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DCS incorrectly modelling mach?


Idle_Wild

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I was watching the following video, where this player fires the AIM54 into the upper atmosphere, and the mach continues to increase all the way to the Karman Line, where the simulation apparently ends at 100,000 metres and the missile abruptly disappears.

 

 

 

 

 

At first I thought, ok the atmosphere will be thinner, so the speed of sound will decrease even if the rocket motor on the missile had stopped providing acceleration, which could explain why the mach was increasing even when the missile was decellerating.

 

 

But looking at wikipedia, the speed of sound only varies by about 25%, cycling up and down all the way up to the Kaman line, at the Karman line it's only about 285m/s which is just slightly slower than the launch altitude of 20,000 metres (68,000ft) where the speed of sound is at 295m/s approx. so a 4% difference, and I might expect a 4% increase on the rockets maximum mach as a result, but as you can see, the mach keeps climbing, and climbing, until the missile disappears.

 

 

 

https://en.wikipedia.org/wiki/Speed_of_sound#/media/File:Comparison_US_standard_atmosphere_1962.svg

 

 

Reading further about the performance of the missile Wikipedia states, the AIM54 is capable of mach 5 at 30,000 metres, where the speed of sound is 310m/s, vs 275m/s at the Karman line, which is a difference of 12%, which brings the potential top mach up to mach 5.6, not mach 9 as per the video. But also, the missile is travelling almost vertically which should reduce the top performance somewhat, not to mention the rocket motor had been exhausted well before this point.


Edited by Idle_Wild
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Regarding wikipedia, in my opinion the notion that is it an uncredible source is a fallacy promulgated by Universities in order to maintain the status quo.

 

 

As long as the information is referenced with credible sources, then at the very least it's a good basis for obtaining summarial information and pointing you to further reading sources.

 

 

 

I suppose one thing I did not consider is that, although the speed of sound changes by only 12% from the altitudes quoted. The actual atmospheric density will be lower, so the speed of the missile could be higher. But 2x faster?

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Regarding wikipedia, in my opinion the notion that is it an uncredible source is a fallacy promulgated by Universities in order to maintain the status quo.

 

This. Wikipedia can be susceptible to errors, but take a look at the bottom of the page. What do you see? Sources. Sources, just like a peer-reviewed, published, research paper. Wikipedia is only marginally more susceptible to errors than an actual academic publication.

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Source, Wikipedia:

 

The local speed of sound, and thereby the Mach number, depends on the condition of the surrounding medium, in particular the temperature. The Mach number is primarily used to determine the approximation with which a flow can be treated as an incompressible flow. The medium can be a gas or a liquid. The boundary can be traveling in the medium, or it can be stationary while the medium flows along it, or they can both be moving, with different velocities: what matters is their relative velocity with respect to each other.

 

If I understand this correctly, Mach is not properly used to measure velocity or acceleration. It signifies how close one is to "breaking" the sound barrier (the "incompressible flow"), or how far one has gone beyond the limit.

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First you have to realize that the Mach value is coming from TacView, not DCS. The ACMI is a record of positions and therefore velocities. TacView's display of Mach is based on its own ideas of what the sonic speed is and since TacView doesn't know a lot of data from DCS like wind and temperature they won't agree.

 

 

What would be meaningful is to investigate the velocity mechanics. If the missile is accelerating in climbing coast then obviously that would be wrong. The missile may very well be decelerating properly given the video. It's impossible to tell.

 

 

Let's have a look at the velocity profile of the ACMI and only then can we discuss anything.

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