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Sunday, May 8, 2011
Outer Space ... Science ...

The Space Shuttles are making their last flights and will soon be safely stuffed and mounted for all to see in museums. Many people look back at them with disappointment. They turned out to be a good bit more expensive, unsafe, and difficult to use than advertised. Most of the experts agree that America’s future in manned spaceflight, if and when there is such a future, will not utilize the airplane-like “space truck” design of the Shuttle. Although the Orion successor program was recently halted, it boldly went back to past designs, i.e. Apollo-like capsules on top of tall rockets. The Russians also experimented with something like the Shuttle, but took a pass and stuck with their old-fashioned Soyuz capsule (which is now the only way to get a human being into space and back).

But I wonder if some of the disdain for the Shuttle is over-reaction. Yes, the Shuttle was not designed for deep spaceflight, it couldn’t go to the moon or to an asteroid, nor could it form the keystone for an eventual Mars mission. But it was a pretty useful tool when it worked; it allowed Americans to place and repair important satellites (including the Hubble telescope), was a mobile space science lab, and made a pretty good delivery and assembly vehicle for the International Space Station. Had it worked better it may well have had important military applications, such as inspecting suspicious space vehicles sent up by not-so-friendly humans. We are probably going to miss it.

So let’s pretend that the USA was still flush with cash, and not tipping on the edge of insolvency. Let’s pretend that we are with NASA, to which Congress has given a blank check to build a new generation of Space Shuttles. We will imagine that the basic functionality will remain the same (launch with re-usable rockets and tank, land like an airplane, carry 7 or 8 people with a lot of room for cargo). The most important thing will be to make it much safer. We have to fix the inherent flaws in the Shuttle design that led to the Challenger and Columbia disasters.

OK, so let’s play engineer. First thing we engineers do is to look at a diagram – so let’s look at a diagram of the basic Space Shuttle launch configuration. Looking back, we know there were several design features that went bad. First, having the solid fuel rockets right next to a big tank full of explosive propellant. If anything bursts at the seams on those rockets, the whole shebang blows to kingdom come (orange arrows). Second, if that happens, the astronauts are sitting ducks; they are positioned right nest to the center of the tank. There’s no way to get them away safely if something goes wrong suddenly; by contrast, the old Apollo and Mercury rockets had escape rocket towers to pull the manned capsule away if the rockets below went boom.

And third, of course, is the ice problem. The tank holds really cold stuff, and during a launch, ice flies in every direction from it. Not much you can do about that, except to try to keep the critical surfaces from being exposed to that ice shower. The blue arrows show what went wrong with Columbia, as a chunk of ice-infused insulating foam bashed a hole in the wing’s front edge during the acceleration of ascent.

So what do we do to fix that? Well, maybe we can make the Shuttle more like the old Apollo-Saturn rocket. Let’s put the new Shuttle on top of the fuel tank and booster rockets. Let’s make the tank into a rocket itself, with liquid fuel engines at its bottom (that way the Shuttle vehicle itself will need less powerful engines, saving some weight and space). We can even put an Apollo-like escape tower on the Shuttle nose, and design the crew area to break-away in event of a launch emergency. So, the Shuttle and its crew would now ride above all the ice and fire, and would have a chance of survival if worst came to worst. The launch support structure will need to be more complex for this arrangement, so it will take longer to set up for a blast off. But otherwise, it sounds good! What was so hard about that?

Unfortunately, there is a fatal flaw with this. The Shuttle’s wings would make this rocket set-up terribly unstable while rising through the atmosphere. It could well spin out of control, despite the fast computers now used to instantly adjust engine thrust vectors. Arrows have their wings at the base, not up near the point, for good reason. Similarly, rockets with wings need to have them down at the base. Another problem: the tank would need to be more structurally rigid to handle the thrust forces from the engine below it, adding weight that reduces the carrying capacity of the Shuttle.

OK, back to the drawing board. We’re going to have to put the fuel tanks and booster rockets along the body of the Shuttle after all. But maybe we can re-arrange things as to mitigate if not completely solve the current Shuttle’s flaws (and keep the wings near the bottom). First off, how about if we put the solid booster rocket and fuel tanks on opposite sides of the Shuttle’s wings. Second, maybe we could split the tanks up into two shorter tanks, such that any falling ice during the ascent would either avoid the wing edge, or hit it with relatively low velocity (wouldn’t fall very far against the accelerating Shuttle wing). Third, how about if we lengthen the shuttle a bit, and still add that break-away crew cabin up front, with an escape tower attached to it. It might not be perfect, but maybe it would give the crew a fighting chance in many launch failure situations.

So, here is my proposal. We put the two tanks up on top of the wings, on the Shuttle body shoulders in effect. We line the two booster rockets up parallel to the tanks, but attached to the heat-shield side of the wing. If anything went wrong with the booster rockets, like a seal failure, the hot burning gas would hit the heat shield, which was built for heat anyway. And indeed we will lengthen the Shuttle such that the crew cabin is up above the fuel tanks and booster rockets, with the escape tower on top. If an explosion occurred during the ascent, the crew may or may not survive, but at least the escape tower would have a clear shot at getting them away from the mayhem.

But of course this introduces new problems. The wings will need to be heavier to assume the loads from carrying the tanks and absorbing the booster rocket thrust. That means lower payload capacity. But even worse, the whole thing would probably go into a tipping-over spin, because the booster rockets put out more force than the main body engines. In the original Shuttle, the booster rockets in effect lifted up the heavy fuel tank, while the main engines on the bottom of the Shuttle’s body supported only its own weight. Now the booster engines are far out of line with the fuel and the overall center of gravity of the vehicle being launched.

So, what we need to do is to make the Shuttle’s body engines bigger and stronger, as to take up more of the heavy lifting relative to the booster rockets. This could even-out the thrust distribution as to avoid tipping out of control. But liquid-fuel engines take up precious space on the Shuttle, and add to weight problems during re-entry. So, we are going to need to add a component, a “first stage” engine segment attached on the bottom of the Shuttle body that sucks in fuel from the tanks, but falls away before reaching orbit. Unfortunately, this new component will probably need to go way up, almost into orbit with the Shuttle. Thus, it might not be recoverable and re-usable, adding to the cost of each launch.

The overall set-up now mostly hides the Shuttle’s wings, so there won’t be much of an instability problem in the atmosphere. But the two-tank arrangement does have a down-side: there is a bigger area from the top-down view (two tanks now versus one before), thus more surface being thrust against the winds during the ascent. There is increased air drag because of this, and our payload capacity will suffer.

Oh, another problem. As to allow a more workable escape arrangement, we lengthened the Shuttle body forward of the wings. This will be OK on launch and in space, but it might cause increased instability during re-entry and atmosphere glide to the runway. We may be able to compensate during re-entry with modern fast computers firing directional rockets in the nose. But once the Shuttle reaches thicker atmosphere, around 20 miles up, the long-body delta aircraft design might cause many control problems. As to compensate for that, we might have to add front canard wings, as in the diagram. These are commonly used in modern delta-wing fighter jets such as the Eurofighter Typhoon, for high-speed stability. Problem: these canards might not make it through re-entry; they would need to be flush with the body for most of the flight, then fold-out once the returning Shuttle reaches dense atmosphere and is gliding for a landing.

So welcome to the world of engineering design compromise. We might now have a slightly bigger and safer Space Shuttle. But we have reduced its lift capacity and increased its costs and complexity. If the previous Shuttle was hard to maintain and prepare for launch, this one will be even worse. And there could be other new problems, such as the possibility that when the tanks are detached after running out of fuel, they could drift upward and damage the tail; if that happened, the Shuttle couldn’t glide in the air to a runway. Now maybe you can see why such an imperfect design for the original Shuttle was settled upon. Such is the real world of engineering.

[With thanks to Ben A. for his corrections. He summed it up well in saying that “the problems that the space program present are not easily solved”.]

◊   posted by Jim G @ 8:26 pm      
 
 


  1. Jim, The most I can say here is beautiful and understandable diagrams–understandable even by someone who knows NOTHING of engineering, specifically me.

    So, sorry I cannot say anything at all about your proposal and/dor comments on re-engineering the space shuttle.

    It does remind me, tho, of one time back in the 1960s when studying for my first masters: For some reason I opted to audit an engineering course. Something about its description caught my attention, and I tho’t I might gain some insight, information, learning that would be useful and valuable to me. After I asked my first question, the prof told me that if I had to ask that question, I should not be auditing his class. Needless to say, I did not attend any more. So, I was even thrown out of auditing an engineering class. Thus all I can say is great diagrams. MCS

    Comment by Mary S. — May 9, 2011 @ 1:31 pm

  2. You would need to re-imagine the whole concept from ground up. A redesign would be insufficient. As a big hint, we would be unlikely to attempt to make another vehicle that was nearly as large again. The shuttle size was dictated by events that were beyond NASA’s control, and spoke to cold war politics and to aspirations of it’s technical capabilities that were ultimately impractical and unattainable.

    [Interesting thoughts. Space shuttles may make a come-back, but could well be much smaller — e.g. Swiss S3, USAF X-37B.]

    Comment by Bud1 — March 20, 2013 @ 6:32 am

  3. The Columbia disaster was caused by a piece of insulating foam that broke off of the external fuel tank [Correct — but the foam was there in the first place because of the inherent ice problem from tanks holding cryogenic liquids; likely that the foam chunk that broke off held frozen atmospheric moisture within its foam pockets, giving it increased mass and thus impact energy; the break-off process could well have been triggered by the icing of penetrating moisture]. In fact, the launch criteria for the Space Shuttle specify that the temperatures be well above freezing, so there is no surface ice. [Launch criteria was for ambient air temperature, as to avoid ring seal problem on solid-fuel boosters, re: Challenger disaster; but the tank skin temp was well below that; top of foam could be 60 degrees F, 1/2 inch down could be below 0 C.] Also, there would need to be a ton of reinforcing done to the shuttle body to reposition the liquid tanks, especially if they are going to be used for thrust. That reinforcing would be heavy, and complicate aerodynamics. [Yep.]

    It’s great to conjecture about things like this [glad you enjoyed it]. But the problems that the space program present are not easily solved [glad you agree].

    Comment by Ben A. — May 28, 2014 @ 5:16 pm

  4. Excellent overview, review and analysis. I agree that the inherent design flaws in the shuttles will ensure that they remain on display in perpetuity. Having said that “less is more” and “if it ain’t broke don’t fix it”. The original Apollo-Saturn and Soyuz rocket designs therefore are the way to go. As the Soyuz capsules hide behind a protective covering for liftoff and extends its solar panels in orbit a redesigned shuttle should sit on top the rocket with a capsule like retry modified and improved for more precise trajectory, counter propulsion, maneuverability and remote guidance redundant capacity for more precise terrestrial real soft landings.

    Comment by LC — March 12, 2016 @ 9:22 am

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