Mach 6 at Low Altitude: Craft Skin, Booms & Visibility

[Aero51]

That is true. Even still, higher pressures would correspond to a a greater rate of heat transfer and more drag (if we are going to compare the tunnel pressure to some altitude). Regardless, its a cool video and it shows what happens if your materials are not sufficiently heat resistant.

 

[chasrob]

...
Or is that a weapon instead? Or the air-breathing phase of a future British spacecraft launcher?

I saw this article on the Shaurya, and wondered if it made much of a boom. Or if it was smaller--man-sized or so--the boom would much less noticeable. A cruise missile that boomed half the neighborhood wouldn't be much use, wouldn't it?

If it could be considered a cruise missile in the first place.

 

[boneh3ad]

By the time a missile moving that fast is heard, it is too late. The missile would likely be miles past by then, not to mention that a maneuverable vehicle moving that fast would be nearly impossible to shoot down. I realize that RVs from ICBMs are moving much faster, but they are on a ballistic trajectory and don't maneuver in the atmosphere, greatly simplifying interception.

 

[Enthalpy]

I think this video will answer some questions with regards to aerodynamic heating at low altitudes. The video is a series of shapes ran in a supersonic wind tunnel between mach 6 and 7. This corresponds to 2040 to 2380 m/s or 4563 to 5323 mph. Not sure what type of metal they are, probably a steal alloy of some sort given that they melt so quickly.

It's an alloy chosen to melt at 158°F or 70°C. O good.

 

[Aero51]

It's an alloy chosen to melt at 158°F or 70°C. O good.

I don't see what you are trying to say here. It still demonstrates what happens if you use a metal that melts below the shock temperature.

And on the topic of this thread: I was reading a paper recently with regards to hypervelocity projectiles launched from a RAM accelerator. Based on the tests that were carried out at standard sealevel temperatures and pressures, some metals (aluminum and magnesium alloys) will actually oxidize and ignite. These tests were carried out with M ~ 6. It was also postulated that titanium would oxidize and react with the fuel air mixture at higher velocities. The name of the paper is:
[Ram Accelerator Operating Limits, Part 2: Nature of Observed Limits]
[A. J. Higgins,* C. Knowlen,t and A. P. Bruckner^
University of Washington, Seattle, Washington 98195]

Its a very fascinating topic!

 

[boneh3ad]

I don't see what you are trying to say here. It still demonstrates what happens if you use a metal that melts below the shock temperature.

What he is saying is that, cool though it may be, it doesn't actually add any insight to the discussion that was taking place.

And on the topic of this thread: I was reading a paper recently with regards to hypervelocity projectiles launched from a RAM accelerator. Based on the tests that were carried out at standard sealevel temperatures and pressures, some metals (aluminum and magnesium alloys) will actually oxidize and ignite. These tests were carried out with M ~ 6. It was also postulated that titanium would oxidize and react with the fuel air mixture at higher velocities. The name of the paper is:
[Ram Accelerator Operating Limits, Part 2: Nature of Observed Limits]
[A. J. Higgins,* C. Knowlen,t and A. P. Bruckner^
University of Washington, Seattle, Washington 98195]

Its a very fascinating topic!

That reminds me of the flame generated on the projectiles fired from the Navy's experimental rail gun.

 

[brenan]

How fast would an ICBM be traveling when it reached ground level after re-entry
I read somewhere they reach speeds around M10? But presumably they couldnt
withstand the temperature (going by this thread and trying to learn) of such
a velocity on the way down but they must still be moving pretty fast on impact?

 

[cjl]

ICBMs reach near orbital speeds - in the neighborhood of mach 20. They're designed to keep as much speed as possible on impact too, so modern ones do hit at mach 6-10. You're right that anything traveling that speed for long in the lower atmosphere would burn up, but they are only in the lower atmosphere for a few seconds (so the heat doesn't have time to do much more than vaporize a thin surface layer). They basically are very high-density, high ballistic coefficient reentry vehicles coming in with near-orbital speed.

 

[Aero51]

Also, if the ICBM has a nuclear payload it will detonation above ground. How much I don't know as I do not have a security clearance and do not want to go to jail.

 

[cjl]

Actually, that part isn't classified, and the answer is that they can be programmed to explode at various heights, depending on the desired effect.

 

[boneh3ad]

Also, if the ICBM has a nuclear payload it will detonation above ground. How much I don't know as I do not have a security clearance and do not want to go to jail.

Unless of course it is a nuclear bunker-buster...

 

[Aero51]

Cjl, that was a very vague sentence with no more information than mine. You do not know the precise effects of radiation and blast waves due to nuclear detonations at specific altitudes. That information is classified.

Bonehead, I did not know we built bunker busting ICBMs. Seems like a waste of money.

 

[cjl]

Cjl, that was a very vague sentence with no more information than mine. You do not know the precise effects of radiation and blast waves due to nuclear detonations at specific altitudes. That information is classified.

Bonehead, I did not know we built bunker busting ICBMs. Seems like a waste of money.

Actually, it does contain more information than yours. Yours indicates that they all detonate that they all detonate a preset distance above ground, and that distance is classified (which is incorrect). Mine indicates, correctly, that the distance can be set, and the distance required depends on the desired effect on the target. The specific effects of nuclear airburst at various altitudes are not classified either. For example, see this website for a couple of specific cases:

http://www.cddc.vt.edu/host/atomic/nukeffct/airburst.html

As for nuclear bunker busters, that is a concept which (to my knowledge) has never been fully developed into an operational weapon, but it has been investigated conceptually several times.

 

[boneh3ad]

Probably because its a ridiculous concept. The physics and engineering are sound, but practicality/morality is another story. I can't imagine a situation that would force that system's use, as you certainly wouldn't want to just go around nuking caves willy nilly.

In related lunacy, DoD at one point considered producing conventional ICBMs. As if you could somehow launch one and convince Russia it wasn't nuclear.

 

[nsaspook]

Also, if the ICBM has a nuclear payload it will detonation above ground. How much I don't know as I do not have a security clearance and do not want to go to jail.

We had a few of these type devices in the safe for doing damage estimates on strike targets. http://www.orau.org/ptp/collection/sliderules/brlnucweapeffcomp.htm

 

[etudiant]

The idea of low altitude hypersonic flight was considered in the late 1950s as a trajectory for a nuclear powered ramjet missile, called the SLAV (supersonic low altitude vehicle).
The design power source was a pebble bed reactor and the expectation was Mach 3+ at low altitude. The shock wave was expected to add to the destructive effect, as pushing a 3-500,000 pound vehicle through the air at that speed is an energetic affair and that energy has to go somewhere.
The concept was scrapped partly because ICBMs offered much better time to impact, but mostly because no one could envision how the vehicle could ever be landed or serviced without frying the airport used.
Your Mach 6 at 5000 ft concept would need even more power than the SLAV. I think you would have more than broken windows to worry about if it ever flew.

 

[chasrob]

Thanks for mentioning the SLAV; I've never heard of it before.

What my question was: could an airframe be built that would survive extended and repeated flights at 4000 mph. IOW, what materials could be used, and what shape would the airframe take at hypersonic speeds. Apparently the SLAV used a type of stainless steel, Rene 41, with a skin thickness of 1/10 to ?? inch.

Suppose it wouldn't stand up to Mach 5.3 low-altitude flight, eh?

 

[Aero51]

Guess I was wrong about the US classifying nuclear explosion effects. Seems extremely stupid. Then again, what could we do with that information?

 

[etudiant]

Hi Chasrob,
Power is the problem of low altitude/high mach number flight.
Barring nuclear propulsion, it is pretty much out of the question to fit fuel for more than a couple of minutes of flight under those conditions. So your question re extended and repeated 4000 mph flight at low altitude is moot, because we cannot engineer the propulsion system that would be required.
Note that the SR-71 cruises at about 80,000 ft at M 3.2, but can not sustain that for more than a couple of thousand miles. I believe the aircraft refuelled twice during its record breaking final run from California to the Smithsonian Air and Space Museum.
Air density at sea level is 30x greater than at 80,000 ft, which means refuelling 60x during a comparable low level cross country run as a first guess. Good news is that the wings become unneccessary for lift, a shaped body would be ample, but control would be a challenge.
The structures issue is a whole different can of worms for such a vehicle. You would obviously use your fuel to cool the most vulnerable parts of the structure, but note also that heating a 200 ft long vehicle from ambient to say 600 Deg F would generate substantial expansion, which in turn complicates the management of the shock waves the vehicle generates in flight, apart from basic integrity issues.
Many years ago, (1981-1982), NASA sponsored a set of workshops on solar power satellites. One of the concepts studied was solar powered airplanes, getting the power for their engines from satellite power receivers mounted on the airplane. It would take that kind of fuel free propulsion concept to make lower altitude hypersonics workable, but even then, I'm unable to think of a justification for operating in that mode. It seems willfully perverse to make the airframe and engine suffer dense air when the system is so much happier at altitude.

 

[etudiant]

Guess I was wrong about the US classifying nuclear explosion effects. Seems extremely stupid. Then again, what could we do with that information?

Au contraire, nuclear weapons effects should be widely publicized and ideally taught in public schools, at least imho.
These things are so insanely destructive that we have unfortunately ceased thinking about them at all.
Ignorance is not bliss, however. If the public were better aware of the challenges they pose, we might have more constructive debate about what to do about nuclear proliferation, before it really takes off.
I'm not eager for a world in which Samsung and the Mafia have the bomb, along with Hezbollah and various unnamed Liberation Fronts, but that is where we are currently heading, eyes firmly shut.

 

[Aero51]

From the perspective of a military power: The less an enemy knows about my capabilities, the better. Personally, I think the new front will be electronic warfare. This is expected considering everything we use nowadays requires a computer.

 

[etudiant]

From the perspective of a military power: The less an enemy knows about my capabilities, the better. Personally, I think the new front will be electronic warfare. This is expected considering everything we use nowadays requires a computer.

It seems clear that conflicts between more advanced states are currently conducted by computer hacks (Stuxnet for instance) rather than by bombs.
That said, there are few secrets about nuclear weapons effects, because the details were disseminated so widely during the civil defense effort of the 1950s and 1960s, so there is not much to hide there. However, there have been hints, notably from the late US nuclear weapon designer Ted Taylor, that it was possible to design nuclear devices to deliver a directed blast. Such a feature would greatly increase the military utility of nuclear weapons.

 

[Aaronvan]

There are videos of a Sprint missle launch going from 0 to mach 10 in a few seconds. It's white hot after only a few seconds at that speed. It had a range of about 25 miles and a typical intercept time was expected to be about 15 seconds.



There is information about the heat shield here:
http://srmsc.org/pdf/004431p0.pdf

Oh man, I love Sprint! You could cool it off with an acetylene torch!

 

[brenan]

... Good news is that the wings become unneccessary for lift, a shaped body would be ample, but control would be a challenge.

Ah - this is what was interesting me - could you or (or anyone) explain how flight works
without wings in such cases?
(any comments on the control issue would be interesting too)

 

[boneh3ad]

You can put just about anything at an angle of attack and it would fly, given enough forward velocity. If you strapped a rocket to a fridge and flew it at an angle, you could make it fly. Shapes like airfoils merely make flight more efficient at low velocities.

Once you reach supersonic speeds, camber and thickness to a surface no longer contribut a thing to lift, only drag. The most efficient supersonic airfoil is an infinitely thin flat plate at angle of attack, so most purely supersonic craft (e.g. many missiles) have no real need for wings. As long as they maintain an angle of attack they generate lift.

 

[brenan]

Yes this was what my earlier comment was about relating to cruise missiles - were they flying (with lift) or being constantly corrected for "falling". As far as I can tell the shaped body etudiant is alluding to is just a shape very similar to a wing. .i.e. nothing new or special just a merging of the body with the wing or shaping the body as a wing.

 

[etudiant]

Yes this was what my earlier comment was about relating to cruise missiles - were they flying (with lift) or being constantly corrected for "falling". As far as I can tell the shaped body etudiant is alluding to is just a shape very similar to a wing. .i.e. nothing new or special just a merging of the body with the wing or shaping the body as a wing.

You can get a sense of what lifting bodies from NASA look like here:
http://www.nasa.gov/centers/dryden/history/pastprojects/Lifting/index.html

 

[chasrob]

@etuciant: you're right, it makes more sense to expend more fuel to get up to altitude and accelerate to speed up there. I was reading about the hypersonic cruise missile Shaurya, and thought it was like most cruise missiles i was aware of, nap of the Earth hugging types. I thought that materials tech had advanced to the point where such profiles were possible, and wondered what those super-materials were and what shape the air frame took.

Although now I suppose the Shaurya first flies to 50km and dives into the target. Apparently the BrahMos sea skims at 3-4 meters doing M 3.