… you know, an M1A1 Abrams isn’t anywhere near as sleek looking as those tanks in “Things to Come” or Popular Mechanics illustrations from the Thirties…
Well, I’m not entirely sure about this, but I know that the reason the Shuttle wasn’t made especially streamlined is for shockwave deflection — a blunt nose will give a much shallower shock cone than a pointy one.
If the Shuttle had a pointy nose, the shockwaves from travelling through the air at some ungodly multiple of Mach would wreak untold havoc on the aerodynamic control surfaces of the wings and tailfin. Also, having the shockwave as sort of a buffer between the Shuttle’s skin and the rest of the atmosphere gives the added bonus of a little less thermal heating on re-entry. At least, this is what I heard; I am not an aerospace engineer.
But those models and illustrations weren’t done by tank manufacturers predicting and promising the future of weaponry – they were done by illustrators who wanted something that looked futuristic and different.
The suggestion of some here is that the rocket engineers were doing a bit of both.
While hunting around for some cites to make a point here, I found this article that A) pretty much covers the OP, and B) completely blew away the point I was going to make.
http://www.astronautix.com/lvfam/vonbraun.htm
It seems that it comes down to the streamlined designs being drawn using Von Braun’s specs by artists for public consumption, and the designs were obsolete even before the last iterations went out. They were all based on assumptions that Von Braun made in 1948 and were updated more for their visual look than real-world engineering advances.
It’s pretty interesting. To be honest, I’d never looked at how and when these images were presented, I’d always assumed that the streamlined designs were either purely artistic creations, or from a time before anyone was really sure of what a rocket beyond the V-2 would really look like.
Excellent work, buckgully!
I think we have the winner. Rocket design is the art of compromise and optimization. If you see an exposed hinge, it’s probably exposed because some engineer did the math and determined that the small amount of additional drag was a better tradeoff than adding the weight of a hinge shroud or increasing the diameter of the rocket. Ribbed sections add a lot of strength with no addition of weight. Or perhaps they are ribbed to allow for expansion and contraction due to the cryogenic fuel. Or perhaps some of the external bumps are valves and various fluid lines and such put on the outside of the rocket because putting them on the inside would require more heavy insulation to protect them from the cold of the fuel, and it was worth trading off drag for weight.
In less optimal designs, you can often declare ‘good enough’ and move on. A rocket isn’t like that. There’s a reason for everything.
Point to any little bump or wrinkle on a rocket, and you can probably find a sheaf of documentation justifying it, sometimes with a reason you would never have imagined. There’s no ‘one big difference’ in how rockets are designed today.
Some of the stuff you see could also be for boundary layer control, control of supersonic shockwaves, and other aerodynamic tricks that we learned about from hard experience, wind tunnel testing, and later on, computational fluid dynamics and the like. Tools, techniques, and observations that may not have been available to Von Braun and his ilk.
Sometimes a change in one aspect of the rocket can have dramatic ripple effects on the design elsewhere. The addition of computer guidance and steerable engines means the elimination of fins, which means the elimination of support structures for fins and their aerodynamic loads, which changes the shape of the lower stage, which effects the shape of the upper stages, etc. Computer guidance means the rocket can be dynamically unstable, which further changes the shape. Airplanes undergo the same kinds of transitions when new technology changes a fundamental characteristic.
This thread is an example of SDMB at its collaborative best.
From the beginning people kept trying to tell you that your assumptions were wrong. As they proved to be. Looking good really was the driving factor even though you kept disclaiming the possibility.
This is not anything new or different. Many GQ threads are based on people making assumptions about answers and then not being able to understand why they’re wrong. They are wrong because they’re assumptions are wrong. Some people come to accept that, as you did, and some refuse to do so which makes for the banging-heads-against-walls threads.
Watch and see how many threads you come upon over the next few days that follow this syndrome. I think you’ll be surprised at how common they are.
CalMeacham, I think one factor to consider is that streamlining is not always energy efficient.
Laminar flow of fluids actually involves frictional losses along each laminar layer as it moves against each other layer. Going for a turbulent flow model, espeicially at the much higer speeds that rockets operate in, compared to aircraft, is going to reduce frictional drag.
I don’t think you’re reading them carefully enough – too many of the early replies seem to be making the assumption that I was asking why the rocklets we came up with didn;t look like the ones in pulp illustrations. wasn’t asking that, and had to point it out several times. My assumptions weren’t wrong – I was asking why the designs being presented by the rocket engineers (not illustrators or even science fiction writers, but the guys who were spending all their time on this, doing it for a living, and who had already been not mrely designing but builduing and flying rockets for up to twenty years earlier) were so dramatically different from the actual working designs used in the 1960s. And you can see it extremely clearly if you look at the Colliers magazine illustrations, or the Duisney shows of the 1950s, both of which I referred to, and were based directly ion the work of von Braun and his collaborators. It’s not merely streamlining per se, but also the overall look that accompanies that streeamlining.
And most of the replies, whether they addressed my question correctly or not were guesses – yours included. I got contradictory suggestions regarding the need for streamlining (and the original V2 designs , built by members of the team, were streamlined, so it wasn’t merely done by look, but was an implemented factor). It was suggested that you couldn’t streamline a multistage rocket, but that;'s precisely what they did do.
I have to thank buckgully for his excellent link (although I’d like to know more abnout where his source came by a lot of that info), and omphaloskeptic and Sam are correct about engineering tradeoffs, but I never did say that there was any one factor explaining everything about all the blips and bulges on the outside of the rocket. But there ARE two that would explain their absence – if 1.) Streamiling WAS of overriding importance (in which case the engineering tradeoff would have impelled the designers to build all those hinges, heat sinks, and other features inside, regardless of greater cost elsewhere) or 2.) It was being done to “sell” the idea to NASA and Congress and the public (I find it interesting that von Braun and company were scamming the Nazis, too, by workling on the A-9 that they were forbidden to do by remaming it the A-4B, as buckgully’ss link states.) Either or both of those factors (both, apparently) can explain why the designs looked the way they duid in the 1950s but ended up being built as they were in the 1960s.
I’m extremely curious about the design details of the capsules that omphaloskeptic refers to so offhandedly, and that I’ve never heard of. It’s bothered me why the Mercury and Gemini capsules had that weird ribbing on them, that he says was a heat sink. I’ve never heard that before, It doesn’t look like any heat sink I’ve seen (but I could see why it would be, in a compromise between efficiency of heat transfer against excessive drag), and would like to know more. How did they do away with that for the heavier Apollo capsule, which looks so streamlined? And it’s also bugged me that the Mercury and Gemini capsules had such flat and “busy” nonaerodynamic fronts, while the Apollo capsule WAS rounded as you’d expect (and as most missile fronts were – look at the front of the Titan without the Gemini capsule on it, or the Atlas or Redstone without the Mercury capsule. Or the Agena that was sent up to rendezvous with Gem,i ni. “Aerodynamic”-style streamlined noses weren’ty just a naive expectation of the uninformed public, they were the norm outside of the earliest manned spacecraft).
So, no, I don;'t believe at all that I was ig noring the persistent suggestions of people. I answered criticisms that were inappropriate because they hadn’t properly read the OP, and I gave my reasons for doubting unsupported suggestions until better-supported and better reasoned posts came along. And I think Walloon’s comment is dead on. Thanks to all who contributed.
I think Exapno was being kind of harsh. I think it is a good question; I just don’t think it has a single answer. buckgully’s link says that the pictures you saw were just outdated versions of pictures of earlier designs, which nicely explains why the pictures all looked the same but doesn’t explain the major changes in shape for the later designs. I’d guess the gross shape changes are (as mks says) due to better understanding and design for supersonic flight; I don’t really understand the supersonic regime, though. My previous post was just pointing out that every little bump has its own story, trading off aerodynamics for something else for a net gain.
It’s pretty hard to find good online technical documentation on the early space missions. The first clue is of course that they are a corrugated, non-glossy black that you don’t see anywhere else on the rocket–it just looks like a heat sink. Here’s the best description I can find; it’s quite a nice summary of the sort of extra engineering problems that come with manned spaceflight. Page down to the section “GEMINI THERMAL RADIATION CONTROL COATINGS” for a discussion of the requirements for thermal control and the various layers used. You can also Google with “Rene-41,” the high-temperature nickel alloy used.
Part of the answer is “heavier”–i.e., another engineering trade-off; and part is technological advances. Apparently Apollo used ablative heat shielding for the conical upper section as well as the bottom disc. Read the link for some of the tradeoffs (higher weight; but probably able to shed more heat, and also providing some radiation shielding to the crew).
Here I think you’re comparing the wrong pictures. The Mercury, Gemini, and Apollo capsules all had relatively flat fronts; these were all covered with some sort of aerodynamic and protective shell (and sometimes a launch escape tower) for launch. (Here’s Gemini 7 from Gemini 6 and here’s Apollo 16 from the LEM.) But of course the aerodynamic front is only needed for launch; after that there’s no reason to keep the top side aerodynamic. The strange cone-cylinder shape of Mercury and the similar Gemini design appears to be mostly a tradeoff between reentry stability, afterbody heating, and parachute stowage.
Contrasting manned capsules with the payload sections of unmanned missiles I think is another apples-to-oranges comparison. For unmanned missiles, all you see is a lightweight aerodynamic shroud. The payload doesn’t need food or water, doesn’t get tired of the dark, and usually doesn’t have to come back in one piece, so the shroud can be installed over the payload days before launch; all that’s required of the shroud is that it be able to separate itself from the payload without damaging it. There’s a lot of extra engineering in making a large payload, easily accessible up to launch time, easily separable at any point in the launch, and able to withstand reentry.
No – I clearly recall that Mercury capsules had a non-rounded tip at launch, and so , IIRC, did Gemini. Apollo definitely had a rounded cover at launch (and was surmounted by that Emergency Escape Tower construction.
My point about the non-manned rockets is that they always had rounded tops. So I don’t think I’m comparing the wrong pictures at all.
I built quiite a few models of these ships as a kid – I got to know the tips quite well.
Ablative coatings were studied at the Avco Everett lasns where I worked for a time. They had (still have, in fact, in their new home) a picture on the wall comparing the ablation observed vs. the predicted ablation from a re-entry tip. The tip is rounded. I was under the impression that they used such material in all manned flights, but I never checked up on it. Kantrowitz’ work wass early enough for it to be applied to Mercury.
No – I clearly recall that Mercury capsules had a non-rounded tip at launch, and so , IIRC, did Gemini. Apollo definitely had a rounded cover at launch (and was surmounted by that Emergency Escape Tower construction.
My point about the non-manned rockets is that they always had rounded tops. So I don’t think I’m comparing the wrong pictures at all.
Picture of Gemini Capsule atop Titan – no cap:
http://en.wikipedia.org/wiki/Image:Titan2.jpg
Mercury Capsule:
Apollo Capsule at launch with rounded cap:
http://www-pao.ksc.nasa.gov/kscpao/history/apollo/apollo.htm
I built quiite a few models of these ships as a kid – I got to know the tips quite well.
Ablative coatings were studied at the Avco Everett lasns where I worked for a time. They had (still have, in fact, in their new home) a picture on the wall comparing the ablation observed vs. the predicted ablation from a re-entry tip. The tip is rounded. I was under the impression that they used such material in all manned flights, but I never checked up on it. Kantrowitz’ work wass early enough for it to be applied to Mercury.
We might be talking about different things. Your Gemini image doesn’t show the top very well (I can’t find any good top-down views of the capsule in launch configuration), but there’s a white cap there that isn’t there in the picture I linked, which I assumed was some sort of aerodynamic cover. Similarly for Mercury and Apollo the escape tower and cover provide an aerodynamic cover but come off to reveal a flat top. Are you talking about the overall cone-cylinder shape of Mercury or two-cone shape of Gemini versus the smoother blunt single-cone shape for Apollo?
What “tip” do you mean? Mercury, Gemini, and Apollo all used ablative shields on the bottom (forward-facing) surface, but apparently (see links in previous post) only Apollo used ablation instead of radiation on the conical afterbody.
I wasn’t trying to be harsh. Just that the OP often sees things through different eyes than the rest of us and it’s sometimes frustrating to have what seems to be an obvious point rejected. Since this is so common around here, it’s something OPs should be sensitive to in their own behavior. That helps everyone.