Mach 6 at low altitude

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.

http://www.youtube.com/watch?v=msXtgTVMcuA

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

chasrob

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

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

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.


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.

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

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...ic_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.

http://www.youtube.com/watch?v=RChlt5wdqBs

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

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

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