F-15 in the interceptor role, hitting limits

[sprstdlyscottsmn]

In my diving through the F-15E-1 (that includes information for a plane with no CFTs and the F100-PW-220 engines essentially mirroring an F-15D) that the placard limit for stores carriage on the body is 1.4M for air to air missiles. This compares to Basic Aircraft Limits for wing mounted air to air missiles. This led me to thinking, what is the best configuration for a scramble?

I tested the standard loadout of 8AAMs as well as body only AAMs (minimum drag) and wing only AAMs (maximum speed limit). I went through the Climb charts (to 35k, lowest alt for best forward speed) Accel to max speed, and fuel flow at said speed for a "cruise" section.

Not surprisingly the body only "clean" loadout climbed and accelerated the fastest and had the lowest fuel burn at 1.4M giving a greater range (by virtue of longer time in cruise). What I did not expect is how little of a difference it actually makes. In a race to 150 nm the "clean" Eagle only beats the standard Eagle by ~20-30 seconds and since they have the same speed the have similar missile flight parameters. It gets to spend almost 4 minutes longer in cruise however giving a max intercept range of 196nm before RTB compared to the standard Eagles 163nm.

In a race to 125 nm the "wing" Eagle beats the "clean" Eagle by almost a minute and due to the greater speed (drag limited to ~M2.04) it increases the range of a missile shot by 10nm. However the massive fuel burn of said speed causes the "wing" Eagle to RTB immediately.

Now this also brought up an interesting position. M2 for the "wing" Eagle compared to M1.4 only buys 90-60sec (12%-8% faster) of time while costing 40-70 miles (29%-55% more) of range for the standard and "clean" Eagles respectively. This also highlights why more powerful engines for the F-15C would be mostly a waste. Even the highest drag load I tested was placard limited 0.55M slower than it was drag limited.

Disclaimers are of course that these times and distances are using the given charts, while a Rutowski profile would reduce climb fuel and acceleration time and fuel. Also standard day conditions were used but as this is a hypothetical comparison only that should be a non-factor. This was fun and I may see if I can do it for the F-16C and the F/A-18E as well.

[eloise]

max intercept range of 196 nm seem really short for something as big as the Eagle

[sprstdlyscottsmn]

Gas gets guzzled pretty quick with afterburner. The range would of course be extended greatly if drop tanks were used and dropped when empty but that is not the metric I was looking at for this.

[eloise]

Gas gets guzzled pretty quick with afterburner. The range would of course be extended greatly if drop tanks were used and dropped when empty but that is not the metric I was looking at for this.

what would be the approximate range of Typhoon, Rafale ,Gripen, F-18E/F, F-16, Mig-29STM for similar mission profile ?
with and without Fuel tank

[sprstdlyscottsmn]

I can get the data for F-16C/C+ and F/A-18E, but I do not have the flight manuals for the rest. I wish I had all the flight manuals. I would LOVE to see the range of the Su-27 and to know what the placard limits are for it and how much drag the various stations carry.

[madrat]

It puts the 'paltry' range of the English Electric Lightning in perspective. I think it did 150 nm in a similar scramble. People just assume it could only fly that far in every profile.

[sprstdlyscottsmn]

Just got the number for the F-16. With a standard air to air load of 4 AIM-120s and 2 AIM-9s it takes 9.9 min to cover 85nm before it is RTB. The Viper slightly outclimbs the standard Eagle to 35,000ft and then the Eagle begins to out accelerate the Viper to its placard limit of 1.4. The Viper continues to M1.86 and hits bingo fuel ~25 seconds later. So as we see, hauling butt at high mach is a gas guzzling venture.

[johnwill]

In my diving through the F-15E-1 (that includes information for a plane with no CFTs and the F100-PW-220 engines essentially mirroring an F-15D) that the placard limit for stores carriage on the body is 1.4M for air to air missiles. This compares to Basic Aircraft Limits for wing mounted air to air missiles. This led me to thinking, what is the best configuration for a scramble?

spurts, I am not questioning your analysis at all, but I am puzzled by the 1.4 mach limit for the body-mounted missiles. I can't imagine any reason for such a low limit. I am wondering if that limit applies to stores mounted on conformal tanks, or with external wing tanks, or perhaps is a launch limit for the missiles. Do you know if the F-15A/B/C/D are limited to 1.4 with body-mounted missiles? One thing about missile launch speed limits - sometimes the launch limit determines the carriage limit. For example, if launch is limited to 1.4 for whatever reason, carriage may be limited to the same speed. Why? Let's say you are zooming along at 1.8 and you get a missile shot opportunity. You don't want to have to slow to a launch limit before you can fire. So you limit carriage speed to the same as launch limit.

Would you mind double checking the 1.4 limit conditions?

[sprstdlyscottsmn]

spurts, I am not questioning your analysis at all, but I am puzzled by the 1.4 mach limit for the body-mounted missiles. I can't imagine any reason for such a low limit. I am wondering if that limit applies to stores mounted on conformal tanks, or with external wing tanks, or perhaps is a launch limit for the missiles. Do you know if the F-15A/B/C/D are limited to 1.4 with body-mounted missiles? One thing about missile launch speed limits - sometimes the launch limit determines the carriage limit. For example, if launch is limited to 1.4 for whatever reason, carriage may be limited to the same speed. Why? Let's say you are zooming along at 1.8 and you get a missile shot opportunity. You don't want to have to slow to a launch limit before you can fire. So you limit carriage speed to the same as launch limit.

Would you mind double checking the 1.4 limit conditions?

Certainly! It is an awfully perplexing issue. So I will type this as I go so that you can "discover" with me.

First stop, the External Stores Station Identification.

With or without out CFTs I see the missile stations are listed as 3(c), 4(c), 6(c), and 7(c). Keeping in mind this is an F-15E chart and I am using a few assumptions, it shows AIM-7F for 3c, 4c, 6c, and 7c. It lists Carriage and Employment limits as 660/1.4 for each.

It only shows AIM-120A on Stations 2 and 8 (wing) with both Carriage and Employment limits posted as BAL (Basic Aircraft Limits). The same is true for AIM-9. They do have different Carriage and Employment limits for G loads (BAL for all a-a loads for carriage and +7.2 or +7.3 for employment).

To the point of the limits being the same when they are not really, the Mk-82 LDGP shows Carriage of 660/1.4 and Employment of 500/.95. So they are clearly differentiating the two.


Now I noticed that in the stores configuration sections they only show CFT loaded options, however in the performance sections the relevant airplane configurations are Clean, (4) AIM-7, and -CFT (4)AIM-9 (4)AIM-7. This seems to me that you can load AAMs on the body itself without CFT, but I have to assume the limits are the same since there is no config limit charts without them.

So, that is my data and those are my assumptions. Let me know what you think.

[Corsair1963]

Just got the number for the F-16. With a standard air to air load of 4 AIM-120s and 2 AIM-9s it takes 9.9 min to cover 85nm before it is RTB. The Viper slightly outclimbs the standard Eagle to 35,000ft and then the Eagle begins to out accelerate the Viper to its placard limit of 1.4. The Viper continues to M1.86 and hits bingo fuel ~25 seconds later. So as we see, hauling butt at high mach is a gas guzzling venture.

I am also confused??? It's my understanding that a F-15C with 4-AMRAAM's and 4-Sidewinder (no external fuel tank) has a top speed of just Mach 1.78 If, true I have a hard time believing an F-16C could reach Mach 1.86 with 6-AAM's.

[johnwill]

Thanks for the good explanation of what you see in the -1. In your explanation, 1.4 limit applies to AIM-7 missiles at 3/4/6/7, while AIM-120 at 2/8 can go to BAL. So my guess is that the 1.4 limit is imposed by the presence of AIM-7. In your OP, I thought when you said AAM, you meant AIM-120, as I assumed (!) the AIM-7 is no longer used. Is there any configuration shown in the -1 for AIM-120 at 3/4/6/7 ? I have a good friend who is a former F-15C pilot, so I'll ask him what he remembers of that loading.

Very interesting analysis, by the way.

[sprstdlyscottsmn]

I am also confused??? It's my understanding that a F-15C with 4-AMRAAM's and 4-Sidewinder (no external fuel tank) has a top speed of just Mach 1.78 If, true I have a hard time believing an F-16C could reach Mach 1.86 with 6-AAM's.

From what I can interpolate from the charts an F-15"D" with that load is drag limited to 1.94M. I have also heard the 1.78M number before but that is not presenting in my charts.

Perhaps it is an issue of engine tune. The F-15C charts list top speed with engine at 97.5% 100% and 102.5% tunes IIRC. It was also my understanding that operationally they are all tuned for 97.5% for maintenance.

Also, the F-15C uses F100-PW-220 engines, the F-16C whose charts I am reading uses the F100-PW-229. The F-16C also has the four underwing missiles on low drag AAM pylons while the F-15 practically has those same pylons mounted to huge wet/heavy pylons. For an idea, the total pylon weight for the F-16C is (4)*(98)=392lb and 24 counts of drag (*300/10,000 for 0.72ft^2 of drag area). The F-15 pylons are (2)*371+(4)*111=1186lb and 11 counts of drag (*608/10,000 for 0.67ft^2 of drag area). Well it looks like the pylon drag aspect was different than I expected, but sound science is to post the results anyway! In fact the total drag area added by the weapons is 1.47ft^2 for the Viper and 1.08ft^2 for the Eagle.



johnwill, I did have to assume that the AIM-7 limits apply to the AIM-120 as well (since nothing mounted to the body has higher carriage limits then 660/1.4) I was referring to the AIM-120 and it is what I used for drag and weight modeling, so you actually assumed correct. There are no configurations shown with AIM-120s at 3/4/6/7.

What I did was take the performance of Clean (DI=0), 4xAIM-7 (DI=7.2), and CFT 4xAIM-7 4xAIM-9 (DI=50) and interpolate the performance for the drag index of the configurations I was using. Obviously anything from an actual F-15C -1 trumps my assumptions.

[sprstdlyscottsmn]

And the F/A-18E results.

Looking at the following configurations: "clean" two conformal AIM-120s and two tip AIM-9s.
"standard" as above plus one additional AIM-120 each on outboard and midboard pylons
"Max" double AIM-120 pylons on inboard and midboard pylons for fourteen AAMs

Climb: Time/Distance to 35,000ft from brake release
clean: 2.25min/13.3nm
standard: 2.6min/15.8nm
Max: 2.8min/18.3nm

Accel: Speed/Time from ToC to speed/Distance from brake release
clean: 1.56M/8.5min/108.7nm
standard: 1.39M/5min/69.3nm
Max: 1.07M/3.8min/52.8nm

Dash: Time in dash/Distance in dash/Distance from brake release
clean: 3.2min/47.9nm/156.6nm
standard: 3.7min/48.5nm/117.8nm
Max: 9.4min/95.9nm/148.8nm

So interestingly the SHornet is drag limited in all loadouts and the drag of the "Max" load keeps the max thrust so low that it has the lowest fuel burn and as such it gets nearly the same range as the "clean" load, but it takes two minutes longer to get there.

Very interestingly the "standard" load of six AIM-120s and two AIM-9s is the same as the standard on the Eagle, both planes have an empty weight within 500lb, both have a fuel load within 500lb (SHornet heavier on both counts), and both planes have a top speed of around 1.4M. The BIG difference is the thrust of the Eagle makes it a placard limit while it is a thrust-drag limit for the SHornet and due to a lower drag design the Eagle goes over 40nm further and takes just over two minutes to do so as it climbs and accelerates faster.

As discussed above the Eagles placard limit speed of 1.4M is based on mounting munitions to CFTs as opposed to base fuselage locations while the thrust limit was 1.94M. If anyone has the placard limit from an F-15A/B/C/D for A-A stores on the fuselage I welcome the input.

[sprstdlyscottsmn]

Well johnwill, I found the missing piece of data. While I was not able to get an F-15A/B/C/D manual, I was able to find the stores loading/limits page for the F-15A that covers AAMs. Carriage limits for AIM-7s on the body is BAL, Employment is 800 KCAS/2.3M/+7.33G. For practical purposes that is still BAL. So this tells me that my previously used 660/1.4 limits applies to CFT mounted loads but NOT to truly fuselage mounted loads. So my initial assumption that CFT and fuselage carriage limits are the same was incorrect. I love getting new data, especially when it proves my assumptions wrong. How this effects my initial analysis is that range and time will drop as speed goes up to 1.94M.

[johnwill]

Good find. That makes much more sense to me.

One other thing, I'm not trying to be a grammar cop, but you (and many others) need to learn the difference between affect and effect. With the high quality of your research and writing, confusing those two words can ruin it.