Aim-54 mk47 / mk60 Specific Impulse is Wrong & Not Accounting for Height Change

[vovochen]

Specific Impulse is calculated like this: https://www.grc.nasa.gov/www/k-12/airplane/specimp.html
It will be called "Isp" from here on.

The Isp therefore calculates from Game Files like this: 

13595N / ((163kg/27s) * 9,81m/s )= 229,554s

As a Comparison, the Space Shuttles Solid Rocket Boosters Isp was 242s on Ground level, 268s in Vacuum.
Source:  https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster?fbclid=IwAR0U76UEI_XSu9uyMmW-ieV7sorH4y-cTmtCPLeXGUVowO2t_1LMh3T8ii4#:~:text=This propellant is commonly referred,%2Fs 

As visible in the Files, it currently works with a specific thrust that is not changing with Air Pressure, even tho it should. 

Source: https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/thrust-force/#general-thrust-equation-for-rocket-engines

Furthermore, the Missile does currently NOT reach close to its Specification, as specified in the two Sources below (see "New Air-Launched Small Missile (ALSM) Flight Testbed forHypersonic Systems" and "SubA_6_2_Phoenix Project Overview").

 Source: "nullNew Air-Launched Small Missile (ALSM) Flight Testbed for Hypersonic Systems"

Source: "SubA_6_2_Phoenix Project Overview"

Furthermore, both Rocket Motors share the same Isp, even tho one is Smokeless and the other one is Smoking (!!!)
This is very unlikely, given smokeless propellants usually have Isp Drawbacks.

Proposed Solution:

Currently, the Aim-54 Isp and Max. Mach Speed are too low compared to the real world without a doubt.

Increasing the Thrust (and therefore Isp) by:

5% for the MK60 Motor at Ground level

0% for the MK47 Motor at Ground level

would put the MK60 Motor at 241,03s of Isp, about on par with the Space Shuttles Solid Boosters.

Most important however is the (currently missing) additional Dynamic Thrust by Altitude:

Both Boosters should gain extra Thrust of 268s/242s = ~11% when in Vacuum, so using the Barometric Formula for Air Density at Altitude both Motors should achieve a higher thrust of:

3,74% burning at 10000 feet (see Math below)

6,6% burning at 20000 feet (see Math below)

8,69% burning at 30000 feet (see Math below)

9,9% burning at 40000 feet (see Math below)

 

Math: 

10000 feet: 1atm - 0.66atm = 0.34atm, 0.34 * 11% = 3,74%
20000 feet: 1atm - 0.40atm = 0.60atm, 0.6 * 11% = 6,6%

30000 feet: 1atm - 0.21atm = 0.79atm, 0.79 * 11% = 8,69%

40000 feet: 1atm - 0.10atm = 0.90atm, 0.9 * 11% = 9,9%

Source: https://www.mide.com/air-pressure-at-altitude-calculator & https://en.wikipedia.org/wiki/Barometric_formula

 

For Testing, I have modified my Game Files to match the Math:
The MK60 Missile would now reach Mach 4.5 instead of its previous Mach 3.8 when launched at 40000 feet, bringing it right to where it should be, according to both Sources ("New Air-Launched Small Missile (ALSM) Flight Testbed forHypersonic Systems" and "SubA_6_2_Phoenix Project Overview").

Thank you for taking the time to read and assess this, Dear Reader
Best Wishes, Florian

(Student and future Engineer of the Aerospace Engineering Branch)

New Air-Launched Small Missile (ALSM) Flight Testbed forHypersonic Systems.pdf SubA_6_2_Phoenix Project Overview.ppt

[vovochen]

ADDITION:

Found this Post, claiming the Isp is indeed off and should be 252s instead of 229s: https://www.secretprojects.co.uk/threads/aim-54-phoenix-motor-propellant-question.15336/

Furthermore does the Author claim that the "Hazard Classification of United States Military Explosives and Munitions USA 2009" (File is attached) lists
"MXU-637/B" for the AIM-54A as containing 459 lbs propellant, (212.73kg which are nowhere to be found in the game files)
and "MXU-637A/B" for the AIM-54C as containing 360 lbs propellant. (163.3kg which are in game right now like this)
I checked the Document, and that appears to be true.
 

HAZARD CLASSIFICATION OF UNITED STATES MILITARY EXPLOSIVES AND MUNITIONS.pdf

Edited July 8, 2023 by vovochen

[JNelson]

1 hour ago, vovochen said:

Specific Impulse is calculated like this: https://www.grc.nasa.gov/www/k-12/airplane/specimp.html
It will be called "Isp" from here on.

The Isp therefore calculates from Game Files like this: 

13595N / ((163kg/27s) * 9,81m/s )= 229,554s

As a Comparison, the Space Shuttles Solid Rocket Boosters Isp was 242s on Ground level, 268s in Vacuum.
Source:  https://en.wikipedia.org/wiki/Space_Shuttle_Solid_Rocket_Booster?fbclid=IwAR0U76UEI_XSu9uyMmW-ieV7sorH4y-cTmtCPLeXGUVowO2t_1LMh3T8ii4#:~:text=This propellant is commonly referred,%2Fs 

As visible in the Files, it currently works with a specific thrust that is not changing with Air Pressure, even tho it should. 

Source: https://www1.grc.nasa.gov/beginners-guide-to-aeronautics/thrust-force/#general-thrust-equation-for-rocket-engines

Furthermore, the Missile does currently NOT reach close to its Specification, as specified in the two Sources below (see "New Air-Launched Small Missile (ALSM) Flight Testbed forHypersonic Systems" and "SubA_6_2_Phoenix Project Overview").

 Source: "nullNew Air-Launched Small Missile (ALSM) Flight Testbed for Hypersonic Systems"

Source: "SubA_6_2_Phoenix Project Overview"

Furthermore, both Rocket Motors share the same Isp, even tho one is Smokeless and the other one is Smoking (!!!)
This is very unlikely, given smokeless propellants usually have Isp Drawbacks.

Proposed Solution:

Currently, the Aim-54 Isp and Max. Mach Speed are too low compared to the real world without a doubt.

Increasing the Thrust (and therefore Isp) by:

5% for the MK60 Motor at Ground level

0% for the MK47 Motor at Ground level

would put the MK60 Motor at 241,03s of Isp, about on par with the Space Shuttles Solid Boosters.

Most important however is the (currently missing) additional Dynamic Thrust by Altitude:

Both Boosters should gain extra Thrust of 268s/242s = ~11% when in Vacuum, so using the Barometric Formula for Air Density at Altitude both Motors should achieve a higher thrust of:

3,74% burning at 10000 feet (see Math below)

6,6% burning at 20000 feet (see Math below)

8,69% burning at 30000 feet (see Math below)

9,9% burning at 40000 feet (see Math below)

 

Math: 

10000 feet: 1atm - 0.66atm = 0.34atm, 0.34 * 11% = 3,74%
20000 feet: 1atm - 0.40atm = 0.60atm, 0.6 * 11% = 6,6%

30000 feet: 1atm - 0.21atm = 0.79atm, 0.79 * 11% = 8,69%

40000 feet: 1atm - 0.10atm = 0.90atm, 0.9 * 11% = 9,9%

Source: https://www.mide.com/air-pressure-at-altitude-calculator & https://en.wikipedia.org/wiki/Barometric_formula

 

For Testing, I have modified my Game Files to match the Math:
The MK60 Missile would now reach Mach 4.5 instead of its previous Mach 3.8 when launched at 40000 feet, bringing it right to where it should be, according to both Sources ("New Air-Launched Small Missile (ALSM) Flight Testbed forHypersonic Systems" and "SubA_6_2_Phoenix Project Overview").

Thank you for taking the time to read and assess this, Dear Reader
Best Wishes, Florian

(Student and future Engineer of the Aerospace Engineering Branch)

New Air-Launched Small Missile (ALSM) Flight Testbed forHypersonic Systems.pdf 3.83 MB · 5 downloads SubA_6_2_Phoenix Project Overview.ppt 3.93 MB · 4 downloads

 

49 minutes ago, vovochen said:

ADDITION:

Found this Post, claiming the Isp is indeed off and should be 252s instead of 229s: https://www.secretprojects.co.uk/threads/aim-54-phoenix-motor-propellant-question.15336/

Furthermore does the Author claim that the "Hazard Classification of United States Military Explosives and Munitions USA 2009" (File is attached) lists
"MXU-637/B" for the AIM-54A as containing 459 lbs propellant,
and "MXU-637A/B" for the AIM-54C as containing 360 lbs propellant.
I checked the Document, and that appears to be true.
 

HAZARD CLASSIFICATION OF UNITED STATES MILITARY EXPLOSIVES AND MUNITIONS.pdf 734.95 kB · 0 downloads

 

 Hello Florian,

Thank you for your detailed research. It's worth noting that what can be seen in the game files can not always be compared with reality because it depends on how Eagle Dynamics interprets those numbers which might not always be obvious from an outside perspective. 

If you compare Figure 7. Launch elevation angle effects on ALSM flight performance (Mach 1.2, 45,000-ft launch conditions). From the document you attached "New Air-Launched Small Missile (ALSM) Flight Testbed for Hypersonic Systems" you will find that currently the performance of the phoenix is very close to that simulated in that paper.

Please rest assured the effective overall impulse and thrust of the missile are very close to their real values for in game and they match sources and match test shots withing 5% error for things like impact velocity, loft altitude and time until impact. 

Finally please see the attached graph which are the same parameters as figure 7 with the NASA simulation overlayed (dashed) with the DCS results (solid).

Edited July 8, 2023 by JNelson

[vovochen]

Thank you very much for going through and honoring my efforts with a Reply, JNelson. 

In the conducted NASA Research a Missile with "Zero-lift trajectories [...] with no missile guidance (fixed fins)" on a ballistic course was used to generate the Simulation Data.
I am going to sim this in DCS and share my findings; did your missile in DCS describe a parabolic trajectory when generating the Graph, or did it go in a straight line until running out of energy ? There is no height data included; is that available, could you post the Graph of it ?

Where in the Code is the Motor thrust being adjusted by height ?
I have been looking into the DCS Code for 4 years now, but was indeed unable to find anything regarding that.

Furthermore: Which Motor did you use generating the Graph ?

[JNelson]

22 minutes ago, vovochen said:

Thank you very much for going through and honoring my efforts with a Reply, JNelson. 

In the conducted NASA Research a Missile with "Zero-lift trajectories [...] with no missile guidance (fixed fins)" on a ballistic course was used to generate the Simulation Data.
I am going to sim this in DCS and share my findings; did your missile in DCS describe a parabolic trajectory when generating the Graph, or did it go in a straight line until running out of energy ? There is no height data included; is that available, could you post the Graph of it ?

Where in the Code is the Motor thrust being adjusted by height ?
I have been looking into the DCS Code for 4 years now, but was indeed unable to find anything regarding that.

Furthermore: Which Motor did you use generating the Graph ?

Yea it's not trivial to get the missiles to fly in a ballistic trajectory. You have to disable some stuff in the lua config like the guidance and the induced drag and lift (the latter two because the stability of the missile in the old schema which the AIM-54 uses does not appear strong enough to keep the missile on a ballistic trajectory, this doesn't actually matter because the test is conducted with a zero lift condition anyway). 

You can see below with those accommodations the missile follows a ballistic trajectory rather nicely (remember this is time not down range distance so the actual trajectory depends on speed).

Edited July 8, 2023 by JNelson

[vovochen]

Thank you for your response !
If the Scale on the left is feet (which I assume it must be), then you are not matching the "Mach 1.2, 45,000-ft launch conditions", but launching at 15000 feet. What is going on there ?

The MK47 does genually behave as advertised in your Graph (although my missile was always going a bit slower, but that may me measuring error), but it does not seem to be able to achieve anything close to the Mach 5.1 (source: SubA_6_2_Phoenix Project Overview) when launched at 52000 feet going mach 2.0 and giving it an angle of around 20°.
The highest Mach I could achieve was 4.5 instead of the 5.1 seen on the NASA chart.

Have you been able to achieve Mach 5.1 with the phoenix being launched at 52000 feet and the Tomcat going Mach 2.0 ?

Edited July 8, 2023 by vovochen

[JNelson]

27 minutes ago, vovochen said:

Thank you for your response !
If the Scale on the left is feet (which I assume it must be), then you are not matching the "Mach 1.2, 45,000-ft launch conditions", but launching at 15000 feet. What is going on there ?

The MK47 does genually behave as advertised in your Graph (although my missile was always going a bit slower, but that may me measuring error), but it does not seem to be able to achieve anything close to the Mach 5.1 (source: SubA_6_2_Phoenix Project Overview) when launched at 52000 feet going mach 2.0 and giving it an angle of around 20°.
The highest Mach I could achieve was 4.5 instead of the 5.1 seen on the NASA chart.

Have you been able to achieve Mach 5.1 with the phoenix being launched at 52000 feet and the Tomcat going Mach 2.0 ?

 

The altitude graph is in metres. The Mach 5.1 you describe comes from the simulated phoenix which has 250 lbs removed. 

Edited July 8, 2023 by JNelson

[vovochen]

Please excuse the feet / meters slip-up of mine. ^^
I was not wearing my glasses and missread meters as mach, thinking it may have been mis-labeled. ^^

But no, I am referring to this, entirely different launch conditions, and without a stripped down missile.
Did Heatblur ever test, if this is currently achievable ? 

[vovochen]

Further,HAZARD CLASSIFICATION OF UNITED STATES MILITARY EXPLOSIVES AND MUNITIONS.pdf how do you solve the discrepancy of the Hazard Classification of the US Military listing the propellant weight of the A Version Motorsas 459 pounds, it being listed on "myskyhawk.com" too as 459 pounds, by a company who is selling Military Parts, and a third Company Specialised in Military Components called PartTarget which also lists the AIM-54A with a propellant weight of 459 pounds ? 

I have further Emailed these Companys, asking if they are allowed to share the Specific Impulse of the used propellant.

( https://myskyhawk.com/parts/guided-missils/guided-missiles/intercept-aerial-guided-missile-1410-00-427-5506 ) 

( https://www.parttarget.com/1410-00-427-5514_1410004275514_1410-PA55.html )
 

[JNelson]

2 hours ago, vovochen said:

Please excuse the feet / meters slip-up of mine. ^^
I was not wearing my glasses and missread meters as mach, thinking it may have been mis-labeled. ^^

But no, I am referring to this, entirely different launch conditions, and without a stripped down missile.
Did Heatblur ever test, if this is currently achievable ? 

No you read correctly it was mach, before I went back and changed it like a minute later. 

In what you post here it seems to suggest that these are hypothetical scenarios and that "Weight reductions improve the performance". The source of this powerpoint is the same study which produced "New Air-Launched Small Missile (ALSM) Flight Testbed for Hypersonic Systems" so I would be surprised if they had different findings on what the phoenix was capable of.

Edited July 8, 2023 by JNelson

[JNelson]

1 hour ago, vovochen said:

Further,HAZARD CLASSIFICATION OF UNITED STATES MILITARY EXPLOSIVES AND MUNITIONS.pdf how do you solve the discrepancy of the Hazard Classification of the US Military listing the propellant weight of the A Version Motorsas 459 pounds, it being listed on "myskyhawk.com" too as 459 pounds, by a company who is selling Military Parts, and a third Company Specialised in Military Components called PartTarget which also lists the AIM-54A with a propellant weight of 459 pounds ? 

I have further Emailed these Companys, asking if they are allowed to share the Specific Impulse of the used propellant.

( https://myskyhawk.com/parts/guided-missils/guided-missiles/intercept-aerial-guided-missile-1410-00-427-5506 ) 

( https://www.parttarget.com/1410-00-427-5514_1410004275514_1410-PA55.html )
 

We have more reliable sources for our information regarding this stuff.

[vovochen]

On the mach/meters: I really thought I had embarrassed myself there.  
 

You so far dodged my question about the increase of thrust by altitude: Where is that happening ? I have attached the .lua, so you dont need to grab it out of the files yourself. 

Well.. since everything's declassified by now, or it wouldn't be in the game.. what are your sources ? ^^

Do you think asking the Iranian Government is worth a shot ?

Weapons - Copy.lua

Edited July 9, 2023 by vovochen

[DSplayer]

15 hours ago, vovochen said:

On the mach/meters: I really thought I had embarrassed myself there.  
 

You so far dodged my question about the increase of thrust by altitude: Where is that happening ? I have attached the .lua, so you dont need to grab it out of the files yourself. 

Well.. since everything's declassified by now, or it wouldn't be in the game.. what are your sources ? ^^

Do you think asking the Iranian Government is worth a shot ?

Weapons - Copy.lua 57.17 kB · 0 downloads

 

I will say that while the documents might be declassified, they are generally not publicly available and getting a hold of the documents is a pain in itself. It's also good to note that a good amount of DCS' missile characteristics are estimates since it is pretty hard to get the aforementioned documentation regarding this type of stuff.

Also, the Isp doesn't change in the way it does like you think within the files. With my limited knowledge (feel free to correct me here JNelson), the way the difference in lower and higher altitude performance is achieved in DCS is via the nozzle_exit_area parameter along with some other potential stuff. So if you were to reduce or enlarge that value, a larger performance delta would appear. Currently with the current AIM-54 simulation with the older schema/API, it doesn't really need any change and sticking with it's current parameters seem to do the job quite well. In the case of missiles like the SD-10 or AIM-120 which use the newer API/schema, the nozzle_exit_area parameter does have a pretty good influence on higher altitude performance.

Imo, the current AIM-54 simulation in DCS is the best that we can get with the extremely limited information available.

[GGTharos]

16 hours ago, vovochen said:

You so far dodged my question about the increase of thrust by altitude: Where is that happening ? I have attached the .lua, so you dont need to grab it out of the files yourself. 

ED has incorporated a thrust increase based on altitude behind the scenes since DCS came out ages ago.  The exact details are not known.

16 hours ago, vovochen said:

Well.. since everything's declassified by now, or it wouldn't be in the game.. what are your sources ? ^^

Everything is not 'declassified' and even for things that are, the information may simply not be available to you and HB might not be able to release what they have.  Why you ask?  In some cases people have purchased this information via FOIA and are unwilling to release it (you can do your own FOIA request), in others the information comes from proprietary sources such as the manufacturer, released to a developer specifically for say, making the module, and they do not allow further dissemination of the documents.

Edited July 9, 2023 by GGTharos

[vovochen]

Thank you everyone for participating,
Especially the HeatBlut Person who took his time, 
I have come to terms with the difficult truth that the Phoenix is currently as it should be in the game.

Best Wishes !

[DCS FIGHTER PILOT]

On 7/22/2023 at 8:02 AM, vovochen said:

Thank you everyone for participating,
Especially the HeatBlut Person who took his time, 
I have come to terms with the difficult truth that the Phoenix is currently as it should be in the game.

Best Wishes !

Would you be able to share with the community how you arrived at this conclusion?