Re: Shuttle fleet grounded again?

Roger Hamlett wrote:

"Martin" <nospam@xxxxxxxxxx> wrote in message news:42e898e3_1@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

"has.mac" <has.mac@xxxxxxxxxxxxxxx> wrote in message news:PA%Fe.4587$_k2.77165@xxxxxxxxxxxxxxxxxx

"Martin" <nospam@xxxxxxxxxx> wrote in message
news:42e81735_4@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx

Sky News are reporting that NASA have decided to ground the Shuttle fleet
again once the current mission is complete. Looks like they are worried
over
the external tank foam yet again.

And with good reason. Big chunks of something were spotted on the initial launch video - the live feed I was watching replaed it within about 10 minute of the launch. Nature seemed not to have noticed.

Does make you wonder why the insulation wasn't put on the inside of the
structure in the first place. 

Too difficult to install it internally in a pressure vessel.

Didn't they used to paint the external tank. Would this help keep the foam
in place, at the cost of weight?

The external foam system is a lot more complicated than you imagine. The leading edges of the tank (and the bits near the engines) are subject to serious aerodynamic and thermal stresses. Several foams are used.

All I know is that an aeroplane has its cabin insulation on the inside of the skin and my loft insulation is inside not stuck on the roof.

However if you think on, your hot water tank (which like the external tank, has to hold materials inside), is insulated on the outside, as is the cold water supply cystern. In fact your 'loft insulation', is 'on' the ceilings, and has the complete extra layer of the 'roof' outside it.

It has to cope with thermal expansion/contraction over a huge range of temperatures and in an environment often at 100% humidity and weaknesses in the closed cell foam structure allow ice patches to form.

Its a design flaw and I'm betting that without a fundemental re-design they may have to live with it. I don't think paint would help as its breaking away from the outer surface of the tank. Could they perhaps fit something (like a giant condom) over the top end of the tank (where bits coming off are more likely to hit the Orbiter) to help hold the foam in place?

The problem is the huge range of temperatures and pressures the 'condom' would have to survive. The odds are that unless it is made of something like aluminium, you would end up with bits of this falling off as well...

The best solution for reentry is the old traditional ablative heat shield. Very robust, low tech and entirely protected from damage during launch.

I think we may well have seen the last shuttle flight.

Me too. And that is a pity since there is nothing even on the drawing board to replace it. The ISS I would happily dump into the ocean tomorrow so that space *science* and astronomy got more funding.

What puzzles me, is that they didn't expect this (maybe they did...). If you watch the old Apollo launches, you have so much ice falling, ignoring any 'insulation' materials, that damage has to be expected. Every shuttle launch has had tile damage during take-off, what was exceptional in the Columbia launch, was not that it occurred, but the size of the piece involved, and that it managed to damage the leading edge structure (which is much tougher than the 'tiles'). Something like an alloy 'net condom', should have been possible, if necessary at the cost of one crew place, to keep the maximum size of pieces involved down to reasonable limits...

They did. There is some circumstantial evidence that the change to non-CFC blown foams for the bulk tank insulation may have made it slightly more brittle. OTOH some of the bits known to have fallen off in the past were manually applied still using CFC-11 blowing agent.

NASA short report online at:
http://spaceflight.nasa.gov/shuttle/rtf/otte/et_thermal_protection_fact_sheet.doc

Intrinsically there is a very nasty problem that huge changes in temperature typically from 300K down to 20K for the LH2 tank has bad effects on plastic to metal bonding. If foam cell integrity gets compromised then cool spots develop and water ice ingress becomes an issue.

Regards,
Martin Brown
.

Relevant Pages

  • Re: GO FEVER IS WELL ENTRENTCHED:(
    ... Schedule pressure propels NASA ... Foam danger is unresolved, but agency moves ahead with launch plans ... NASA has delivered a revamped external fuel tank to Kennedy ...
    (sci.space.shuttle)
  • Re: Shuttle Rocket Booster Foam
    ... >> The foam on the external tank performs three functions: ... >> 1) Protect the ET from aerodynamic heating during ascent. ... > A small subset of the larger group called insulation on the Tank. ...
    (sci.space.shuttle)
  • Re: wrap Et in mesh?
    ... >>attaching it to the ground instead of the tank. ... > insulation and the tank is not an easy problem -- it was troublesome ... >>the surface of the last layer of foam? ...
    (sci.space.shuttle)
  • Re: Shuttle Rocket Booster Foam
    ... > The foam on the external tank performs three functions: ... A small subset of the larger group called insulation on the Tank. ... Is the foam protecting the huge heat sink, or is the huge heat sink ...
    (sci.space.shuttle)
  • Re: wrap Et in mesh?
    ... >>attaching it to the ground instead of the tank. ... > insulation and the tank is not an easy problem -- it was troublesome ... >>on the surface of the last layer of foam? ... NASAs problem with the mesh in the foam idea. ...
    (sci.space.shuttle)