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You Think The Hypersonic Region Is Too Big?

Sat Jun 05, 2004 8:41 pm

According to different publications including the standard dictionary, it supposedly begins at around Mach 5 and goes on up to orbital speed. That's a lot of mach.

But that other thread about scramjets got me thinking. Officially, there is no barrier to the first break of Mach (1.0) just that historically is had been really hard to get past it unless you either had enough thrust or enough acceleration (same thing).

Compressibility begins as soon as you start moving, just it is extremely negligible until you reach speed near the average critical Mach of most jet airliners, roughly M0.8; you are within the region known as transonic by then until you pass that barrier and become supersonic.

This is where I get confused somewhat. It had been said that the hypersonic Mach number is just an average, being that some effects can begin at speeds of Mach 3 or as late as Mach 7. Some engineers won't consider hypersonic unless those speeds in question are well above Mach 10.

I remember (could be wrong) that one of the original reasons that NASA choose the speed of M2.4 for the ex-HCST was that if it went past that there would be a sudden increase in friction heating that was not generally linear.

I believe that should be the initial barrier to hypersonic flight as its general characteristic is excessive heating and M2.4 is where it begins. Granted that decision may and probably is due to their researched shape and is not the same for all shapes.

What about an uppermax? I do not know how it makes sense that meteorites can travel at hypersonic velocities of Mach 100+ as they burn up in our atmosphere. Meaning that word should not be used for the noun in question. Though the word hypersonic in not always used for ICBM's or orbital space flights, I think as long as we are talking about flights within the atmosphere, it should be restricted to such.

For example, if say the flight was accelerated by an air breather, the uppermax would occur at a point where the engine cannot further accelerate the body. Right now it is suggested that a scramjet can accelerate a space plane of sorts into orbit. I figure the point at which there is maximum velocity does not occur at thrust = drag. Rather it is where the engine does not get enough oxygen and thus the thrust drops, and then thrust equals drag.

How is this possible? Two words: Oxygen disassociation. Fact: if the heat around a hypervelocity vehicle gets hot enough, the diatomic molecules separate into ions.

As you accelerate to just under Mach 3, the atmosphere so far is a calorically perfect gas. From that point to about Mach 7, you get vibration excitation within the atmosphere. From there to about 5 kilometers per second, there is oxygen disassociation. Which means the engine is not going to be able to burn diatomic oxygen as it continues to accelerate. It is at this point I think the word hypersonic has it's upper maximum, which depending on a bunch of ambient circumstances, can be anywhere from M15 to M20.

If at this speed you continued to orbital speed within the atmosphere, you would be experiencing nitrogen disassociation. And if you for whatever reason decided to accelerate to escape speed within the atmosphere, around 16 kilometers per second, you would be experiencing complete atmospheric ionization. Basically you're flying in a superheated acid-particle soup.

Perhaps this region past hypersonic should/could be named "ultrasonic" or "astrosonic" cuz since scramjets can't work out here only rockets can continue your acceleration. On the other hand, sound and space have nothing to do with each other so using the word sonic at all is dumb.

I usually refer to this region as some fraction of orbital speed. Like if orbital speed is 18000 mph at some altitude and you're going 16200 mph, you would be going 0.90 Orbital.
The meaning of life is curiosity; we were put on this planet to explore opportunities.
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RE: You Think The Hypersonic Region Is Too Big?

Sun Jun 06, 2004 7:24 am

With hypersonic vehicles I think that because there are a so few the term "hypersonic" isn't broken down into sub-regions because there isn't really any point. What do we have? SR71 at M3 and above, the Space shuttle, the X15 if you go back far enough and some missiles.

The limit on Scramjets is the ability to throw stuff out of the back faster than you're moving forwards. Basically the reaction process in the propulsion system does not produce a fast enough jet out the back, or the time taken in the combustion process means that it happens outside the craft and can't generate any useful force.

As far as dissociation is concerned, the space shuttle would be in a lot of trouble if it didn't happen. During dissociation, energy that would usually increase the temperature is absorbed by the oxygen/nitrogen as they dissociate and this has the effect of reducing the temperature compared to an "ideal gas". The downside is that the atoms recombine into the various molecules further down the fuselage and can lead to high local heating. This problem is exacerbated by contaminants in the surface (paint, salt) that cause recombination sooner than may be predicted. This is one of the reasons that people stand clear of the shuttle after it's landed. The recombination process leads to various NOx compounds as well as other nasties.

Is it true that oxygen dissociation is a problem for propulsion? I'd have thought that oxygen atoms would be a lot more reactive than molecules and give a better performance, unless they react with other things before they get to the fuel. In the combustion process, energy would normally be absorbed in breaking the oxygen bonds so they could react with the fuel.

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RE: You Think The Hypersonic Region Is Too Big?

Sun Jun 06, 2004 7:44 am

...if you for whatever reason decided to accelerate to escape speed within the atmosphere, around 16 kilometers per second...

Make that 11.2 km/s...  Big grin

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RE: You Think The Hypersonic Region Is Too Big?

Fri Jun 11, 2004 4:31 am

For an airbreather, oxygen is O2 (diatomic) and only it should be reacting. The general formula is that a hydrocarbon with oxygen results in mainly CO2 and H2O vapor and a bunch of methane, nitrious and sulfuric oxides.

I would guess, I had chemistry a looooong time ago, that anatomic oxygen would mean more fuel usage and high inefficiency. Chemically, in most burn reactions, oxygen is always diatomic before it is reacted with a reactant (fuel). NOS is an interesting example, first it is an NOx then separates into diatomic N and O while hot with gasoline and air, together producing lots of horsepower in cars.

That said, for example, Ozone (O3) can be used but it's bad for the atmosphere. Concorde did suck in some at 60 kft that resulted in nitrogen compounds in the exhaust (think about any over-oxygen burn) , which was what some of the hoopla was about.

Qantas332, I knew it was 11 something, just not the units attached.  Smile
The meaning of life is curiosity; we were put on this planet to explore opportunities.
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RE: You Think The Hypersonic Region Is Too Big?

Sat Jun 12, 2004 6:29 am

Now that I am back from my prolonged vacation after piling it higher and deeper, I can contribute again.

Couple of odd theoretical bits.

1. The theoretical maximum speed of a reaction based propulsion system is a direct function of its Isp (specific impulse). A perfect gamma ray/antimatter engine has an Isp that gives it a maximum velocity of the speed of light.

2. One of the many, but highly accepted definitions of hypersonic region is where the difference between the actual Mach number and M=infinity is sufficiently negligible, i.e, around 10%. Conversely there is a "hyposonic" region, i.e., where the Mach number is sufficiently close to zero that incompressible assumptions can be used, below Mach 0.3.

As to the ability of a propulsion system to work, it isn't disassociation of the incoming fluid, that is actually a plus since separating O2 into 2-O requires a fair amount of energy, that kills you it is the lack of recombination after the fact. If you only have OH and CO after combusting CH4 and H2O you are missing out on a tremendous amount of the energy extraction.

As an aside those flame trails you see behind afterburing jets are OH to H20 combinations that are lost energy. You can solve this problem by increasing combustion pressure, but this adds to the NOX problem. The biggest problem with Scramjets is combustion length, of which a sufficient length is necessary to allow for recombination. This is, more than anything else, what defines the operational speed range. Hydrogen requires a significantly shorter combustion length than hydrocarbons, and produces about twice the Isp.

The 2-H2 + O2 reaction is quite complex with several stages especially on the recombination side.

If you want to do your own Isp calculations I recommend looking up the heats of disassociation and recombination in the JANAF tables. Hill and Peterson's jet propulsion book is a good reference for the actual equations.

Your friendly gas-dynamicist:


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