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

[chasrob]

A cruise missile, small--say 10 feet long, making 4000 mph at 1 mile altitude-

Would it make a boom loud enough to break windows? The smaller the craft, the less the boom, correct? Probably would be a tough one to answer; no aircraft I'm aware of is that small, making that kind of speed.

The X-15 could do Mach 6, but at what, 20 miles up. Any ideas about what the skin of a craft built for low altitudes could be made of? Something made of the same material as the shuttles tiles?

At that speed and low, one mile altitude, would the air be ionized and the skin glow? How would it appear from the ground, in other words?

Also how fast is mach 10?

The speed of Mach 10 is ten times the speed of sound, which is approximately 7,660 miles per hour (12,345 kilometers per hour) at sea level and room temperature (about 68°F or 20°C). Therefore, Mach 10 is:

Mach 10 = 10 times the speed of soundMach 10 ≈ 76,600 miles per hour (123,450 kilometers per hour)

It's important to note that the speed of sound varies with factors like altitude and air temperature, so the specific speed of Mach 10 may differ slightly depending on these conditions. This calculation provides a rough estimate for typical conditions at sea level.

 

[boneh3ad]

It's hard to say how strong the boom would be in that case, but despite the small size, the strength of the shock is enormous. It depends a lot on the design of the nose, and the most relevant vehicle to what you are asking would be the X-51, but the design data for it to my knowledge is not publicly available. Various assumptions can be made, though. Let's say, just for grins, that the nose is essentially a wedge with a 12-degree half-angle. Under those conditions, the pressure ratio across the shock is 4.538. That's a pretty substantial pressure increase so the intensity of the boom relative to the weight would likely be pretty high.

As for temperature and the air, the temperature ratio across the shock assumed before is 1.694. At a mile altitude, the average temperature is about 278 K, so the temperature behind the shock for most of the shock would be around 471 K, which is nowhere near the temperature needed to ionize the air. However, right near the nose tip where the shock is quite possibly detached, the shock is essentially a normal shock, and the temperature ratio behind that at Mach 6 would be 7.941, giving a temperature of 2207 K, which certainly could start to ionize the air, though in a very tiny region. In other words, it would likely look like the tip was glowing but the rest of the vehicle would look largely like it does while not moving.

 

[chasrob]

Thanks for the info, much appreciated.

I realize much of the Waverider tech is classified, but do you think a larger vehicle able to carry several passengers is doable with today's tech? The structure... the body of the aircraft, I mean, not the propulsion method to get up to that speed.

 

[The Jericho]

Well, no. An aircraft traveling at Mach 6 at an altitude of 1 mile would have a very short life because of the structural fatigue-induced failure as well as excessive thermal fatigue in the outer skin. And the sonic boom could shatter a single glazed garden shed window :)

 

[boneh3ad]

Fatigue wouldn't be noticeably more problematic for a hypersonic vehicle than it is with a standard airliner except in the sense that more exotic materials are less likely to be fully understood in terms of fatigue life. The real issue is having materials which can withstand the temperatures without being extremely heavy or fragile, since most materials that we know of that come close generally are quite dense or else are very brittle. Propulsion aside, I don't think we have the technology right now to produce a large, sustained flight hypersonic vehicle. The materials are just as far if not farther from being ready than the propulsion.

 

[chasrob]

[...]Propulsion aside, I don't think we have the technology right now to produce a large, sustained flight hypersonic vehicle. The materials are just as far if not farther from being ready than the propulsion.

How about a structure like the Waverider, no wings, wedge/bullet shaped; say, disregard the small fins/stabilizers too. Still able to carry several passengers; the size of a small civil aircraft. Also, say weight does not matter, so you could have a 3 inch thick titanium alloy skin.

 

[Borek]

How about a structure like the Waverider, no wings, wedge/bullet shaped; say, disregard the small fins/stabilizers too. Still able to carry several passengers; the size of a small civil aircraft. Also, say weight does not matter, so you could have a 3 inch thick titanium alloy skin.

This is a question of the "how do the laws of physics look like when there are no laws of physics" type. We don't speculate here.

 

[boneh3ad]

How about a structure like the Waverider, no wings, wedge/bullet shaped; say, disregard the small fins/stabilizers too. Still able to carry several passengers; the size of a small civil aircraft. Also, say weight does not matter, so you could have a 3 inch thick titanium alloy skin.

Good luck with stability of that craft. You have just removed all control surfaces. I wouldn't ride it. It also doesn't solve the issues of thermal protection either, even in this "ideal" case you posit.

 

[Astronuc]

How about a structure like the Waverider, no wings, wedge/bullet shaped; say, disregard the small fins/stabilizers too. Still able to carry several passengers; the size of a small civil aircraft. Also, say weight does not matter, so you could have a 3 inch thick titanium alloy skin.

Aerodynamic heating would likely be prohibitive at low altitude for high mach. One can look into the SR-71 for those issues. Some of the highest speed flights of X15 used an ablative covering to remove the heat.

The waverider concept has already been proposed under the National Aerospace Plane project.

The aerodynamic configuration was an example of a waverider. Most of the lift was generated by the fuselage by compression lift. The "wings" were small fins providing trim and control. This configuration was efficient for high-speed flight, but would have made take-off and slow-speed flight difficult.

Temperatures on the airframe were expected to be 980 °C (1800 °F) over a large part of the surface, with maximums of over 1650 °C (3000 °F) on the leading edges and portions of the engine. This required the development of high temperature lightweight materials, including alloys of titanium and aluminium called gamma and alpha titanium aluminide, advanced carbon/carbon composites, and titanium metal matrix composite (TMC) with silicon carbide fibers.
. . .

http://en.wikipedia.org/wiki/Rockwell_X-30

The outer surface would need have strength and oxidation resistance at high temperature. Not too many materials like that. And thermal fatigue would be an issue.

http://www.rand.org/pubs/reports/2007/R3878.1.pdf

The craft needs 'wings' or control surfaces with lift capability at 'low speed', if one wishes to land safely after flight. Otherwise, the craft is a ballistic missile.

 

[Enthalpy]

The thermal stress is not much worse at low altitude than at high altitude. It's more a question of mechanical loading, but if one is willing to build sturdy, no reason.

If all leading surface are swept they'll keep bearable temperatures. We're no more in 1950. Only the nose needs better materials, but just use the proper one, it's not that hot.

The limits I see are not of pure feasibility:
- Why fly low when you can fly high? You're pulling our leg with passenger transport.
- How to make money with a thick heavy plane?
- How to convince airframe designers to do it? It will not be built like traditional planes.

 

[chasrob]

Aerodynamic heating would likely be prohibitive at low altitude for high mach. One can look into the SR-71 for those issues. Some of the highest speed flights of X15 used an ablative covering to remove the heat.

The waverider concept has already been proposed under the National Aerospace Plane project.

http://en.wikipedia.org/wiki/Rockwell_X-30

The outer surface would need have strength and oxidation resistance at high temperature. Not too many materials like that. And thermal fatigue would be an issue.

http://www.rand.org/pubs/reports/2007/R3878.1.pdf

The craft needs 'wings' or control surfaces with lift capability at 'low speed', if one wishes to land safely after flight. Otherwise, the craft is a ballistic missile.

Great links, thanks.

Those temperatures in your quote--wouldn't the craft hold its speed down until it reached altitude, to limit stress and thermal issues? The big problem with my scenario is the low altitude, thicker atmosphere, to my thinking. I was also thinking about multi-hour 'flights'.

Regarding the passenger vehicle, I simplified it(with no control surfaces, etc.) in order to ask about its very general shape, outer surface composition, but mainly I was wondering if such a dumbed down arrangement could be built with today's tech.

 

[chasrob]

Well, no. An aircraft traveling at Mach 6 at an altitude of 1 mile would have a very short life because of the structural fatigue-induced failure as well as excessive thermal fatigue in the outer skin. And the sonic boom could shatter a single glazed garden shed window :)

The small cruise missile I posited? Other folks I asked were not so sure.

I read somewhere where boom intensity does not appreciably increase above mach 1.3.

 

[chasrob]

Good luck with stability of that craft. You have just removed all control surfaces. I wouldn't ride it. It also doesn't solve the issues of thermal protection either, even in this "ideal" case you posit.

You're absolutely right, I wouldn't go anywhere near such a beast!

So there's no metal alloy or multi-layer ceramic/alloy that can handle the thermal issues, especially an extended flight of a couple hours? You said, above, 2200 Ks--wow.

 

[chasrob]

The thermal stress is not much worse at low altitude than at high altitude. It's more a question of mechanical loading, but if one is willing to build sturdy, no reason.

If all leading surface are swept they'll keep bearable temperatures. We're no more in 1950. Only the nose needs better materials, but just use the proper one, it's not that hot.

The limits I see are not of pure feasibility:
- Why fly low when you can fly high? You're pulling our leg with passenger transport.
- How to make money with a thick heavy plane?
- How to convince airframe designers to do it? It will not be built like traditional planes.

You're right its not feasible with those issues.

I was just wondering if it could be built with recent tech, and simplified it so the many problems about lift, propulsion, could be made manageable for an informed guess about materials that could handle stress and heating issues. And if, say, a general wedge shape would be ideal for the overall structure.

Weird question, I agree.:grin:

 

[boneh3ad]

Not even the superalloys like Incoloy or Inconel come close to the temperature resistances needed for those types of temperatures. Things like tungsten and iridium could withstand it, but all those sorts of metals are either exceedingly heavy or exceedingly rare and expensive or both.

Like I said, the problem isn't inventing or discovering materials that can withstand the heat. The problem is finding materials that can withstand the heat that are light enough, strong enough and cheap enough to make it economical, and that we just don't have.

For what it's worth, the difference in temperature at 1 mile vs. 8 miles or 10 miles. The temperature difference would be deadly to a person, but in an absolute sense, it isn't that much.

 

[chasrob]

Not even the superalloys like Incoloy or Inconel come close to the temperature resistances needed for those types of temperatures. Things like tungsten and iridium could withstand it, but all those sorts of metals are either exceedingly heavy or exceedingly rare and expensive or both.

Like I said, the problem isn't inventing or discovering materials that can withstand the heat. The problem is finding materials that can withstand the heat that are light enough, strong enough and cheap enough to make it economical, and that we just don't have.

For what it's worth, the difference in temperature at 1 mile vs. 8 miles or 10 miles. The temperature difference would be deadly to a person, but in an absolute sense, it isn't that much.

So its possible, but not economically feasible--thanks!

Wouldn't heat build up over a couple hours through conduction, made worse by the low altitude?

 

[boneh3ad]

So its possible, but not economically feasible--thanks!

Wouldn't heat build up over a couple hours through conduction, made worse by the low altitude?

How is that what you have taken from this? Plain and simple, it is not possible given current technology. Even those exotic metals, while they won't melt, would experience notable expansion and weakening at those temperatures. Oxidation would be a problem. Thermal fatigue. It just wouldn't work.

And heat will build up over time to a point. Once the surface reaches the temperature of the air around it though, no more heat gets transferred.

 

[nsaspook]

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

 

[chasrob]

How is that what you have taken from this? Plain and simple, it is not possible given current technology. Even those exotic metals, while they won't melt, would experience notable expansion and weakening at those temperatures. Oxidation would be a problem. Thermal fatigue. It just wouldn't work.

And heat will build up over time to a point. Once the surface reaches the temperature of the air around it though, no more heat gets transferred.

Oh, OK, it's impossible, I misunderstood your tungsten and iridium remarks. I'm still under the Saturday slows. Gotta get me that second cup of java.

 

[chasrob]

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

Awesome, as the kids say. 130 g's acceleration! Mucho thanks for the pdf, I've downloaded it and will read later.

 

[The Jericho]

It was ambiguous to ask if the boom intensity was enough to "shatter windows" so I was being sarcastic ;). That's interesting though, I imagine the relative distance is the biggest factor to boom intensity however it's not something I've given much attention to.

 

[chasrob]

Heh, heh, the whole question was kind of ambiguous, eh?

Wasn't the question of altitude and boom studied back in the 1960's in prep for SST designs and it was found out that that how high you were didn't alleviate it that much? I seem to have a hazy recollection of that.

 

[Enthalpy]

1800°C isn't that hot. Too much for the old banal nickel alloys, but we have plenty of ceramics that handle it without any cooling, like stabilized yttria, magnesia, and marginally alumina. They would resist the heat indefinitely. Then, you have to live with materials more brittle than a metal - on the other hand, lift is strong enough at such speed that the airframe can be sturdy. Essentially a lifting body with stabilizers.

By the way, electric ovens produce this temperature and more, for unlimited duration, just to put the difficulty in context. What was extremely difficult with the materials of Apollo and Minuteman time is now just difficult engineering.

Stability remains a concern, more so than temperature. You end with a cone more or less, and fins protruding at the aft.

 

[boneh3ad]

1800°C isn't that hot. Too much for the old banal nickel alloys, but we have plenty of ceramics that handle it without any cooling, like stabilized yttria, magnesia, and marginally alumina. They would resist the heat indefinitely. Then, you have to live with materials more brittle than a metal

Which is still quite a problem. Imagine a bird strike in a situation like that. This is precisely why I mentioned that our current materials that can handle the heat are generally too brittle, too heavy or both. Ceramic-metal composites are one promising solution, though we'll see if they are ever economical.

By the way, electric ovens produce this temperature and more, for unlimited duration, just to put the difficulty in context. What was extremely difficult with the materials of Apollo and Minuteman time is now just difficult engineering.

Ceramic ovens can certainly produce the temperatures you would see at Mach 6, but they don't come anywhere close to Apollo-like temperatures. A typical ballistic re-entry will see around Mach 22 at 200,000 ft altitude. At that altitude, the temperature is roughly an average of about 240 K. That speed with those temperatures translates to an air temperature behind the bow shock of around 23,000 K. I don't know of any material that can withstand that. That is nearly 4 times the temperature at the "surface" of the sun.

Stability remains a concern, more so than temperature. You end with a cone more or less, and fins protruding at the aft.

I would argue both are still a huge concern. Stability really isn't all that difficult at those speeds. The problem is having a vehicle that is stable at those speeds and at the low subsonic velocities required to get to those hypervelocities.

 

[Enthalpy]

A typical ballistic re-entry will see around Mach 22 at 200,000 ft altitude. At that altitude, the temperature is roughly an average of about 240 K. That speed with those temperatures translates to an air temperature behind the bow shock of around 23,000 K.

Far less than 23,000K because the dissociation of oxygen molecules absorbs so much heat. If this didn't suffice nitrogen would also dissociate.

To put an Earth atmospheric reentry into perspective, the atmospheric entry probe from Galileo survived a dive at 48km/s in Jupiter, not 11km/s as Apollo did, nor 7.5km/s when entering from low-Earth orbit.
http://en.wikipedia.org/wiki/Galileo_( spacecraft )#Atmospheric_entry_probe

Anyway, the thread is about 4000 mph = 1800 m/s.

I'm not too much anxious about a bird strike. Zirconia for instance isn't a brittle ceramic, and to fly at this speed the frame needs not be very light. The other help is that the plane can brake from its 1800m/s rather quickly if a detector feels heat (that is, disappears...) and in the meantime an underlying ablative protection can work. Like: hold ceramic tiles before massive wood and the leading edges.

 

[Enthalpy]

In case thick wood behind a broken ceramic tile doesn't give enough time to brake:

Put behind the first tiles an impact absorber that doesn't have to resist the heat - something like a crushable honeycomb, or a compact ceramic felt, or get inspired from a bullet-proof jacket - and behind this impact absorber, a second layer of tiles that resist heat but not necessarily a bird impact.

To imagine (or even simulate to a limited extent) the effect of a broken tile, you can use a blowtorch, set very oxygen-rich. It cuts varied materials quickly (ceramic felt less so) but takes much longer than an impact.

Marc Schaefer, aka Enthalpy

 

[boneh3ad]

Of course there are the real gas effects of oxygen and eventually nitrogen. It certainly won't actually be 23,000 K as it would in an ideal gas. However, that isn't going to be enough to make our current slate of materials useful here. Otherwise, it would have been done already, particularly since there has always been an interest in reusable reentry vehicles and our only somewhat successful attempt at that was the shuttle, and its tiles were notoriously brittle. Of course there is the TUFROC system they just employed on the X-37B, which is pretty neat but probably not something that has been made available outside of that program as far as I know. I am unsure if something like the TUFROC would be suitable for level flight, however. It was designed for reentry, so for all I know, it may be uniquely suited for that application.

At any rate, the problem is as I said before, we have materials that can withstand Mach 6 type heating, but none that can do so that have the proper mix of weight and toughness. Zirconia, for example, is closer in density to steel than to aluminum. It is even some 25% heavier than titanium. It certainly doesn't seem like an ideal candidate, even if it can stand the heat and is relatively tough.

I wish there was a current solution, but there simply isn't yet. If there was an easy solution, they wouldn't be actively trying to solve that exact problem. In fact, a few years ago, the Air Force made a fairly large investment in solving hypersonics-related problems by forming 3 hypersonics research centers to cover three big challenges in the field: materials, propulsion and laminar-turbulent transition. The materials center, led by Teledyne, is trying to solve this exact problem. The technology just isn't there yet.

I also don't share your optimism with things such as bird strikes. Reentry or high altitude flight is one thing, but birds tend to do their migration at 1 to 2 miles altitude, so the possibility exists for a bird strike even in cruise, even if it is unlikely. If the goal is to make a fast transport as the OP suggested, then this would have to be accounted for.

I also don't agree that the frame needn't be light. Given the original post, it's safe to assume this vehicle would have to take flight and reach cruise altitude and speed on its own power, which would require detailed attention to weight.

 

[Enthalpy]

1800m/s shouldn't be confused with a reentry speed. The resulting 1800K are bearable by many ceramics, and the layered construction I suggested should answer the broken tile worry.

An airframe that flies at Mach 6 in the dense atmosphere won't take off much slower, just because the wing area can't make both, so weight isn't a big issue. Anyway, we're speaking about protections at the leading edges. Other skins can use lighter materials.

What doesn't work well up to now is propulsion, and far less economic propulsion.

I don't see why an airliner should fly fast at a low altitude. The first design choice should be to define a flight high enough that it reduces drag and accepts a wing area that reduces the take-off and landing speeds to manageable figures.

High altitude also reduces the sonic boom, but not to levels acceptable over a continent.

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

 

[Aero51]

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.

 

[boneh3ad]

That doesn't really correspond with low-altitude, FWIW. Those conditions there are about p=0.008 atm and T=61 K. That's pretty high altitude type numbers.

 

[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.