Community driven CFD for R-27 family of missiles (virtual wind-tunnel testing)

[GGTharos]

IIRC ED used to make up for the motor 'on' with extra thrust in the past, in some cases.

[BlackPixxel]

5 hours ago, nighthawk2174 said:

It already got a partial treatment and the drag nearly matches my own CFD stuff for it, within a few %.  Biggest concern I have is that the motor burn time fell by a decent amount.

 

The big improvement for the R-77 will be the loft and the autopilot update.

 

Similar to how the Aim-120 fired without loft has quite a poor performance compared to the R-27ER.

 

The loft of the 120 in DCS gives it an gigantic range increase, it feels like it gets twice the engagement range.

5 hours ago, GGTharos said:

IIRC ED used to make up for the motor 'on' with extra thrust in the past, in some cases.

 

Extra thrust in the thrust config, or extra thrust in the missile flight calculation?

 

R-27ER certainly has the correct thrust values, not enlarged, in the config.

[BlackPixxel]

5 hours ago, nighthawk2174 said:

Biggest concern I have is that the motor burn time fell by a decent amount.

 

On this timestamp is a video of a R-77 launch. It is hard to tell when the motor stops exactly, due to the angle and the camera movement, but it appears to be somewhere around 5 seconds.

There are also some other nice launches in the video.

 

 

[nighthawk2174]

42 minutes ago, BlackPixxel said:

 

On this timestamp is a video of a R-77 launch. It is hard to tell when the motor stops exactly, due to the angle and the camera movement, but it appears to be somewhere around 5 seconds.

There are also some other nice launches in the video.

 

 

TBF as well the exact burn time is also dependent on the motor temperature as well.  And it may not be a small variation either.  Also either way the Adder will still suffer more though due to higher drag.  And its not like it has ton of weight when its empty either, iirc both the 120B and 77 are similar in weight once both of their motors burn out.

Edited January 15, 2021 by nighthawk2174

[Cmptohocah]

17 minutes ago, nighthawk2174 said:

Also either way the Adder will still suffer more though due to higher drag.

 

Where are you guys getting these conclusions/information from? Could you please share some references (charts, papers, whatever). This is the second post of this type on this thread that is a "black box".

[nighthawk2174]

5 minutes ago, Cmptohocah said:

Where are you guys getting these conclusions/information from? Could you please share some references (charts, papers, whatever). This is the second post of this type on this thread that is a "black box".

 There are two main reasons

 

A) larger diameter which will increase drag by 10% over the 120B

B) the grid fins on the adder, these are well known for being a lot draggier in the M.8-1.6 area and similar but still a touch more draggy than the conventional (and smaller) fins on the 120B at speeds above and below this region.

 

[Cmptohocah]

13 hours ago, GGTharos said:

 

It isn't that important.  This will affect top speed, but you could generate a graph just for the motor burn-out part.   The other trick is to reduce all drag coefficients by a ballpark of 22% when the motor is on.  It's not super-duper accurate but it'll get you close.

 

 

Look at the graph (Scroll down):  https://space.stackexchange.com/questions/22907/modelling-spacexs-lift-and-drag-versus-angle-of-attack-and-mach-number

 

This is what a typical coefficient graph looks like for an AAM.   Most missiles, really.   What you have now looks more like going up a slight incline and then you fall off the cliff.  You need to climb the mountain, then go down the mountain.   The exact shape of the mountain is what varies.

This is a graph for a German V-2 rocket. Missiles have different length-to-diameter ratios and this will vary the Cd greatly, so it's not valid to make an assumption that this fits "most missiles".

At best, it can be used to verify/disprove the current results which is a very nice to have actually. I will do some tests on the V-2 and see how the results fit or not.
Also it says nothing about the air density used to obtain these graphs, so I will assume sea level.

Edited January 15, 2021 by Cmptohocah

[BlackPixxel]

51 minutes ago, nighthawk2174 said:

TBF as well the exact burn time is also dependent on the motor temperature as well.  And it may not be a small variation either.  Also either way the Adder will still suffer more though due to higher drag.  And its not like it has ton of weight when its empty either, iirc both the 120B and 77 are similar in weight once both of their motors burn out.

 

 

It is about 110 Kg for the empty 120B and 122 Kg for the empty R-77. Both should have almost the same fuel fraction.

 

Not sure about the grid fins, there is a reason they are still used on the R-77-1, I doubt it is only about actuator size.

[Cmptohocah]

18 minutes ago, nighthawk2174 said:

 There are two main reasons

 

A) larger diameter which will increase drag by 10% over the 120B

B) the grid fins on the adder, these are well known for being a lot draggier in the M.8-1.6 area and similar but still a touch more draggy than the conventional (and smaller) fins on the 120B at speeds above and below this region.

 

So they perform poorly a cross the speed range, or are they better than the conventional fins in one regime and worse in the other?
There has to be some tradeoff, since I can't see designers choosing an inferior design for no reason.

 

Yup, larger cross-section will increase the total coefficient of drag, if comparing same missile bodies. Don't know how the fins play here.

[GGTharos]

3 hours ago, BlackPixxel said:

 

It is about 110 Kg for the empty 120B and 122 Kg for the empty R-77. Both should have almost the same fuel fraction.

 

Not sure about the grid fins, there is a reason they are still used on the R-77-1, I doubt it is only about actuator size.

 

Grid fins allow more useful AoA, or same AoA at lower electrical power, thus reducing demand on the battery for an ARH (motive power is the #1 consumer, not the radar).   Of course it's all a guess.

 

Also, the R-77 has a surprisingly lower fuel fraction IIRC.  There was a list with the fuel weights for these missiles, but it was from a Russian book and I no longer know where to find it.  Still, going by long memory.

4 hours ago, Cmptohocah said:

This is a graph for a German V-2 rocket. Missiles have different length-to-diameter ratios and this will vary the Cd greatly, so it's not valid to make an assumption that this fits "most missiles".

At best, it can be used to verify/disprove the current results which is a very nice to have actually. I will do some tests on the V-2 and see how the results fit or not.
Also it says nothing about the air density used to obtain these graphs, so I will assume sea level.

 

Pretty much most missile graphs look like that.  It's about the shape, not the exact values.   It doesn't matter if it's a V-2 or a Sparrow.   But certainly the shape and values will vary - for example, some missiles may have a long 'mach hump' if you will that seems to go on from M0.95 to 1.5 and only then it drops off.

 

My point was simply that you had a very high value for M=0.5, unless I've misread something.   Basically I would expect this value to be lower than a Cd0 at M=2.0.

[Cmptohocah]

22 minutes ago, GGTharos said:

 

Grid fins allow more useful AoA, or same AoA at lower electrical power, thus reducing demand on the battery for an ARH (motive power is the #1 consumer, not the radar).   Of course it's all a guess.

 

Also, the R-77 has a surprisingly lower fuel fraction IIRC.  There was a list with the fuel weights for these missiles, but it was from a Russian book and I no longer know where to find it.  Still, going by long memory.

 

Pretty much most missile graphs look like that.  It's about the shape, not the exact values.   It doesn't matter if it's a V-2 or a Sparrow.   But certainly the shape and values will vary - for example, some missiles may have a long 'mach hump' if you will that seems to go on from M0.95 to 1.5 and only then it drops off.

 

My point was simply that you had a very high value for M=0.5, unless I've misread something.   Basically I would expect this value to be lower than a Cd0 at M=2.0.

Oh I see, you were talking about the chart shape and not the values them selves.

[GGTharos]

Yep - sorry I wasn't very clear about that

[Skysurfer]

I can appreciate the work you are doing dude but you are really wasting your time here. ED is not going to use any of your data, nor did you have an accurate 3D model for each missile (ET will be different from ER) to have real accurate values. There are unclass. and published flyout and EM graphs for these missiles. Given the fact ED are still tinkering with the AMRAAM and Sparrow and breaking it ever other update I wouldn't have too high hopes they'll re-visit the russian missiles again. Let alone the whole thing with the russian DOD, even though the missiles in talk aren't actually of russian design and production.  We'd all love to see all mainline missiles in DCS abide by the same physics and be on the same API but this is sadly not going to happen anytime soon, and just like with the amraams, 3rd party cfd work will not be taken into account. 

 

Again, great work from a pure research standpoint but I don't see what the goal is with all of this?

[Coxy_99]

1 hour ago, Skysurfer said:

I can appreciate the work you are doing dude but you are really wasting your time here. ED is not going to use any of your data, nor did you have an accurate 3D model for each missile (ET will be different from ER) to have real accurate values. There are unclass. and published flyout and EM graphs for these missiles. Given the fact ED are still tinkering with the AMRAAM and Sparrow and breaking it ever other update I wouldn't have too high hopes they'll re-visit the russian missiles again. Let alone the whole thing with the russian DOD, even though the missiles in talk aren't actually of russian design and production.  We'd all love to see all mainline missiles in DCS abide by the same physics and be on the same API but this is sadly not going to happen anytime soon, and just like with the amraams, 3rd party cfd work will not be taken into account. 

 

Again, great work from a pure research standpoint but I don't see what the goal is with all of this?

Goal is that the R family missiles are ****, We dont all rely on the 120 to get kills.

[GGTharos]

1 hour ago, Skysurfer said:

Again, great work from a pure research standpoint but I don't see what the goal is with all of this?

 

The goal is to compare to what the in-game missiles do, and I'm sure on the OPs end it's also about learning.  There are no really useful flyout profiles published for he R-27 family.   In the end a CFD gets you details that you couldn't get otherwise.

[dundun92]

5 hours ago, GGTharos said:

There was a list with the fuel weights for these missiles, but it was from a Russian book and I no longer know where to find it.  Still, going by long memory.

I believe this is the doc in question, though I could be wrong (it doesnt list fuel mass directly though):

If i'm interpreting this right, the bottom row seems to be total impulse/missile weight. Assuming an Isp of 250s, it gives 133kg propellant mass for the ER, and aroumd 56kg for the R-77, which lines up very closely with EDs values.

For fuel fraction, for the ER it would be 0.37, and for the R-77 its 0.32. AMRAAM for comparison is 0.32 for the C, and 0.3 for the B, so not that much of a difference actually.

Edited January 15, 2021 by dundun92

[BlackPixxel]

Interesting, it says that the R-77 can be used on the Su-33

 

 

I only found this (user made?) table in some fourm. Aim-120 values seem off, at least their motor configuration is not how we have it in DCS. But maybe the R-77 value is correct to achieve the 79.

Edited January 15, 2021 by BlackPixxel

[GGTharos]

The values are pretty good.  One inaccuracy is probably in the AIM-120B weight, it should be lighter (there is a lot of weight back-and-forth between the variants).

 

This looks like the old FM configurations, I haven't checked how fuels have changed ... the masses should have remained the same.

[BlackPixxel]

20 minutes ago, GGTharos said:

The values are pretty good.  One inaccuracy is probably in the AIM-120B weight, it should be lighter (there is a lot of weight back-and-forth between the variants).

 

This looks like the old FM configurations, I haven't checked how fuels have changed ... the masses should have remained the same.

 

But in DCS the 120B has a dual stage motor, and the 120C a single stage one. In the table it is the opposite. Which is why I am not sure about the accuracy of the table.

Edited January 15, 2021 by BlackPixxel

[GGTharos]

You're right about that, too ... AIM-120A/B/C1-4 are using the same boost-sustain motor, AIM-120C5 and after is all-boost.

[nighthawk2174]

On 1/15/2021 at 4:28 AM, Cmptohocah said:

So they perform poorly a cross the speed range, or are they better than the conventional fins in one regime and worse in the other?
There has to be some tradeoff, since I can't see designers choosing an inferior design for no reason.

 

Yup, larger cross-section will increase the total coefficient of drag, if comparing same missile bodies. Don't know how the fins play here.

Drag wise their worse than conventional across the board except at high Mach and subsonic where their pretty close.  Their main advantage is making the missile VERY maneuverable at subsonic and supersonic speeds (allows for lower aoa for similar maneuverability which as was pointed out above could be a way to reduce power drain just a theory though).  And as such you don’t need as large actuators for them so they will fit in a much smaller form factor (which iirc was one big reason for using them).  

Edited January 16, 2021 by nighthawk2174

[nighthawk2174]

On 1/15/2021 at 4:28 AM, Cmptohocah said:

So they perform poorly a cross the speed range, or are they better than the conventional fins in one regime and worse in the other?
There has to be some tradeoff, since I can't see designers choosing an inferior design for no reason.

 

Yup, larger cross-section will increase the total coefficient of drag, if comparing same missile bodies. Don't know how the fins play here.

It seems you were curious as too why they perform badly in certain regimes, well I just finished up my high res cfd run for the Adder and this is why:

Spoiler

 

Mach 1.0:

Mach 5.0:

[GGTharos]

Could you do mach 1.0-2.0 in 0.2 increments, or better yet 0.1?

[nighthawk2174]

1 minute ago, GGTharos said:

Could you do mach 1.0-2.0 in 0.2 increments, or better yet 0.1?

Yeah I did but I didn't take screenshots lol I should have XD,one day out of curiosity I will.

Edited January 20, 2021 by nighthawk2174

[IVAN01rch]

On 1/15/2021 at 6:11 AM, nighthawk2174 said:

Concerning motor on and motor off differences this is what were talking about:

it's about a 20-22% difference.

 

 

It already got a partial treatment and the drag nearly matches my own CFD stuff for it, within a few %.  Biggest concern I have is that the motor burn time fell by a decent amount.

 

Edit: graphs

To simplify, silver = my cfd with new ED thrust, blue = ED stuff with new thrust.  Yellow is Old ED drag with new thrust.  Orange is my CFD with old thrust.

 

Hey nighthawk,

 

Naturally Cd coefficient can be defined in a several ways. Maybe you dug into these questions and can help me:

1. How do you take into account working engine when you calculate drag coefficient? Do you simply neglect rocket back wall when integrating pressure over the surface to get the drag force? Obviously you do not want to simulate supersonic jet with nozzle all the time. Do you know how it is made in industry or how does ED make it? Cd is mostly a value for engineering applications so it looks logical to separate drag and engine forces.
2. Do you use cross sectional area of the rockets as the reference area in Cd equation? Are you sure everybody use the same value?
3. Do you take into account thermal effects? What ED did?
4. Do you use ideal or real gas models for supersonic cases? What ED did?
5. Obviously rocket has a lot of control surfaces. And it is useless to apply any angle of atack with all control surfaces at 0 angle as it is not a real situation. Do we know any data about control surfaces movement during flight and manouvers?

Feel free to share any thoughts about the topic.