Why are scramjets limited to Mach 25?

Question

I know that ramjets are limited to about mach 8 because when you go faster, the fuel will not burn up properly. However, that does not explain why a scramjet is limited to mach 25. Can't you just make the compressed air flow through the engine core faster with scramjets?

Answer 1

The answer is quite amusing, actually.
Mach 25 in the atmosphere correspond to orbital velocity. At that speed, going any faster means gaining altitude, getting out of the atmosphere, and actually slowing down!
The only way to go any faster while staying in the atmosphere would be to have wings providing *downforce* instead of lift -- essentially flying inverted -- so as to be forcibly kept in the atmosphere.
But then, at the rarefied pressure associated with the kind of atmosphere that would permit moving faster than Mach 25 (including the problems associated with having an airframe shaped as a function of the shock wave, and made of material able to cope with the kinetic heating) any further speed gain would translate into more problem that it is worth it. Remember: at Mach 25, you go around the world in 90 minutes; so technically, the flight time at Mach 25 to reach any point on the planet should be 45 minutes (more than that, you should have flown in the *other* direction). Of course, a significant portion of the flight time of a Mach 25 capable scramjet used as a point to point transportation (as opposed to a orbital booster used to loft object into space) would be to climb and accelerate to Mach 25, and decelerate from that speed in time to not overshoot the destination. Going faster in "cruise" would decrease the least significant part of the flight profile, as far as elapsed time is concerned.

("tk" below is a bit off implying that the speed of sound is a constant above 40000 ft. The stratosphere does not extent all the way to space, and the speed of sound starts increasing again above 66000 ft. The "Mach 25" as orbital velocity is used as a quick reference, it was never intended to be a very exact figure. Here is a link to an interactive standard atmosphere model:
http://www.digitaldutch.com/atmoscalc/tableoptions1.htm
At 86 km altitude, the speed of sound has creeped up so that orbital velocity is Mach 27, based on the local conditions.

He also makes a rather bold statement bringing up "escape velocity" which is the speed one needs to have to be able to move away from earth *to infinity*. I mentioned "orbital velocity" which is still deep in earth's gravitational well. Any rocket already in orbit would have to gain speed to reach escape velocity)

Answer 2

This Site Might Help You.

RE:
Why are scramjets limited to Mach 25?
I know that ramjets are limited to about mach 8 because when you go faster, the fuel will not burn up properly. However, that does not explain why a scramjet is limited to mach 25. Can't you just make the compressed air flow through the engine core faster with scramjets?

Answer 3

First lets clear up some incorrect answers posted in terms of mach spped and escape velocity

The speed of sound is a variable number. The best numbers are as follows:

sea level = 761 mph
20,000 ft = 707mph
30,000 ft= 678 mph
40,000 and higher = 660 mph


escape velocity = 25,039 mph

Next.....scramjets

Scramjets might be able to accelerate from approximately Mach 5-7 to around somewhere between half of orbital velocity and orbital velocity (X-30 research suggested that Mach 17 might be the limit compared to an orbital speed of mach 25, and other studies put the upper speed limit for a pure scramjet engine between Mach 10 and 25, depending on the assumptions made). Generally, another propulsion system (very typically rocket is proposed) is expected to be needed for the final acceleration into orbit.

Now for your answer - which is at best theoretical given the state of scramjets today.

Projections for the top speed of a scramjet engine (without additional oxidiser input) vary between Mach 12 and Mach 25 (orbital velocity), but the X-30 research gave Mach 17 due to combustion rate issues. The fuel we have today can not burn quick enough to produce energy to go beyond these theoretical estimates.

Answer 4

This Site Might Help You.

RE:
Why are scramjets limited to Mach 25?
I know that ramjets are limited to about mach 8 because when you go faster, the fuel will not burn up properly. However, that does not explain why a scramjet is limited to mach 25. Can't you just make the compressed air flow through the engine core faster with scramjets?

Answer 5

do the math - if the sr-71 was white hot at mach 2.x, and the shuttle re-enters at mach 10 or so (not sure exactly), then mach 25 is really really fast.

maybe it's just not practical for a sub-orbital vehicle to get up and get down, there's just more energy and heat involved than can be handled.

Answer 6

It is way more complex than that. The fuel needs to burn properly as well. Also there is a problem with heating of the exoskin of the aircraft at high speeds . Some aircraft have serious safty issues when operating at such speeds. Vibration heat g force heating or icing of the fuel and or wings and tail etc .Going faster even if possible is considered to unsafe to risk by many.
There are better aircraft with faster speeds. Many however are not publically recognized. They are called black aircraft and many speculate that they are of course stealth and much quicker than any aircraft in public knowlegde. Though they can only guess at the actuall speed.Of couse the military denies the existance of such aircraft.

Answer 7

there would have to be modifications in aerodynamics , friction plating and a system to deliver "fuel" and liquid o2 at an adjustable rate depending on altitude/o2 saturation . if extraorbital is desired then a full ratio of fuel to o2 would have to be 100% with only an exhaust opening . of course the fuels would have to be liquid or "slush" and the tanks kept at a constant temperature for such an objective .

Answer 8

any air-breathing engine need to compress the airflow, and airflow increase its temperature when compressed. and usually in combustion chambers you want the airflow to be subsonic.
At very high speeds if you obtein subsonic speeds in combustion chambers the pressure increases so much that the air gets at a temperature superior than the burning temperature of fuels.
Scramjets solve this problem using combustion chambers with a supersonic flow inside(so air temperature is not increased too much),( I try to guess: SCRAMJET stands for Supersonic Combustion Ramjet?) but going faster and faster the combustion will shift backward(fuel needs a little time to burn), and at one point the combustion will occour outside the engine and the thrust will be 0.
And imagine that fuel needs 0.01s(i don't know if this value is realistic) to burn, if the speed in combustion chamber is 800m/s, you would need a combustion chamber length of 8m ! !

Answer 9

there ain't nothing going 25 mach the spaceshuttel does only in space

Answer 10

My guess would be air resistance(drag limited).