK, Titan IIIM was a long pole, with flight tests not even started as late as 1969.

Actually, the UA-1207 SRM for the III-M was test-fired in April 1969, according to astronautix.com.

[Blackstar]

It wasn't the Titan IIIM that killed MOL.  It was high costs and continued program slips.  Yeah, a smaller spacecraft would have required a smaller rocket.  But your alternative sounds more complex.

[Danderman]

It wasn't the Titan IIIM that killed MOL.  It was high costs and continued program slips.  Yeah, a smaller spacecraft would have required a smaller rocket.  But your alternative sounds more complex.

I would imagine that a redesign of Gemini to allow for transfers through the nose/top would eventually have been cheaper and easier than all these through the heat shield design ideas.

Having said that, the Russians slavishly copied MOL and executed missions with landing craft featuring hatches in the heat shield, although they never risked a crew landing with this architecture.

[Proponent]

[T]he Russians slavishly copied MOL and executed missions with landing craft featuring hatches in the heat shield, although they never risked a crew landing with this architecture.

A heat-shield hatch sure seems scary, but I wonder whether it's really as risky as it looks.  The Air Force did perform a successful flight test, after all.  Also, IIRC, on an early, unmanned Soyuz flight a plug at the center of the heat shield actually failed.  Although the crew cabin lost pressure, the temperature inside stayed within reasonable limits (I think I read about this in a Jim Oberg piece).

[Hoonte]

A heat-shield hatch sure seems scary, but I wonder whether it's really as risky as it looks.

Not that risky.. It pretty much welded shut on reentry. Hatches have proven to work pretty fine. The shuttle has hatches for it's gears (Which fortunally doesn't get welded shut :-))

Modify: added some additional pictures I found

[Archibald]

Gemini docking per its nose ? would be a new spacecraft. I would try it differently.
As far as I remember the astronauts entered Gemini by two large swinging doors on the capsule side. I propose to replace these doors  by a side mounted docking ring (the crew would enter the capsule by this docking ring, even on the ground)
No hatch in the heatshield then, and Gemini would dock by its "side", a bit like a shuttle or biconic.

The Oberg article was entitled Soyuz 5 flamming return, and it is SCARY.

About hatches in the heatshield: I've read somewhere (can't remember where) that Chelomei TKS-VA had some clever design where the hatch was literally sealed by heat reentry and reentry pressure altogether.

[Jim]

I would imagine that a redesign of Gemini to allow for transfers through the nose/top would eventually have been cheaper and easier than all these through the heat shield design ideas.

Huh?   No way.  Though the nose would be impossible with keeping within a Gemini design.   The diameter is less than the tunnel width.  Where would the RCS and parachutes go?  Add a docking mechanism, rendezvous equipment, 3 axis ACS, translation engines, etc. It would be a whole new spacecraft.

The MOL Gemini just required a hatch and tunnel interface and it had many unnecessary systems removed. 

MOL without a Gemini still required a T-IIIM and adding a docking system  would add back some weight.

[Hoonte]

The hatch through the heat shield was the chosen method for the crew to move from the capsule to the lab.  This raises a question in my mind.  How was the tunnel to the lab connected to the capsule and how did it provide an airtight seal?

I believe that it didn't need to be airtight. The tunnel was merely a passage way to the MOL. The Gemini was depressurized and the floated in their suits through the tunnel to the MOL and sealed and pressurized the MOL..

[Jim]

I believe that it didn't need to be airtight. The tunnel was merely a passage way to the MOL. The Gemini was depressurized and the floated in their suits through the tunnel to the MOL and sealed and pressurized the MOL..

Where does it say the MOL flew unpressurized?  Where is the "backpack" and oxygen for the crew while they move from the capsule to the MOL?  The Gemini was to stay unpressurized for the duration of the crew stay?
And the MOL would have to depressurized at the end of the mission so the crew can get back into the Gemini?