Opto-Mechanical design 1

[pappe]

Hello!

I found the forum as I was searching for some answers to opto-mechanical design for aerospace applications. Perhaps someone here can help me to find them...

I have to decide if an optical system which will be used in aerospace should be open or close. There are some concerns about outgasing of the bond materials, pressure and temperature. Now there are two approaches:

1. Open design:

We design the system open so that every cavity is connected to the environment. No pressure differences can occur.

2. Close design:

We design the system close so that hole system is sealed against the environment.

If anyone have some ideas it would be great. If there anyone who have some experiences superb!

Thanks!

Martin

 

[IRstuff]

Use low-outgassing bond materials; there are plenty of those.



TTFN

FAQ731-376

 

[drawoh]

pappe,

Seal your system against the environment and install a breather desiccator. Check out Drytech's website.

A sealed device in an unpressurized aircraft is a pressure vessel. Ambient air pressure drops 4psi at 10,000ft, and 6psi at 15,000ft. That translates to a surprising amount of force on your covers. This could be anywhere from an annoyance, to a significant safety hazard.

Your system probably will have to be certified as safe for installation in an aircraft. Do not piss off your Designated Airworthiness Representative.

Outgassing is an issue in the vacuum of outer space. I am not aware of problems in an aircraft at normal altitudes.

JHG

 

[JQCF1]

Drawoh,

You cited the problem OK but unless I'm confused about the situation I think you need DER help not DAR help, Right?

 

[IRstuff]

In small, confined optics, the escape path for outgassing might be quite tortuous, resulting in the deposition of organics on the lens surfaces. End result is a kind of haze, not unlike what you get on the inside of your windshield in southern California.

TTFN

FAQ731-376

 

[drawoh]

JQCF1,

When you design something to be installed in an aircraft, it must be approved by a DAR as noted above. Daughters of the American Revolution are not much use to us here, unless they have been trained by the Federal Aviation Administration.

I actually am not sure if the OP's application is going into an airplane or being sent out into space. Outgassing is an issue in outer space. Weight is an issue in outer space, which makes designing a pressure vessel highly undesirable. Again, you are going to have to convince some engineering type that your design is safe and functional.

What is a DER?

JHG

 

[KENAT]

Hang on.

DAR applies if the FAA is involved.

If on the other hand it's military kit or in another juristiction they may have little or nothing to do with it.

I'd pay attention to what IRstuff is saying from what I know of what he does for a living and who he works for.

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(probably not aimed specifically at you)
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[drawoh]

KENAT,

My design background is mostly with hardware in civilian aircraft, flying at low altitudes. I am not aware of problems with outgassing. I am aware of crash safety concerns. If the FAA is not concerned, someone else is.

If outgassing were my potential problem, I would try very hard to not use materials that outgas.

JHG

 

[KENAT]

Sorry drawoh, didn't mean to imply you should be ignored.

Posting guidelines faq731-376

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(probably not aimed specifically at you)
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[IRstuff]

> Optical outgassing typically comes from improper selection of adhesives, so choose wisely. There's a PDF floating around that describes RTV outgassing.

> Lots of normal stuff outgasses; that's the genesis of "new car" smell. FEMA is famous for its toxically outgassing travel trailers it provided after Katrina. I would hazard a guess that a new plane from Boeing or Airbus has lots of outgassing, initially; we just think of it as "new plane" smell.

> Conclusion: outgassing is fairly normal, but its effects are at best a temporary pleasure, mostly a nuisance, and occasionally dangerous

TTFN

FAQ731-376

 

[JQCF1]

Drawoh,

Please do your homework.....In FAA speak a Designated Airworthiness Representative is a DAR and a DER is a Designated Engineering Representative. Typically neither has much to do with any revolutions.

Primarily, the DAR is involved with Airworthiness issues and a DER with Engineering issues.

Go to Order 8100.8C Designee Managemant Handbook for our marching orders and specifically to Order 8110.37D for DER guidance. I happen to have both delegations.

Good luck with your project and thanks Kenat for your comment.
John Q.