DCS: F-14 Development Update - AIM-54 Phoenix Improvements & Overhaul - Guided Discussion

[tavarish palkovnik]

A bit more about impulses, nozzle expansion ratios etc etc. Once upon a time Phoenix in DCS if I'm not wrong, you guys will confirm, had specific impulses in range of 250s. Internet said it is specific impulse of CTPB and HTPB types of propellants and how not to accept it. However, there is so many behind it and simply it was wrong. Developers either figure it out alone, or somebody draw attention to them or simply with time some documents of various motors appeared where miraculously specific impulses were more about 230 than 250 and propellant inside were CTPB and similar. Hmmm...what to do now...all right it will be 230 or something like that in Phoenix as well...wrong again

 

    

 

Let's see something about these AP aluminized propellants with polybutadiene rubber as binder. Here I drew two RDS propellants, one with 14% of aluminium and 69% of ammonium perchlorate and second with 4% of aluminium and 82% of ammonium perchlorate. This second one is what I believe similar to what was in Phoenix. So these are theoretical values of specific impulses for such composition, maximal values, but only if ratio between chamber pressure and atmospheric pressure is 68. This is from American literature so it is chamber pressure of 1000psi vs atmospheric pressure of 14,7 psi (1000/14,7=68) or 69 bar / 1,013 bar. Russian standard is 40/1.

 

All right...maximal theoretical specific impulse for 4% aluminized CTPB would be 250s. We need not theoretical value but delivered value, what exactly nozzle gives, and it is hard study, but generally bigger percentage of aluminium is always with bigger losses or better to say with lower nozzle efficiency. It is not huge difference but few percent and for 4% it shouldn't be more than 1 or 2%. Let's say 1% so delivered specific impulse would be 247,5s ... again ... only with ratio of pressures as 68.

 

 

 

 

Some imaginary motor with chamber pressure of 69 bar to fully expand all to the atmospheric pressure of 1,013 bar needs to have nozzle with expansion ratio of 8,8. I don't have diagram for 8,8 ratio but this one for 8 could be easily used. What we see, if chamber pressure would be 1000 psi (69 bar) such nozzle at see level (atmospheric pressure 14,7 psi or 1,013 bar) will deliver specific impulse of something like 245s. Great, that is what we need. By the way, this delivered specific impulse is also with additional reduces, major reason is losses in divergent zone, again only about few percent we are talking. You can see that on these 3 nozzles and 3 chamber pressures, delivered specific impulse at see level vary a lot, from 220 to 250s but neither of these 9 cases, except marked one, is full expansion, either over expanded either under expanded nozzle.

 

All right, so nozzle should be with expansion ratio 8,8 that chamber pressure of 69 bar fully expand to 1 bar

 

    

 

That is case of these ''red'' marks, and this Phoenix propellant like most of other similar propellants is with heat ratio of 1,2 or very close to 1,2. This is perfect, line of 8,8 nozzle expansion ratio and curve of pressure ratio 69 perfectly hit curve of optimum. Let's see Phoenix with its 18,5 nozzle expansion ratio. To hit curve of optimum chamber pressure should be 175 bar to have full expansion to atmospheric pressure. Chamber pressure is not even close to 175 bar but average pressure is something like 46 bar...take a look how much it drops in overexpanded zone at see level. Thrust coefficient is only 1,4 ... 1,4*4,6*55,372˄2*3,14/4=15500 N ... looks familiar isn't it ... actually now I remembered that it was 45 bar in my calculation but it doesn't matter or change things in principles.

I've already showed how much this motor is different from other motors of tactical missiles, this is just other form of same. This ''blue'' area is where most of other motors fit and take a look how much Phoenix is far a away of that.

For questions at disposal of course, what ever is in my power and level of knowledge I will try to explain. This has nothing with sources but just common theory of rocket propulsion, sorry guys but without this it will be hard to understand how Phoenix motor worked 

 

[IronMike]

I think it is time to lock this thread, at least for the time being.

The fact remains that we have data on the motor, which we will not share, which we deem more accurate than what has else been presented here. Please be so kind and accept that.

The remaining issues with the phoenix are something that both ED and us will continue to work on. But we are starting to turn in circles here, and a back and forth will not get us anywhere.

Thank you all for your kind contributions and understanding. Once more progress has been made, we will open a new thread, or re-open this one.