Slashdot Mirror
Shuttle Atlantis Launched Without Incident
forkazoo writes "Space.com is reporting the successful launch of the space shuttle Atlantis. There were no major incidents or problems during the launch, except that there was some concern about the weather at the two European abort landing sites. The weather cleared up and the launch was pretty much perfect. 'Preliminary analysis of images taken by onboard cameras revealed expected "popcorning" foam loss during ascent but none that appeared to strike the orbiter. NASA has kept a close watch on the shedding of fuel tank foam insulation during shuttle launches since the 2003 Columbia accident, which claimed the lives of seven astronauts, and made modifications to reduce the amount of debris shed during liftoffs.' The launch was broadcast live NASA TV stream."
I believe the foam, or its adhesive, had to be replaced with something less-effective due to environmental regulations. That's when the trouble started.
The problems all started when they changed the foam formula. The old foam (BX 250) didn't break off. However, it contained CFC 11 so was deemed bad for the environment by the EPA. NASA applied for an exemption, but was denied. In 1996, NASA switched to an HCFC 141b based foam. The new foam isn't as good an insulator, so it has to be thicker, and it isn't as strong. This means it tends to break off, and in large pieces. /Waiting for NASA to be forced to use lead free solder. And the fireworks that would cause.
The "old" foam wasn't any better. In fact, Columbia rode an old tank on her ill-fated STS 107. The fact is, Shuttles have been returning with TPS damage for the entire program. But much like the erosion in the SRB o-rings pre-Challenger, the threat was never perceived to be so great that action was necessary to prevent a loss of vehicle/crew. Now we know better, and now they take every ounce of foam loss very seriously.
You're insulating the hydrogen at -250C (~22K); the Lox is almost irrelevant, since it has such a higher heat of vaporization and density, and therefore low boiloff. Hydrogen insulation can be and has been done both inside and outside the tank -- the Saturn V upper stages put it inside. Inside the tank, the attachment between insulation and tank material is kept warm, which makes that problem easier. However, it's hard to make a lightweight insulation that the hydrogen doesn't soak through, and hydrogen has a very high thermal conductivity, so that destroys the insulating properties. The foam doesn't have to take pressure, but it has to be sufficiently sealed that you don't get conductive flow past eg your interlocking bricks.
Insulating hydrogen tanks is a decidedly non-trivial task, especially when you want ultra light weight for a rocket. It's rather far from obvious what the best answer is.
IMHO, the solution to the problem is very simple -- don't use hydrogen! Kerosene, propane, and methane are all better alternatives. They actually have higher performance by many relevant metrics, too. Hydrogen is *so* light weight (0.07 g/cc) that the tanks get big. The lower Isp of hydrocarbon fuels is more than compensated for by the better fuel / tank mass ratio in the vast majority of applications. And that's even before you count the high cost of handling hydrogen and designing engines to work with it -- it's enough colder than LOX to make a difference, and it has myriad other handling concerns that make the development programs expensive.
And yes, I do build rockets for a living. No, I haven't ever worked with hydrogen, but there's a reason for that...
Not true on all counts. Aspen Aerogels makes a felted insulating blanket out of the stuff; I've worked with it for Lox insulation. It's not even all that pricey -- $4/sq ft, 1/4" thickness. It works *great* on the ouside of the tank, when it doesn't have to have any strength (I wouldn't want to expose it to any sort of aerodynamic loading, though). However, it's quite porous (aerogels are inherently open-cell structure) and soaks up liquids quite well. I've personally experimented with immersing it in LN2; it's obviously not the right choice on the inside of the tank.
Oh, and vacuum isn't a possibility -- the structure required to hold vacuum is *far* heavier than that required to hold pressure. Vacuum is the insulator in standard cryo shipment (dewars), but there weight isn't a concern.
1. Materials. The alloys used to make the tank are designed for cryogenic service. they will not exhibit metal fatigue or stresses. They are also designed not to react with they hydrogen or the oxygen. many materials start acting funny (funny boom, not funny haha) when exposed to pure environments of either propellant. thermal cycling on an ET is very limited anyway, perhaps a handful of tankings ahead of a launch depending on the number of scrubs, so the shrinking and reexpansion of the tank when cryo temperatures are applied or removed does not occur enough to cause metal fatigue. they have people keeping track of such things.
2. Manufacturing complexity. Most of the ET foam is applied with a robotic sprayer once the tank is completely built. Retooling the assembly line to spray the inside of the tank would be an expensive proposition in and of itself, not to mention requiring the tanks to not be completely assembled when the foam is applied. the tank seams would thus not be as well insulated, causing ice to form. The spray on insulation isn't what killed Columbia anyway. the robotic process allows the foam to be sprayed uniformly with few voids in the foam. the CAIB concluded that hand applied foam applied to reduce aerodynamic loading at the orbiter attach points as well as prevent orbiter killing ice formation at those same points is what brought down Columbia. the hand applied foam cannot be applied with nearly the same uniformity as the spray on foam. The tanks were redesigned to eliminate most of the need for hand applied foam. This doesn't prevent mission managers from being paranoid about anything coming off the tank and causing a problem.
3. Foreign object debris. the popcorning seen on liftoff is due to aerodynamic stress and vibration that the vehicle experiences during the climb. there is no reason to believe this won't happen if the foam is inside the tank. (an additional cause of foam shedding in this case is mentioned in 1 above)
The turbopumps on a shuttle engine are very powerful and built to tight tolerances. Even a very small piece of debris entering these pumps can tear an engine apart when it is operating at full capacity. Filters are placed in the fuel lines ahead of the pumps to help prevent such things from happening, but they're meant to catch the odd piece or two. You can see from the launch video how much popcorning can occur during a flight, so placing the foam inside the tank where it can access the fuel line creates one of two scenarios. 1. the filter clogs, starving the engine of fuel, shutting it down and creating at best an abort scenario, which, depending on the point of the climb at which it happens increases the risk to the crew and at best forcing NASA to spend extra money from its dwindling budget to retrieve the shuttle from Africa. 2. the filter fails, allowing FOD into the engine, blowing the back end of the orbiter off and creating a very bad day for everybody.
In order to prevent this FOD, a liner, perhaps made of a metal alloy would be needed. This 1. increases manufacturing complexity even more. 2. to borrow your argument, increases risk due to metal fatigue and stresses. and 3. adds a gigantic amount of mass to the vehicle, reducing the payload capacity. when you're getting to orbit, payload is king. the more payload you can get to orbit the better. the success of many programs, manned or otherwise, can hinge on tens of pounds in the mass budget either way. that's the nature of the game. the first few flights of the shuttle, the tank was painted white. This created a nice, pretty, uniformly white vehicle. Then someone pointed out that the paint served no engineering purpose and was costing 500 pounds. The paint requirement was then deleted and 500 pounds more payload could be sent to orbit.
I can probably think of more engineering arguments, but it's late and Iv'e had to retype this once already.
I am sorry, this is not accurate. The foam was changed over to a new environmentally friendly one some time ago, even though the subcontractor was granted a waiver by the EPA to use the older/tested material. It is this new foam that is the problem. The decision to switch foam types was some what political as the controlling subcontractor was changed from Rockwell to Lockheed at that time. As I understand it, this is egg on the face of a particular colonel, and he seems to be very good at obfuscating his involvement.
According to this article:
http://www.capmag.com/article.asp?ID=4448
The foam that broke off was from the old foam....but it was stock piled BX 250. This foam weakens with age.
Gorkman
What the hell are you talking about? Yes foam and ice has always fallen off and yes it was always a risk, and yes TPS tiles have been damaged on previous shuttle flights. The new non-Freon based foam however did significantly exacerbate these risks. And Columbia most assuredly did NOT fly with an "old" tank, as the contractor who makes the tank had switched to the new foam several years earlier and they don't keep these things stockpiled. They're built to order for the mission. Do you have some reference to back that claim up? Otherwise whoever marked you as informative has done a severe disservice to the moderation system.
And as far as Challenger, the O-rings didn't erode, they became stiff and non-resilient when reduced to near freezing. Funnily enough the O-rings were replaced for a political reason much like the foam on the Shuttle's tank, and in both cases that lead to a loss of vehicle and crew. Where exactly are you getting your information?
Maxim: People cannot follow directions.
Increases in truth directly with the length of time spent explaining them
Carbon footprint? Are you trying to be funny? The shuttle burns liquid oxygen and liquid hydrogen. The resulting exhaust is not carbon dioxide, it's water vapor. Get a life.
Maxim: People cannot follow directions.
Increases in truth directly with the length of time spent explaining them