Transterrestrial Musings

Comments

Mr. Simberg wrote:
"and in particular, the notion that it shouldn't be allowed because it's a nuclear-powered ship is extremely spurious..."

There's another issue as well, which is safety. Russian nuclear propulsion at sea has an atrocious safety record and this includes their atomic powered icebreakers. There is the notorious story about the cook on the first Soviet atomic icebreaker who cleaned his pots and pans using the radioactive steam line and contaminated the entire kitchen. These vessels have never been safe, to their crews or to the environment. The environmentalists have a legitimate point here, which is that the Russians have amply demonstrated that they cannot be trusted to operate nuclear reactors at sea without causing significant environmental and safety problems (look at the Bellona website for more info: http://www.bellona.org/en/international/russia/icebreakers/index.html )

But you raise the interesting point about how much these revenue projects cover actual costs. My guess is that it is probably impossible for the Russians to figure out how much various things, like a Soyuz launch, actually cost, because they have no tradition of financial management. The US government finds this hard to do in the best of times, so nobody should expect the Russians to be capable of full cost accounting.

But as Jim Oberg is fond of pointing out, there is also the thorny issue of corruption. When a private entity purchases services from the US government (for instance, a studio renting military aircraft for a movie), there is no question that all of the money is going to the government and is not lining the pockets of public officials. But in Russia, it is entirely possible that when a Western company or individual brings over hard currency, some of that gets siphoned off into private pockets.

However, I would also point out that when it comes to the Soyuz tourist flights, there is no need for the tourist ticket to cover all or even most of the flight. The vehicle is already paid for and the tourist ticket only helps to offset those costs.

Posted by Dwayne A. Day at July 6, 2004 09:20 PM

"one of the few things that Russia can surpass
the US at, at least by some criteria"

Rand: so exactly what criteria are you
considering? Leaving human hamsters in
orbit for a ton of time on Mir. OK, Ill
give you that one, but what else, Buhran (sp?)?

BTW: Im curious about the "shock-free" supersonic
sniper bullets. I want to buy some right after
I buy the magnetic fuel filter that'll get me
100 mpg and a perpetual motion machine. So
where's the link for that?

It's pretty funny to listen to you bitch about
movie physics.

gkn

Posted by greg at July 7, 2004 12:04 AM

"one of the few things that Russia can surpass
the US at, at least by some criteria"

Rand: so exactly what criteria are you
considering? Leaving human hamsters in
orbit for a ton of time on Mir. OK, Ill
give you that one, but what else, Buhran (sp?)?

It's pretty funny to listen to you bitch about
movie physics.

gkn

Posted by greg at July 7, 2004 12:05 AM

"...exactly what criteria are you considering?

Ummmm...how about the ability to get crew and supplies to our space station?

Im curious about the "shock-free" supersonic
sniper bullets. I want to buy some right after
I buy the magnetic fuel filter that'll get me
100 mpg and a perpetual motion machine. So
where's the link for that?

And just what law of physics is it that requires that supersonic bullets create shock waves?

Posted by Rand Simberg at July 7, 2004 06:14 AM

>"...exactly what criteria are you considering?"

>Ummmm...how about the ability to get crew and supplies to our space station?

Here's a couple more:

The ability to dock spacecraft without human intervention.

The ability to allow paying, non-governmental astronauts on your spacecraft without complaint.

-S

Posted by Stephen Kohls at July 7, 2004 07:40 AM

And just what law of physics is it that requires that supersonic bullets create shock waves?

I'll handle this science lightweight Greg. The answer to your question Mr. Simberg is F=ma, Newton's second law of motion. Using nothing more than that plus some math that you couldn't possible understand one can derive Euler's equation, which is elliptic for subsonic flow and hyperbolic for supersonic flow, which then guarantees a shock. What do you have to say to that?

Posted by Brian at July 7, 2004 10:33 AM

Brian, you're joking, right?

Posted by Rand Simberg at July 7, 2004 10:58 AM

"And just what law of physics is it that requires that supersonic bullets create shock waves?"

Proabally more of an expediency of engineering as the design is somewhat constrained by the need to engage the rifling in order to spin stabilize the projectile. Therefore, a certain part of the projectile will consist of a cylinder deformed by four or six curved troughs where the rifiling has engraved the projectile.

BTW, Did ya'll know the reason for the boattail base on modern spitzer bullets was to enhance long range accuracy by minimizing any instabilities induced by the transition from supersonic to subsonic flight downrange?

This was first used to improve the accuracy of long range machine gun fire in the early part of the 20th century.

Posted by Mike Puckett at July 7, 2004 03:47 PM

Important Internet lesson: It is dangerous to declaim on a subject that you THINK you understand because it is probable others REALLY do. Everybody does an "oops" now and then, of course, I've done plenty of them.

I know just enough about the subject that I'd be pretty careful with the definition of "shock" and "shock waves" here.

What might a "shockless" bullet look like? Some type of cylinder or circular airfoil?

Posted by VR at July 7, 2004 05:45 PM

You got it, VR.

Just think of a DeLaval nozzle in flight, with low-entropy flow throughout. Just make sure you contour it so it doesn't choke.

Posted by Rand Simberg at July 7, 2004 06:01 PM

How do you keep the propellant charge confined behind a flow through cylinder?

What about the engraving of the rifling and the effects on airflow?

Assuming you get around that using sabots, you still have the problem that sabots aren't silencer/noise suppressor friendly that you still have to have to cover the muzzle blast and assuming you overcome this limitation, you would still have a supersonic crack from the sabots as they leave the muzzle and seperate.

Posted by Mike Puckett at July 7, 2004 07:09 PM

This seems a decent place to post this ...

Just noticed that Wired had an interview with Rutan. They say they are ON for the X prize flight and plan on THREE flights in the two week period! See

http://www.wired.com/news/space/0,2697,64123,00.html

As for the bullets, as long as we are speculating wildly, why not use a rail or coil gun design and forget all that chemical stuff? Then remember the question was if a bullet, in theory, could avoid shockwaves while flying and actually launching the thing is a mere detail ... :)

How did that discussion get started, anyway? I can't seem to find any references before "greg" asked about it.

Posted by VR at July 8, 2004 12:02 AM

How do you keep the propellant charge confined behind a flow through cylinder?

That's a good question, Mike, and one that I don't know the answer to. That may be one of the reasons that it never came into wide-spread use.

VR, this discussion started a few posts down, here in reference to my TechCentralStation column.

Posted by Rand Simberg at July 8, 2004 04:52 AM

Rand: So you never answered the question:
Where can one buy these wonderful shock-free
supersonic bullets? Ive got some down time
since the "time displacement"
equipment I bought is on back order.

Posted by greg at July 8, 2004 08:48 AM

The answer is nowhere, Greg. They are not for sale on the retail market (like much military equipment), they would probably require a gun designed to shoot them, and as far as I know they're not now, nor have they ever been manufactured for operational use. But it's not because they defy physics.

Posted by Rand Simberg at July 8, 2004 08:55 AM

I should add, as an aside to "Brian," who foolishly accuses me of being a "science lightweight," that F = ma is not Newton's second law of motion. That is a special case for constant mass. Newton's second law (which is not even close to a sufficient basis on which to make an argument as to whether or not the laws of physics require shock waves) is F = dp/dt.

Posted by Rand Simberg at July 8, 2004 09:00 AM

Regarding Newton's second law of motion, F=ma is just fine for all physics short of things traveling greater than 0.95 the speed of light, or quantum effects. Are you claiming that these bullets fit into one of those 2 categories? So yes F=ma plus a few handy rules like conservation of mass and energy are sufficient to describe the physics of fluid dynamics, and those physics require a shock for anything traveling supersonically. Did you receive a university degree in something resembling physics? From where?

Posted by Brian at July 8, 2004 10:16 AM

Regarding Newton's second law of motion, F=ma is just fine for all physics short of things traveling greater than 0.95 the speed of light, or quantum effects.

Really? How about for subluminal (and even subsonic) rockets, in which F = m*a + v*dm/dt? I think we'd have trouble working out how to figure out thrust, or the rocket equation, if we just said F = ma.

Not that it should matter, but I have multiple engineering degrees, which required lots of science and lots of math, from the University of Michigan. It's been a quarter of a century or so, though, so maybe the laws of physics have changed since then.

I'm still awaiting an explanation of what law of physics requires that (contrary to many wind tunnel tests) supersonic flow through a converging/diverging nozzle has to generate shock waves, and why.

I suspect that I'll wait in vain. Maybe because I'm just too much of a "science lightweight" for my betters to explain it to me. Or perhaps they're too much of a "science lightweight" to be capable of doing so...

Have you ever heard the old saying about Rule Number One when in a hole?

"Stop Digging."

Posted by Rand Simberg at July 8, 2004 10:30 AM

One word for Brian: schooled.

Posted by Nick at July 8, 2004 03:32 PM

F = m*a + v*dm/dt

Oh god, this really so painful. What you’ve written in this equation is that Force is equal to the sum of 2 (ma)s, which I agree with and can even derive from Newton’s second law of motion and some simple calculus, can you? I don’t have time for a simple calculus lesson, but v*dm/dt is the same as ma (think of you pushing on something to accelerate it then having your friend help you push on it too). One can manipulate expressions such as these with some higher math and a few other simple laws like conservation of energy and mass to find exactly what I stated originally Euler’s equation, and it demands a shock for anything traveling supersonically. It’s not just a good idea, it’s the law. The University of Michigan is a fine school; didn’t you learn any fluid dynamics there? And yes it should matter what degree you have, because either you are lying about that degree or you didn’t learn anything, or maybe you’re just having fun with the masses that still believe in 100 mile per gallon carburetor conspiracies, and perpetual motion machines. I guess I hadn’t thought of that.

Posted by Brian at July 8, 2004 04:14 PM

Here’s a way I have been successful in teaching undergraduates about shock waves sometimes. If an object is traveling supersonically the fluid ahead of it doesn’t know it’s coming because “information” (if you will) in a fluid, propagates at the speed of sound. So when the bullet gets to the next “element” of fluid, the fluid is “surprised”. The shock wave is the fluid’s response to being surprised. The fluid must move out of the way very rapidly, and that very rapid change (we call it a discontinuity sometimes) is the shock wave. I hate to explain it to you this way, because the physics and math really speak for themselves, but for you I make an exception. Liepmann and Roshko write an excellent text on this subject, please review that (assuming you really have an engineering degree) and let me know if you think they have made a mistake.

Posted by Brian at July 8, 2004 04:46 PM

Oh, god, this really so painful.

What you've written is that m * dv/dt is the same as v * dm/dt.

Do you really want to stand by that? If so, you don't just need to take a remedial course in calculus--you need to take a remedial course in basic algebra. I'll ignore the rest of your condescending post.

As to your other one, which is at least slightly more respectful, let me tell you that in fact I have read Liepmann and Roshko, and they don't back you up.

While a shock is a result of the fluid's not knowing that the airplane is coming, this only applies when it is hit by a blunt body that can't smoothly separate the flow.

However, a sharp leading edge isn't sufficient. While shock-free bodies can be made with a sharp leading edge and proper contours (see Ferri et al), lifting bodies cannot. This requires the invocation of Crocco's Relation, which implies mathematically that the only way to eliminate the entropy term necessary to balance the wing circulation is to add energy to the flow field.

Fortunately, this can be achieved by proper ducting of the engine thrust to provide a counter circulation needed to balance the wing circulation with induced vortices (and eliminating the need for the inefficient and highly entropic downstream vortices provided by the intersection of the expansion fans and the shocks waves), resulting in shock-free lift.

The devil is in the details, and the CFD, but there is no violation of any law of physics, including Newton's laws (the full ones, not the pablum passed out in high school, like F=ma).

By the way, does Lockheed Martin know that you're posting ignorant nonsense on the web with their email account? They'd be particularly amused (or not) considering that they've applied for patents using this very concept, and demonstrated it in CFD sims in the early nineties...

Posted by Rand Simberg at July 8, 2004 06:27 PM

By no means am I qualified to enter this discussion, but it did prompt a question: Rand, is what you're describing related to some of the work with using synthetic jets to inject momentum at certain points of an air flow to change its characteristics dynamically? Here's a reference:
http://www.aiaa.org/aerospace/Article.cfm?issuetocid=35&ArchiveIssueID=7

There's some work being done by a few guys at Georgia Tech (local to me so I know about it) that can give a tractor-trailer truck the same flow signature as a passenger car. Neat stuff...

Posted by Michael Mealling at July 9, 2004 08:31 AM

Looking at that paper, it is and it isn't. It's an entirely different application than any of those, but it does involve injecting a supersonic jet into a supersonic airflow (most or all of the applications that I see there look subsonic).

Posted by Rand Simberg at July 9, 2004 08:42 AM

And now just to clear up a few (intentional?) misunderstandings.

F = m dv/dt + v dm/dt
Whenever one sees an equation like this where 2 terms are equated to a force one can be certain that those 2 terms share the units of force i.e. Newton, so thank you Professor Hall for agreeing with me on this, and for your comments on the derivation of the rocket equation using nothing more than F=ma (forgive me for using a simplified form)

Now you might explain to Mr. Simberg that shocks generated by a supersonic body in flight can be controlled, bent, curved, reflected, and dissipated, and minimized, but they can’t avoided. Perhaps a discussion about how hyperbolic PDEs predict that sort of behavior.

Regarding Newton's second law of motion, F=ma is just fine for all physics short of things traveling greater than 0.95 the speed of light, or quantum effects.
He's apparently confused….
Of course I wasn’t confused, while the above is exactly correct, it was intended to be amusing in a sarcastic sort of way.

And finally, a putz, Professor Hall? Name calling when everything I posted was correct (even if I simplified it a bit for general consumption)? I’m shocked (and not in the gas dynamic sense). I expect so much more from a professor. What next? Are you going to ask me to step outside to settle this like a man?

Posted by brian at July 9, 2004 10:38 AM

Good lord. Of course they both have the units of force. That doesn't mean that they are the same thing--only that they have the same units.

I suspect that what Chris was primarily objecting to (as was I) was your condescending tone ("science lightweight", "math that you couldn't possible understand").

As for the the impossibility of shock-free bodies, I suggest you look up the work of Ferri and Busemann.

Posted by Rand Simberg at July 9, 2004 10:54 AM

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