Air-to-Air Missile Discussion

[GGTharos]

I have seen the 40g quote often in the past for AIM-120, and more for R-77. I don't know where they come from, but I hear that they should be 'more than older missiles'.

 

I know AIM-9L had 40g.

 

I mean not overload, but max G-load (probably, bad machine translation) for missiles - for example, 24g for R-27 or 40 for actives. And for some other missiles.

[danilop]

...

 

You also need to know the parameters of the rocket nozzle. Depending on the shape of the rocket nozzle, and the velocity of the gas, you can achieve a lot more thrust than you would guess by using mass * Isp (you can already see this with AIM-7 rocket. The rocket parameters in the lua file are from the real rocket thrust profile).

 

...

 

 

:thumbup:

 

Couple of questions though.

 

Does Thrust Profile (rocket parameters) mean extrapolation (SFM) based on empirical and reported values, or this data makes basis for the new dynamic modelling (AFM)?

 

And speaking of all important Isp ...

 

Is there any exact and valid data on Specific Impulse values for R77 & AIM120 (or any other current military missile in production)?

 

IIRC, Isp is particularly sensitive to propellant mixture recipe (and they are tweaking it from time to time), so getting current data is a bit of a challenge, to put it optimistically.

 

Cooling arrangement and heat performance of the rocket enclosure is also huge part in rocket efficiency. Any data on this? Is this modelled in DCSW?

Edited March 1, 2013 by danilop

[GGTharos]

:thumbup:

 

Couple of questions though.

 

Does Thrust Profile (rocket parameters) means extrapolation (SFM) based on empirical and reported values, or is this a basis for new dynamic modelling (AFM)?

 

It's AFM ... SFM had acceleration time + top speed as parameters.

AFM is given amount of force applied for given amount of time.

 

And speaking of Isp ...

 

Is there any exact and valid data on Specific Impulse values for R77 & AIM120 (or any other current missile in production)?

 

There is for AIM-9L, and IIRC it was 218.

 

IIRC, Isp is particularly sensitive to propellant mixture recipe (and they are tweaking it from time to time), so getting current data is a bit of a challenge, to put it optimistically.

 

Cooling arrangement and heat performance of the rocket enclosure is also huge part in rocket efficiency. Any data on this?

 

Nope, zippity.

 

But then again, we're not simulating a rocket motor, that's why we just need thrust. It does make a difference of course, because the amount of thrust changes with atmospheric pressure etc ...

[danilop]

Without exact parameters of the rocket engine you will definitely have hard time simulating thrust fluctuations over time of missile flight.

 

So kudos to ED's dev team!

 

I'm now starting to really appreciate the effort involved. It's relatively "easy" to model basic rocket engine behavior (there are mathematical models in public domain) if you have crucial data. If you don't, it's a trial and error until the model start behaving according to RL observations - it's slow process so we will probably have several versions before missile model settle down.

[volk]

Is there any exact and valid data on Specific Impulse values for R77 & AIM120 (or any other current military missile in production)?

Есть кое-что.

Р-27 - 62 кг*с/кг (однорежимный РДТТ)

Р-27Э - 94 кг*с/кг (двухрежимный РДТТ)

Р-77 - 79 кг*с/кг (однорежимный РДТТ)

Р-33 - 73 кг*с/кг (двухрежимный РДТТ)

Все это и многое другое выкладывалось в теме http://forums.eagle.ru/showthread.php?t=93834 Там правда, много страниц :)

[danilop]

Большое спасибо! :thumbup:

[GGTharos]

Where did you find data for R-77 rocket?

[GriffonBR]

So, this is what we will have with DCS 1.2.4?

 

 

Found it by accident right now rsrs.

[GGTharos]

You already have this in 1.2.3 I believe.

 

The video is a demonstration of available vs. required g loading to hit a target. Basically the autopilot figured out how many g's the missile needs to pull and gives you what's available.

 

If what's needed is less or equal to what is available, great, if not, you get up to available g.

[volk]

Where did you find data for R-77 rocket?

Из книги "Авиация ПВО России и научно-технический прогресс", под редакцией академика Е. А. Федосова ( http://www.gosniias.ru/head-fedosov.html), стр. 284.

 

[GGTharos]

That is a problem, it's material that I cannot read :/

Although it appears that rocket motor of R-77 and R-27RE are much more efficient than R-27.

[Rusty_M]

Wow! This thread really is a great read when I understand it. That's not a dig at volk. It's because I have basically no knowledge of the subject matter.

[yodosha]

GG, you said the word "academic" quite a few times, what does it mean? Are you working off academic material or involved in it? (research/studies/whatever) Because most universities/colleges have nice CFD software which could answer the "blow on it and find out what's going on" question. You know what the missile looks like (even got models), you could just slap it in there, do a few steady state simulations at varying velocity/alpha and fit a curve on it.

[GGTharos]

I am working off a few academic things, and I know someone or a couple of someones who are involved in this.

 

I don't have CFD simulators available to myself. However, right now they're also not that important. More important is knowledge about the rocket motors right now.

 

GG, you said the word "academic" quite a few times, what does it mean? Are you working off academic material or involved in it? (research/studies/whatever) Because most universities/colleges have nice CFD software which could answer the "blow on it and find out what's going on" question. You know what the missile looks like (even got models), you could just slap it in there, do a few steady state simulations at varying velocity/alpha and fit a curve on it.

[Exorcet]

After doing some testing on maneuvering targets, my feeling is that the only working MRM on playable aircraft is the AIM-7. It's extremely efficient compared to everything else as you can see from the chart I posted. It retains speed much better than everything else whether flying straight or maneuvering.

 

I could reliably beat the AI with a well timed AIM-7 up to about 15 miles. The AIM-120 on the other hand can be defeated into WVR range - almost into gun range. Use of the 120 online seems to confirm this.

 

I feel that the other missiles need to have drag reduced, and the 120 specifically needs to get a lot of thrust back. Not 1.2.2 level (I could get Mach 5-6 AMRAAM's in the right conditions) but closer to that patch than 1.2.3.

 

Unfortunately, missile data is something I'm lacking, so it's hard to know how close I am.

[TZeer]

Found this one on the net:

 

There has been a lot of arguments over the performance of A2A missiles here --

some of which are well backed by facts, some are so far out there it is like claiming that

they fly on hyperdrive.

 

I want to take this opportunity to introduce everyone to a very simple formula that

can be used for estimating the performance of a missile. It goes like this:-

 

Change in Velocity (Delta V) = 10 x Specific Impulse x LN (initial weight / final weight) m/s

 

This assumes that all the fuel is used to get the missile as fast as possible and

none is used to provide just enough thrust to sustain a given velocity.

In otherwords, it assumes an all-boost motor not a boost sustain motor.

 

For example, let'a take a look at the AIM-120A AMRAAM which we have some decent info on...

 

Launch weight = 335 lbs (Published stats)

Motor weight = 156 lbs (WPU-6/B HTPB rocket motor weight as per Raytheon)

Approximate specific impulse = 245 seconds (typical of HTPB solid motors)

Approximate fuel fraction of motor = 85% (typical of robust aluminum cased aerospace rocket motors)

 

OK... if 85% of the motor's mass is the fuel, we have about 132 lbs of fuel in the AMRAAM-A

-- roughly a 39.4% fuel fraction (sounds about right). So let's run the numbers...

 

Delta V = 10 x 245 x LN(335/(335-132)) = 1227 m/s

 

The formula predicts that the AMRAAM will go about 1227 m/s (~Mach 3.7) faster than it started.

If it is launched at say Mach 1.5 it'll be going Mach 5.2.

In reality the AMRAAM doesn't go that fast.

The reason is that not all the fuel is used to get it as fast as possible.

The AMRAAM's motor is a boost-sustain design.

It is probably grained to take the weapon to abut Mach 2.5~2.8 faster than it started at

(Mach 4+ in a typical Mach 1.5 release).

The rest of the fuel is shaped to burn much more slowly to keep it's velocity at

or near the achieved maximum out to a longer range before the motor burns out.

 

 

Well, for any given fuel fraction and specific impulse,

a designer can decide how fast he wants to go and how long he wants to stay at

or near the peak velocity achieved. For instance, if a missile carries 40% of its launch weight

as fuel and uses the typical a modern HTPB propellant motor, it can:-

 

(1) Spend 25% to get an approximate Mach 2.1 delta V and 15% on sustaining that speed for a relatively long while.

(2) Spend 30% to get an approximate Mach 2.7 delta V and 10% on sustaining that speed for a shorter while.

(3) Spend 40% to get an approximate Mach 3.8 delta V have no sustain burn time at all.

 

BTW, in reference to the above comment on deceleration... it doesn't really work that way.

If a missle starts at Mach 4 at burn out and decelerates 25% to Mach 3 after 10~15 seconds,

it WILL NOT decelerate to Mach 2 (another 33% from Mach 3) after 20~30 seconds.

This is impossible because aerodynamic drag (Fd = Cd x A x 0.5 x P x V^2) is a function of

the square of velocity.

As velocity decreases, drag force decreases exponentially in relation to it.

Hence, if the drag for at Mach 4 causes a 25% loss in velocity in 10~15 seconds,

there is no way a much lower drag force at Mach 3 will cause a 33% loss in velocity after

another 10~15 seconds.

What happens is that deceleration is non-linear;

you start off steep and the slope flattens out over time as velocity and hence drag drops.

It'll take a missile a heck of a lot longer to decelerate from Mach 4 to Mach 2 compared to

say Mach 2 to Mach 1 for instance.

 

 

 

Actually it also depends a heck of a lot on altitude (air density)...

Let's plug some numbers shall we?

 

Question: How much thrust is needed to sustain Mach 3.0 in an AAM like the AMRAAM?

 

Drag force (Newtons) = 0.5 x P x V^2 x Cd x A

 

P = Density of Air (kg/m^3) ; ~1.29 kg/m^3 @ sea level; ~0.232 kg/m^3 @ 12,000 m

V = Velocity (m/s) ; Mach 1 = 340 m/s @ sea level; ~295 m/s @ 12,000 m

Cd = Co-efficient of Drag ; ~ 0.6 to 0.95 for rockets depending mostly on finnage,

nose and tail profile

A = Sectional Area (m^2) ; ~ 0.025 m^2 for a 7" diameter missile.

 

For an AMRAAM like AAM going at high altitudes (40,000 ft)...

 

Drag Force @ Mach 3 = 0.5 x 0.232 x (295x3)^2 x 0.70 x 0.025 = 1590 Newtons = 357 lbs

Drag Force @ Mach 2 = 0.5 x 0.232 x (295x2)^2 x 0.70 x 0.025 = 707 Newtons = 159 lbs

Drag Force @ Mach 1 = 0.5 x 0.232 x 295^2 x 0.70 x 0.025 = 177 Newtons = 39.8 lbs

 

The same missile going Mach 3 at Sea Level...

 

Drag Force @ Mach 3 = 0.5 x 1.29 x (340x3)^2 x 0.70 x 0.025 = 11,744 Newtons = 2640 lbs

Drag Force @ Mach 2 = 0.5 x 1.29 x (340x2)^2 x 0.70 x 0.025 = 5,219 Newtons = 1173 lbs

Drag Force @ Mach 1 = 0.5 x 1.29 x 340^2 x 0.70 x 0.025 = 1,305 Newtons = 293 lbs

 

Assuming that there is no sustainer,

the deceleration experienced at Mach 3 by the 203 lbs (empty) missile is

 

Deceleration @ Mach 3 = -F / mass = -1590 / (203 x 0.454) = -17.3 m/s^2 = - Mach 0.059/sec @ 40,000 ft

Deceleration @ Mach 2 = -F / mass = -707 / (203 x 0.454) = -7.67 m/s^2 = - Mach 0.026/sec @ 40,000 ft

Deceleration @ Mach 1 = -F / mass = -177 / (203 x 0.454) = -1.92 m/s^2 = - Mach 0.0065/sec @ 40,000 ft

 

Deceleration @ Mach 3 = -F / mass = -11744 / (203 x 0.454) = -127 m/s^2 = - Mach 0.39/sec @ sea level

Deceleration @ Mach 2 = -F / mass = -5219 / (203 x 0.454) = -56.6 m/s^2 = - Mach 0.17/sec @ sea level

Deceleration @ Mach 1 = -F / mass = -1305 / (203 x 0.454) = -14.2 m/s^2 = - Mach 0.042/sec @ sea level

 

OK... enough of the math and the formulas... what does all these mean?

Well, it means that while coasting at Mach 3 an AAM is going to lose about less than 2% of

its velocity a second at high altitudes while it stands to lose about 13% of its velocity at

sea level! Huge difference isn't it?

Remember though that the rate of deceleration actually DECREASES as the

missile's velocity decreases.

It is easy to see that one can claim that a missile can burn out burn out its booster

and sustainer and be effective out to over 100 km at high altitudes or be useful only

against helos after 10km on the deck!

 

Also, we can make a pretty educated guess as to how much thrust the sustainer has to make.

An AMRAAM class missile with a 400 lbs sustain thrust will be able to stay

above Mach 3 at high altitudes and stay about Mach 1.2 at sea level.

An AMRAAM class missile carrying about 10% of its launch weight as sustainer

grained propellant will be able to keep this level of thrust lit for 20.5 seconds

in addition to whatever the boost time was using the 30% of its fuel to get a

roughly Mach 2.7 Delta V after launch.

A missile like this when fired at Mach 1.5 will reach Mach 4+ and keep

above Mach 3 for the duration of the sustainer at high altitudes.

It will also reach about Mach 2.5 and keep above about Mach 1.2 at sea level.

A motor grained for this thrust profile can have a 10 second boost at ~ 2460 lbs thrust and

a 20 second sustain burn at 400 lbs thrust -- this is a 5:1 boost sustain ratio.

This is also about right for thrust profiles of star grain vs

core burn solid propellant burn rate profiles.

 

 

 

 

Another rough rule of thumb:-

 

The time it takes for a missile to lose 25% of its velocity after burn out at supersonic speeds.

 

Never @ > 100,000 m (~300,000 ft) ; in space

~150 seconds @ 24,000 m (~80,000 ft)

~70 seconds @ 18,000 m (~ 60,000 ft)

~25 seconds @ 12,000 m (~ 40,000 ft)

~10 seconds @ 6,000 ft (~20,000 ft)

~5 seconds @ Sea Level

 

Remember, fractions over time are not additive.

In otherwords, if a missile loses about 25% of its velocity in 10 seconds,

in the 10 subsequent seconds (t =20s) the missile loses approximately another 25% of

the remaining 75% not a 100%. Total velocity loss is ~43.75% not 50%.

 

This is highly collated to the fall in air density.

Drag = 0.5 x P x V^2 x Cd x A.

Holding everything else constant Drag falls proportionally to density.

Drag also falls exponentially with Velocity which accounts for the loss in velocity

in the given time slices being about 25% instead of closer to 40%.

[Mandrake5]

After doing some testing on maneuvering targets, my feeling is that the only working MRM on playable aircraft is the AIM-7. It's extremely efficient compared to everything else as you can see from the chart I posted. It retains speed much better than everything else whether flying straight or maneuvering.

 

I could reliably beat the AI with a well timed AIM-7 up to about 15 miles. The AIM-120 on the other hand can be defeated into WVR range - almost into gun range. Use of the 120 online seems to confirm this.

 

I feel that the other missiles need to have drag reduced, and the 120 specifically needs to get a lot of thrust back. Not 1.2.2 level (I could get Mach 5-6 AMRAAM's in the right conditions) but closer to that patch than 1.2.3.

 

Unfortunately, missile data is something I'm lacking, so it's hard to know how close I am.

 

For me, AIM-7 is useless @ > 8nm, unless the target is head on with very high closure. The missile simply runs out of energy and falls out of the sky...

[Exorcet]

I only tried the AIM-7 vs the AI, so I'm not surprised that the range would be less against players. If you're only getting 8 nm against AI you might not be flying fast enough or I may have to do more diverse testing.

[dores893]

Did they nerf the AIM-120 in FC3?

 

I just went head to head with a Mig 29 in an F-15C. At about 20 miles I started firing AIM120Cs at the guy. By 14 miles I had fired 6 at him. He gets one missle shot off and I turn and go defensive and he kills me. One hit and its over. All six of my missles miss him. How is that even possible? He would have been pnwed in FC2 in that same situation. And, why does it take two 120s to bring to a Ka-50? I'm thinking the AIM120 has been seriously nerfed.

[EtherealN]

Missile "amount" is not operative. Attack geometry is. It is very much possible to defeat slammer shots at those ranges.

 

However, AAM tuning is continuing throughout the FC3 beta and from my information the AIM-120 is indeed less capable in 1.2.3 than in 1.2.2.

[dores893]

Almost the same situation going head on with an SU-25 at less than 4 miles. He dodged multiple AMRAAMs and killed the F-15 with one missle. Since each missle has its own radar, spacing out missle shots every 10-20 seconds should make a difference but it doesn't.

[EtherealN]

You spaced out multiple slammers with 10-20 second interval from a 4 mile initial? That's... interesting. ;)

 

The fact that the enemy did something with one missile means nothing. He waited for a good shot.

 

But as I mentioned, missiles are being tuned. AAMs now obey an entirely new flight engine (they are now using AFMs). This takes a LOT of tuning to get right, which is ongoing during beta.

 

If you think a behaviour is incorrect, I would suggest including track files showing the behaviour. Then it can be used towards looking for errors and thus allow separating actual model errors from pilot error.

[dores893]

Just seems unrealistic to me.

[dores893]

Probably FC2 dynamics for the russian planes that didn't nerfed in the reboot.

[Pyroflash]

Try firing one at around 14 miles. 20 miles wasn't a greatly effective range even in 1.2.1.

 

Also consider that you might want to try waiting until one missile goes pitbull or the TTI runs out before you start rippling off more.

 

Also a lot of tuning is being done, and R-77/AIM-120C's are pretty bad in the current iteration.