correct as is Incorrect AIM-7 MK-58 Rocket Motor Model

[Prez]

Preface: The main purpose of this post is not to strictly improve the missile performance itself, but to correct values in the AIM-7 missile Lua code that are not accurate to the publicly available information. Any improvement to the performance of the missile should be seen as a side effect and not the intended purpose. This post seeks to compare several public sources on the AIM-7's Hercules MK-58 rocket motor to what is actually modeled in game, and contrast them accordingly. Tests were done using a modified AIM-7F utilizing the "corrected" values to showcase the differences to the vanilla missile. Only the AIM-7F was used for testing as the kinematics between each variant carrying the MK-58 rocket motor are practically negligible. The Tacview files will be provided in this post.  

The ED AIM-7 vs Public Data:

 1. Rocket Motor Burn Time
   According to ED's missile code (Fig. 1), the AIM-7 has a booster stage burn time of 3.7 seconds, and a sustainer burn time of 10.8 seconds for a total run time of 14.5 seconds not including the 0.2 second delay for the boost ignition and 0.2 second delay for the sustainer ignition. However, according to the "Raytheon AIM-7F Standard Missile Characteristics" data sheet (Fig.2), "AD-A-142508" (Fig.3) and "Gallery of USAF Weapons, 2010 Almanac" (Fig. 4) the stated burn time for the boost stage is 4.5 seconds and the sustainer stage is 11 seconds for a total run time of 15.5 seconds. This is one full second, not including ignition delays, of difference between the ED code and public information. This may be a negligible difference, but there is a conversation about edge cases where the one extra second may matter.
 

Figure 1: Default Eagle Dynamics AIM-7 MK-58 motor code

Figure 2: Raytheon AIM-7F Standard Missile Characteristics Engine Ratings

Figure 3: AD-A-142508 AIM-7F data sheet

Figure 4: Gallery of USAF Weapons, 2010 Almanac AIM-7 data sheet

2. Boost and Sustain Fuel Mass and Mass Ratios
   Looking at the ED code (Fig.5) boost stage for the MK-58 has a fuel mass of 38.48kg (84.83lbs), and the sustainer stage has 21.82kg (48.10lbs) of propellent for a total mass of 60.3kg (132.9lbs). The sustainer-to-booster propellent mass ratio is approximately 63/37. However, comparing this to the Raytheon propellent masses (Fig. 6) the boost phase contains 52.0lbs (23.6kgs) of propellent and the sustainer fuel mass is 83.0lbs (37.6kg) for a total of 135.0lbs (61.2kgs). The sustainer-to-booster propellent mass ratio with this information is approximately 39/61. The actual masses of propellent in each stage are practically the same with only an error of 7.5%(smaller mass) and 2.3%(larger mass), but reversed in ratio. According to a Canadian study of the Mk-58 Mod 5 rocket motor, there are no listed measurements for the individual propellent masses except for a total fuel mass of 61kg (134lbs). However, the stated sustainer-to-booster propellent mass ratio is 70/30 (Fig. 7) which leads to a booster fuel mass of roughly 18.3kg (40.3lbs), and a sustainer fuel mass of roughly 42.7kg (94.1lbs). Neglecting that the Mod 5 version of the motor may have slightly different masses for each stage, the point stands that the ED MK-58 is coded to have a complete reversal of the publicly available data on the propellent mass and mass ratios for each stage. This will lead to noticeable differences in the performance of each stage as we will see in the test portion of this post.
 

Figure 5: ED AIM-7 fuel mass code

Figure 6: Raytheon AIM-7 SMC Fuel data

Figure 7: Canadian MK-58 study

3. Boost and Sustain Specific Impulse Values
   As stated in the ED AIM-7 code (Fig. 8.), the specific impulse for the boost phase of the MK-58 motor is 247 seconds, and 209 seconds for the sustainer phase. Using the ED values, and a gravity value of 9.8 m/s², we get a boost phase thrust of approximately 25.174kN (5659lbf), and a sustainer phase thrust of approximately 4.138kN (930lbf).  Now, according to both the Raytheon AIM-7 SMC (Fig. 2) and the AD-A-142508 (Fig. 3), the stated thrust of the boost phase is 5750lbf (25.577kN), and 1018lbf (4.528kN) for the sustainer. Note that the ED values are underperforming, but can be considered negligible as there is only an error of about 1.6% and 8.6% and motor performance varies with many factors. Using the ED values for both thrust and fuel mass, but correcting the fuel ratios and burn time for each stage, we get a boost phase specific impulse of 530 seconds and sustainer phase of 121 seconds. This is quite a dramatic difference in performance between the actual ED missile and this theoretical "corrected" motor with the boost phase being over twice as efficient, but the sustainer being nearly half as efficient. Now, this may seem as though the performance of the missile may also dramatically improve, but, as we will see in the tests, this is not the case. This is a minimum however as the ED missile uses a lower thrust and fuel mass compared to the public data, so there may be performance left on the table. If we use only public data to formulate the specific impulse of each stage then we get a boost phase of 498 seconds and a sustainer phase of 135 seconds. We will see how this version of the missile compares to the ED one as will.
 

Figure 8: ED AIM-7 specific impulse code

In-Game AIM-7F Tests:

Test Parameters:
   Game Version: 2.9.1.48335
   Map: Marianas
   Weather, Date, Time: Mission Editor defaults
   Launch Platform: F-15C
   Launch Altitude: ~40,000ft
   Launch Speed: ~1.5 IMN
   Target Platform: MiG-19P
   Target Altitude: ~40,000ft
   Target Speed: ~1.2 IMN

Test 1: Control:

Test 2: Corrected Burn Times:

Test 3: Corrected Burn Time, ED Fuel Mass, Corrected Ratio:

Test 4: Corrected Burn Time, ED Fuel Mass, Corrected Ratio, and Specific Impulse:

Test 5: AIM-7F Public Data Only:

Conclusions:
   First, comparing the control missile to the missile from Test 2, we see that the change in burn time yields practically zero difference in peak energy states. However, it can be said that the extra time in the boost phase and overall longer burn time will help in edge cases where a slightly longer burn time would result in better terminal energy. Looking at Test 3, we see that the acceleration observed at each stage is dramatically different where the majority of the acceleration of the missile is felt during the sustain phase rather than the boost phase, and the overall acceleration of the missile is much smoother reminiscent of the AIM-54's motor. Yet, again, the missile does not yield any noticeable improvement to peak energy state, but it can be said that this could perhaps drastically improve the performance of the AIM-7MH and P missiles as the loft maneuver is less than optimal which depletes the effectiveness of the MK-58's boost stage. This is just speculation though. Next, we see that in Test 4 we finally yield some peak energy improvements however small. It is worth noting that the energy graph has now returned to shape of the vanilla missile with the boost phase taking the brunt of the acceleration leg work. Although the gains are small, this would still result in a better missile with the extended burn time and slightly higher peak energy state, however marginal. Finally, we arrive at Test 5 using only public data. This missile yields the best peak energy state gaining as much energy over Test 4 as Test 4 gains over Test 1. Again, the energy graph is practically a replica of the vanilla one in terms of shape. Although the specific impulse of the boost phase is lower than that of Test 4 the sustainer is higher, and this results in the higher energy state as Test 5 would have higher actual thrust values compared to the ED numbers. It is important to note that Test 4 and 5, where the sustainer specific impulses are lower than the vanilla, would indicate that overall performance may be worse against a maneuvering target, but, again, this is just speculation since the test was done against a cooperative target. Overall, ED has done a fair job at replicating the AIM-7's MK-58 motor performance using "incorrect" fuel ratios and specific impulses, but it is unfortunate that it has been done with "incorrect" information. I believe that the closer we can get to real values (as long as they are public, of course) the better the simulation will be.

References:
1. Raytheon AIM-7F Standard Missile Characteristics: http://aviationarchives.blogspot.com/2016/08/raytheon-aim-7f-standard-missile.html
2. Gallery of USAF Weapons, 2010 Almanac: https://www.airandspaceforces.com/PDF/MagazineArchive/Magazine Documents/2010/May 2010/0510weapons.pdf
3. AD-A-142508: https://apps.dtic.mil/sti/pdfs/ADA142508.pdf
4. Canadian MK-58 Mod 5 study: https://apps.dtic.mil/sti/pdfs/ADA596430.pdf

AIM-7F Corrected Burntime.acmi AIM-7F Vanilla.acmi AIM-7F Public Data Only.acmi AIM-7F Corrected Burntime+Fuel Mass+Impulse.acmi AIM-7F Corrected Burntime+Fuel Mass.acmi

Edited November 27, 2023 by Prez
Minor grammatical error

[Prez]

A slight error on my part with this: the Canadian MK-58 study states a 70/30 “volume” ratio, but I misread it as a mass ratio. I believe I overlooked it since the ratio was fairly close to the mass ratio anyways. I apologize for that error.

[Asphelite]

Reading through all of this I do have to say: Incredible research work! Nice job Prez, I hope the team at ED will honor it in some way.

[Hiob]

Impressive work! 

And interesting insights into both, the inner workings of the missile and DCS.

[BIGNEWY]

Hi @Prez

thank you for making the effort to post here on the forum we appreciate it, however the information we have shows different values. At this time we have no plans to change this. 

Here is a previous topic for the AIM-7 with some information

 

[Hog_driver]

2 hours ago, Asphelite said:

I hope the team at ED will honor it in some way.

Honored with the "Correct As Is" badge. Just kidding

[Prez]

8 hours ago, BIGNEWY said:

Hi @Prez

thank you for making the effort to post here on the forum we appreciate it, however the information we have shows different values. At this time we have no plans to change this. 

Here is a previous topic for the AIM-7 with some information

 

Would it not be worth it to at least take another look at the actual thrust values? Especially, since the ones in DCS are underperforming, albeit a few percentages, compared to not only my references, but the references of Maestro who is behind the code itself. Burn time as well is corroborated between my sources and all but one of Maestro’s. Even if this is not changed immediately or shelved for the time being, I think it’s still worth taking a look at especially when the corrections I made resulted in a missile that is not so far off what is currently in game. Thank you.

[BIGNEWY]

10 hours ago, Prez said:

Would it not be worth it to at least take another look at the actual thrust values? Especially, since the ones in DCS are underperforming, albeit a few percentages, compared to not only my references, but the references of Maestro who is behind the code itself. Burn time as well is corroborated between my sources and all but one of Maestro’s. Even if this is not changed immediately or shelved for the time being, I think it’s still worth taking a look at especially when the corrections I made resulted in a missile that is not so far off what is currently in game. Thank you.

Hi,

I have run it by the team and the sources we have are what we are going to be using in DCS. 

thank you 

[KenobiOrder]

Not even an explanation of why ED thinks it better. Just "correct as is".

 

 

[Tholozor]

15 hours ago, KenobiOrder said:

Not even an explanation of why ED thinks it better. Just "correct as is".

There was more in-depth discussion in the linked thread, you can find out more there.

[DSplayer]

On 11/30/2023 at 1:35 AM, Tholozor said:

There was more in-depth discussion in the linked thread, you can find out more there.

Unfortunately, that thread never got an official ED response regarding the incongruencies between our motor values and those listed in the SMC.

[SkateZilla]

On 12/1/2023 at 7:29 AM, DSplayer said:

Unfortunately, that thread never got an official ED response regarding the incongruencies between our motor values and those listed in the SMC.

suggest you actually click the link, several ED Team Members and devs responded with their source for data.