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From Virgin Galactic on SpaceShipTwo:

Windows in the passenger compartment will be 13 inches (33 centimeters) wide by 17 inches (43 centimeters) tall.

IMO that's enormous for an airplane, especially for a spaceplane! But, wouldn't such large windows cause problems? The Concorde had particularly small windows (pax say that it's almost passport book sized) to maintain the integrity of the airframe and cabin pressure if a window happened to crack at FL 600 AMSL.

But wouldn't such a large window cracking at 110 km AMSL on SpaceShipTwo definitely cause cabin pressure to equalise to atmospheric pressure in seconds? What redundancy does SS2 have to prevent catastrophe in such an event?

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the windshield of a spaceshuttle is also pretty large –  ratchet freak Feb 28 at 11:11
    
But the astronauts have the option of wearing space suits! (but space suits are useless if not worn during event of sudden decompression). Virgin Galactic SS2 will require shirt-sleeves only. –  shortstheory Feb 28 at 13:47
    
Concorde is not really a good example - that technology was a long, long time ago. –  Rory Alsop Feb 28 at 15:43
    
IMO the Concorde's idea was very simple and failsafe. The area of the outflow valves matches the area of two windows, so if two windows blew out, you could just close outflow valves and retain (most) of the cabin pressure at altitude. –  shortstheory Feb 28 at 15:47
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My guess? They are really, really thick (and very possibly have two windows just in case one cracks). And they want them big because this is a sight-seeing flight! –  Lnafziger Feb 28 at 17:25

3 Answers 3

up vote 16 down vote accepted

Here are a few ideas:

  • First and foremost, we're not handling metals here. Composites have the advantage of being able to be made stronger by adding on a few layers around high-stress area like windows. This was difficult with metals since this needed some sort of fasteners or adhesives to do. This reinforcement is seen in the photo below.

  • The windows are designed to keep an even pressure all around in the context of the fuselage shape at that point. You'll notice the flange design which keeps the fuselage shape and stop it from popping open. This is a little like the thicker part of a bottle neck. This is very difficult (if possible) to do well in a metal fuselage to this extent.

SpaceShipTwo

  • The usage is completely different. Concorde was a commercial aircraft, intended for many years of operation. This is a very low-cycle aircraft which will see a fraction of that flying. The number of pressurisation cycles is a lot lower, putting much less force on the windows.

  • As for safety, the style of flying is completely different. Probably flown by test pilots, the aircraft might never even experience rain and will fly under very predictable conditions, with very few flight cycles.

  • Periods of elevated temperatures gradually degrades many materials, this is much less a concern with short flights. If I gather correctly this is the main issue with sustained high-speed flight, as seen on Concorde and SR-71 and other aircraft.

profile

  • Composites have a lot lower thermal expansion (CFRP vs. Aluminium is an order of magnitude lower) which reduces issues with expansion in the fuselage.

  • Concorde was a commercial aircraft with weight and operating price in mind. This aircraft is built for this purpose, with the windows considered as a main feature from the start. You could probably have been able to make the windows a lot larger on Concorde, at the unacceptable price of weight, production and operating cost.

  • Needless to say that knowledge of materials and structures has advanced substantially, all our these features can be modelled in detail on computers, not necessitating the traditional conservative approach.

  • As for redundancy, predecessor SpaceShipOne had double pane windows and double seals everywhere. For the passengers without suits, if a window were to blow entirely, you're most probably out of luck.

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@JanHudec there might be a seal failure or similar. Not catastrophic. I don't see why they would take the chance to go without one. I agree it's pretty dangerous for the high altitude segments though. Passengers aren't going to wear pressure suits though. –  MikeFoxtrot Feb 28 at 22:57
    
"I'd guess the pilot has his mask on the whole flight.": Pressure masks are not sufficient above around 14km/46000ft (and that's most of the flight). Above that altitude, the total pressure is less than the normal partial pressure oxygen in human blood (14kPa), so breathing pure oxygen above 14km is not sufficient to keep one's blood saturated with oxygen. Above about 19km, the boiling point of water drops below 37°C causing further severe problems with various body liquids boiling (blood will not boil, because it has 8-10kPa higher pressure). –  Jan Hudec Feb 28 at 23:19
    
@JanHudec: Masks can work above 46000 feet AMSL. Again, Concorde had CPAP masks to deliver air to the pilots. It must've been uncomfortable to use! –  shortstheory Mar 1 at 3:30
    
@Manfred: this is a good answer and +1. I'm interested to know, why did you say "the aircraft might never even experience rain and will fly under very predictable conditions, with very few flight cycles." Is rain highly detrimental to performance and structural integrity of supersonic aircraft? Also, would the Concorde have worked as well as it did if it was made of CFRP (i.e., in terms of fuel economy, lifetime number of pressurisation cycles, and sustained Mach 2 performance)? –  shortstheory Mar 1 at 3:40
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@shortstheory Water is of course fine in CFRP aircraft if it doesn't enter the composite where it may degrade down the resin, but this is no bigger problem than metal corrosion. It built today, primarily better engines would save a lot of fuel. Concorde was 27 years old when retired. Supersonic composites is used on for instance the F35, so I can't see why a new Concorde would have problems. Metals were a pain as well due to thermal expansion. Lifetime I think should no issue as well. I'm no certainly no expert at composites, but I see no reason why it would not work. –  MikeFoxtrot Mar 1 at 8:58

I think ultimately it's a business question. Virgin Galactic is selling tours at a premium price where as Concorde was sold to move people from point a to point b.

With Concorde, since the point of the craft was to get people from point A to point B very very quickly, they put money into big engines and efficient aerodynamics. The view doesn't really matter in that equation, but you do need structural integrity. So they went with the cheapest way to get that structural integrity, small windows.

Virgin Galactic, on the other hand, is selling an experience. And frankly, if you're going into space the two biggest selling points are going to be zero gravity and, honestly, the view. Hence, even though it will make the craft more expensive to reinforce properly, they are installing very large windows.

As a note, airliner manufacturers could also install big old huge windows if they wanted as well, but do you want to pay another $50 a flight just to have a bigger window? For most people the answer is no...hence small windows.

Basically it's just a matter of figuring out what you are trying to sell and, then, putting money into that selling point. It's what gets you a good return on your investment :).

PS- lol, and after all that writing I appear to have answered the "why" and not the "how". Well, let the internet judge me as it will, I'll leave this answer up for now :).

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For what it's worth, the windows on the Boeing 787 are even bigger than the ones referenced here (19" tall): gizmodo.com/5829395/… –  Lnafziger Feb 28 at 19:05
    
Yeah, the tech changes aren't really what I was trying to speak too, hence my final little note there... But yeah, as the tech has improved you can make bigger windows for the same price as the smaller ones of yesteryear. –  Jay Carr Feb 28 at 19:14

The Concorde had a lot of thermal expansion due to air/skin friction, that may have impacted window size. SpaceShipTwo travels at much lower speeds, and doesn't have that issue.

Ultimately, the issue is whether the window is designed to withstand the pressure differential of cabin to space. As long as it is thick enough, made out of strong enough materials, they should work safely.

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Good point, Space ship 2 is a composite airplane so won't have the issues of thermal expansion like a metal airplane does. –  Lnafziger Feb 28 at 20:29
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According to this diagram, it looks like the Concorde actually travelled slower than the SpaceShipTwo. –  flyingfisch Feb 28 at 21:56
    
What!? SS2 travels at Mach 4 through the atmosphere, and the Concorde only reaches Mach 2.04. SS2 is much faster than the Concorde. –  shortstheory Mar 1 at 3:34

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