The AIM-120 'AMRAAM' - History, Design, Performance & Discussion

Machinery of War Discussion Aircraft
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I did not say the R-77s rocket motor is proportionately larger than the AMRAAM’s. I said the SCAS on the AIM-120C-5 reduces the control actuator section to a size similar to that on the R-77. This was to imply the AIM-120C-5 was able to lengthen the motor propellant casing and match the R-77’s propellant mass but in a slimmer (albeit slightly longer) frame.

https://cdn.discordapp.com/attachments/955829235493273680/1137237547664285796/jwSeB.png
The actual motor of the R-77 is approximately 58.5 inches long in the section you’re measuring based on this image (I believe from an actual manual). Found on this website.
When I measured it appears the motor is 41.3% of the overall length of the missile at the seams.
(3.6 x 0.413 = 1.4868m) or 58.53 inches.

The AIM-120A motor cutaway given by Hughes.
https://cdn.discordapp.com/attachments/955829235493273680/1137241746447798392/1.png
Both missiles have approximately 41% of their length dedicated to the motor. The math works out to a similar figure if we compare it to the R-77 based on the casing, but the internal design of the missiles differs so the comparison isn’t perfect. As mentioned, the diameter of the missile is 12.4% larger on the R-77. Propellant types and ISP also differ.

What you’re not accounting for is that the battery section on the back of the rocket motor (and the control actuators that surround the nozzle) house over top of the remainder of the rocket motor iirc. This further extends the length greater than that of the AIM-120A/B motor. Unlike the AIM-120A/B, the batteries and such were housed at the rear closer to the lattice fins which are a relatively small section in comparison to the AMRAAM as mentioned and shown.

So yes, while the physical motor section of the missile appears to be a similar length (albeit on a missile that is wider)… the overall propellant housed inside is thought to be more. The publicly known burn time is approximately 4.5-6s as opposed to the 6-8s of the AIM-120B which suggests a higher initial acceleration as well. The R-77 will most definitely benefit more from a launch at higher speeds and altitudes than the AMRAAM, and performance at lower altitudes will differ.

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The F-14 does not have multi-launch capability wherein he can actively guide via TWS towards 6 separate selected targets. He has to individually move the PIP to each target and fire, he cannot select 6 and then fire in sequence. As soon as he selects a separate target to launch a missile he will stop providing guidance for the last missile fired I think. The J-8F will only receive 2x PL-12, the Yak-141 may only receive 2x R-77 and other fighters will receive less of their Fox-3 missiles as well early on.

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Your image for AMRAAM gives propellant casing of 58.9 inches though(which is longer than I thought)? And your R-77 cutaway would give a propellant length of 62.5 inches. At the end of it the R-77 is considerably heavier than AMRAAM so to match it it must have a bigger motor which it seems to have and maybe a 5-10% greater fraction. But this is not a very big difference. And R-77 in addition to grid fins is also simply bigger and therefore draggier, it’s larger mass does not make up for this.

Initial acceleration profile impact on range will depend heavily on the efficiency of both missile’s loft profiles which is virtually unknowable. AMRAAM eventually adopted a single thrust profile, but that might have become optimal as a result of changes in behavior.

AFAICT the F-14 does have guidance capability on non-primary targets. The issue is that if the track is lost the missile will not reconnect if it is later regained, so it is important to avoid locking targets with sigificant separation and sometimes the ingame radar behavior will screw you. I don’t know why you’re talking J-8 or Yak-141 WRT AMRAAM, AMRAAM specifically is almost always carried in greater numbers. Part of that is that it is a smaller, lighter missile than R-77 or PL-12/SD-10.

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If the target you fired at last is still visible on TWS as a dot when you switched to the next target, it keeps the TWS track, but as soon the it disappears, you lose it. That is why a lot of players are angry at Gaijin for not fixing the TWS track hop bug, that entirely disables this, because it is so frequent.

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82

This one is a separate bug, giving guidance for multiple targets is not added for air yet , but two spaas have it , TOR M1 and pansir S1

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83

The AIM-120 motor is actually around 59.01 inches. The propellant casing is 667 pixels long, divide that by the length of the missile (1624 pixels) we get the number 0.4107. We multiply 3.65 x 0.4107 and get the total meters length of 1.4991, we can convert this to inches by multiplying by 39.3701.
The AIM-120 propellant section is approximately 59.01 inches, however the portion that leads to the control actuator section tapers off, leaving us with less overall propellant as that entire space is not filled. We can expect it is closer to 58 inches total propellant or even a little less.

For the R-77, the motor is approximately 58.27 inches as shown here.
https://cdn.discordapp.com/attachments/955829235493273680/1137380319624974436/2.png
Motor length of 967 pixels / 2352 pixels = (0.411139 x 3.6) = 1.480102 x 39.3701 (convert to inches) = 58.27 inches.

So the AIM-120’s true inner motor section tapers towards the end for a total of approximately 58 inches, the R-77 has unknown internal design but highly likely that it extends further towards its’ own control actuator section (smaller size).

I don’t know where you got 62.5 inches from the R-77 image. The R-77 is 12.36% larger than the AMRAAM in diameter with a similar length motor (which again, I never said was proportionally larger than the AMRAAM, only that it carried more propellant which is readily evident). The grid fins are not necessarily draggier, requiring far less AoA for small maneuvers and corrections.

We can assume Gaijin will implement an arbitrary loft profile which leaves some things up in the air regarding range and performance. We know the AIM-120C-5 added more propellant and switched to an all-boost motor of approximately 8s burn time, this is similar to the profile used by the R-77 of an all-boost motor with 4.5-6s burn time. The R-77 is expected to have higher acceleration, likely to get it beyond the troublesome transonic region mentioned for grid fins. Overall, I think the R-77 is going to have better time to target, maneuvering after losing energy due to the grid fins, and also more range at optimal firing conditions at higher altitudes… which are reachable thanks to the MiG-29 and Su-27s massive ability to climb and get speed quickly… comparable only really to the F-15 on the American side.

The AMRAAMs adoption of the single thrust profile may have had more to do with assisting the missile in high-angle off-boresight endeavors… which allowed it to do such maneuvers without totally wasting the boost phase on a maneuver. This allows it to “chase” targets that it may have been fired at from off-bore angles at closer ranges. (My theory).

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The F-14 stuff was covered by others but again I’ll reiterate the AIM-54 still being updated on targets that you don’t actively have soft-selected on TWS is a bug, among other issues it is not indicative of how multi-target launches should be done.

The point of discussing other nations Fox-3s is not so easily dismissed and you know very well why I brought them up. If the J-8F is coming with just 2x Fox-3 and if the Yak-141 is getting 2x R-77, it would be generally acceptable if those weapons performed better than the weapons of which other aircraft can equip 4-6x of (like R-27ER and AIM-7F). I know the community doesn’t like it, but this is Gaijins thought process on the matter so it seems.

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84

You guys are also completely ignoring the increase in drag of the R-77, which is likely in the ballpark of 10-20% greater than the AMRAAM without accounting for the grid fins due to the increased diameter of the missile. It gets even worse for the R-77 if you do account for control surfaces.

The longer burn time of the AMRAAM also means it has even less drag as we know that projectiles have SIGNIFICANTLY more drag when the cavity behind the projectile is not being filled by a gas.

For example, base bleed shells (which ill point out do not actually have ANY thrust) have 20-35% greater range than standard shells of the exact same dimensions.

The R-77 is a significantly draggier missile in every way over the AMRAAM.

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85

The R-77 is not much larger than the AMRAAM in terms of size, the caliber goes up to 200mm from 178mm…

The AMRAAM has less than a 2s booster with approximately 5-6s sustainer with significantly less thrust. The R-77 has a 4.5-6s booster only, which is similar to what they do later with the AMRAAM.

Grid fins have significantly higher drag at transonic speeds due to shock waves forming within the lattice of the fins. The drag is known as “wave drag” (for others reading who want to look this up further). At speeds above mach, the grid fins have lower drag and provide greater maneuverability than conventional tail sections like seen on the AMRAAM. At subsonic speeds they have equal drag and greater effective control than conventional fins due to their much higher angle of attack before a stall occurs. They can maneuver better at slower speeds when attempting to hit targets near the end of their flight envelope.

And at subsonic speeds, per aerospaceweb

Another important aerodynamic characteristic of grid fins concerns drag, although it can be an advantage or a disadvantage depending on the speed of the airflow. In general, the thin shape of the lattice walls creates very little disturbance in the flow of air passing through, so drag is often no higher than a conventional fin. At low subsonic speeds, for example, grid fins perform comparably to a planar fin. Both the drag and control effectiveness of the lattice fin are about the same as a conventional fin in this speed regime.

The comment “The R-77 is a significantly draggier missile in every way over the AMRAAM” is simply not true, it can maintain high speeds for longer due to reduced drag from the grid fins at high supersonic speeds. The way of maximizing it’s kinematic range is to simply launch it above mach 1.3 where it will not suffer the transsonic wave drag from the grid fins.

In short:
A 4.5-6s boost will help the missile go faster than the AIM-120 and the grid fins will help it keep that speed for longer if launched above mach 1.3. Further, the AIM-120 will not be able to maneuver as well as an R-77 when attempting to reach a target at maximum ranges because the grid fins allowing almost double the maximum AoA can help to maneuver the missile better towards the end of its’ kinematic range.

Something we should also note, the R-77 is larger and has more room for batteries. It’s been said that it has 8-9 batteries in the base after the rocket motor and before the control actuator section. The control actuator requires less power to move the servos than on the AIM-120 because the grid fins require far less torque. The maximum guidance time could be far longer than the AIM-120 allowing it to coast into targets. This is also assisted by the fin design on the mid-body of the R-77.

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86

Considering the statee missile dimensions (200mm diameter, 3.6m long R-77 vs 178mm diameter, 3.65m long AMRAAM) the math puts the R-77 frontal area at 0.031m^2 vs 0.025m^2 which is almost 20% larger. The overall surface areas of the missiles are closer, with the R-77 at roughly (assuming a cylinder and ommiting the rear surface) 2.289m^2 vs 2.065m^2 which is just about 10% more surface area (ommiting fins for both).

Considering the rest of the drag equation to be roughly equivalent, and therefore canceling itself out, the R-77 are roughly 10-20% draggier than the AMRAAM, and as stated before, thats ommiting control surfaces and ignoring the effect of base drag that the AMRAAM gains an advantage in due to the longer sustainer.

You YOURSELF state they reach similar speeds after launch from their boost phase. Considering all known factors, its pretty simple to figure out the any advantage in propellant the R-77 may have is negligeable simply due to the drastricly increased drag.

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87

Can you make a similar in-depth research into both AIM-7F and R-27ER? Ignore Warthunders implementation of the missiles or their performance charts. Could you take a look at their airframe and mass sections? I’d be interested in hearing your conclusions.

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The R-77 has approximately 63.03 inches long area wherein propellant is stored if we do not go off the seams, and rather off the possible area filled with propellant casing.
Using the formula for cylinders to calculate total area used for propellant yields approx. 88.998m2

After revising my look at the AIM-120A propellant it seems the area where propellant is stored has around 56 inches of motor. The total area used for propellant comes out to around 74.703m2.

The R-77 could yield somewhere between 10 to 19% more propellant, reaches a similar or higher speed than the AIM-120A despite a larger caliber (albeit with reduced drag from the control surfaces).

You were not accounting for the radome shape much in the same way I do not account for the fact that the propellant casings are not perfect cylinders, but
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Essentially,
The R-77

  • More body drag from caliber (12.36% increase in caliber).
  • Significantly less drag at high supersonic speeds from grid fin design.
  • 10 to 19% more propellant.
  • Significantly more thrust / acceleration. (Similar or higher top speed with less burn time).
  • Better maneuvering capacity near end of kinematic flight range due to higher CAS AoA.
  • Significantly hampered by launches below 1.3 mach.

The AIM-120

  • Better drag (initially) due to slightly longer burn time. (~1-2s more).
  • Significantly worse acceleration and top speed as altitude decreases (2s boost, 5-6s sustain).
  • Publicly known to use lofting.

My conclusion:
The R-77 may have slightly better straight line performances, but to my knowledge did not (at least initially) have any kind of lofting profile. The all-boost motor lends itself to reaching significantly higher speeds from launch at lower altitudes in comparison to something like the AMRAAM, provided it can be launched at or near mach 1.3.
At high altitude where drag matters very little, the R-77 will likely outperform the AIM-120 due to the better acceleration, better speed retention at high supersonic, and having obviously more space for more batteries.

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As a comparison, the AIM-54A (according to this sheet) has 1224.86 m/s deltaV. The AIM-7F has 955.54. This means the AIM-54A has 28.2% more deltaV than the AIM-7F. The AIM-54A has a caliber of 380mm, whereas the AIM-7F is 200mm. This is a 90% increase in caliber. In my testing (see chart below) the AIM-54A can travel a distance of 54.97km before it can no longer chase an 1100 m/s target at 10,000m. That is the furthest it can go and remain within lethal warhead radius and successfully kill the target. Likewise, the AIM-7F can only go 37.26km in the same scenario. This is a 47.5% increase in overall distance traveled and 48.4% increase in separation distance from target where the missile was able to successfully hit it with just 28.2% more deltaV and 90% larger caliber.

Another comparison that can be made is the R-27ER and the AIM-7F. The R-27ER has 1195.38 m/s DeltaV, the AIM-7F remains at 955.54. This is a 25.1% increase in DeltaV but only a 14.7% increase in overall distance traveled (albeit limited by guidance duration). The missile had a lot of velocity leftover to hit this target, but failed to at further ranges because it self destructed. This shows how important it is to accelerate quickly and maintain a higher speed, which the R-77 can likely do.

https://cdn.discordapp.com/attachments/955829235493273680/1137465005718110228/image.png

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Gaijin does not implement different “drag” stages for missiles, like subsonic, transonic and supersonic. If they are about to add R-77, they have to create it and so far it seems difficult (like modeling lowered drag when the motor is burning). I think we will need a lot of bug reports for new FOX 3 missiles when they come.

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90

I don’t think they model the drag from post-burn when there is no gas behind the motor section either, the R-77 will be favored in all of these simplifications. As long as the missile under certain conditions has the correct kinematic range and average time to target it will be considered good to go.

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91

Annoying how all game simplifications of gaijins tend to favor ruaaian vehicle and weapon systems. Sure must be convenient…

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Favors all missiles equally, but alright. The R-77 will still have it’s drag and ranges optimized for altitudes 1-5k meters as Gaijin does with all other missiles so it won’t get much of an advantage in comparison to the AMRAAM in that regard.

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This is the AMRAAM (most likely AIM-120B) sequence of events diagram from the Tornado F.3 manual:

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96

0.8s after it is ejected from the frame it starts guidance, and 0.2s after ejection from frame does it start motor… interesting. I know that on the F-16 wingtips it is fired off the rail, I wonder how they will model these differences. Perhaps two separate missile files with changes in the guidance delay and such?

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I guess that’s how they would do it. But then again they don’t have separate versions of the Sparrow for rail vs ejection launch.

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98

I think it would be a pretty easy fix to have the two separate files and just change the guidance delay and all that. Would be a neat feature.

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Would be a nice detail. Maybe it just never occurred to them with the Sparrow.

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You have to report it, it is like if you do not report stuff yourself they will never add it. Happened with a lot of features and neat things.