Forum: Technology

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

I know that the f-4 phantom used a turbojet engine... not a bad one i might add if you don't count the smoke.

Than the f-14, f15 , f-16,f-18, f-22 move to turbofans which are more fuel efficient.

I guess that a modern jet equip with turbojet engine would not have so much low level agility and range would be 25% smaller but performance at high speed and altitude would be greath for air superiority. Probably a lighter airframe than f-4 could by build today (( rafale weight is 9 tones empty) engine trust = 75 kN) j79of the phantom had 79.3 kN

Is there still a chance for the turbojet ? Question

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Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

The difference between an low-bypass afterburning turbofan engine and an afterburning turbojet engine is minimal. The former performs better in just about every way which is why it's used over pure turbojets.

Salute!

Not only that, Rap, but look where most A2A occurs. It ain't up in the ionosphere.

The Viper blip in the performance curve was about Mach .85 and 20K or so. No kidding, there was an noticeable bump in the drag curve, gee-available, et al.

If I were planning to fight at 80K, I would choose a ramjet!!! Down low, it would be a high-bypass fan with a burner.

The pure turbojet has definite advantages in terms of throttle movement versus actual reaction by the motor. But I'll take a slower reaction time to gain fuel economy when cruising to the fight or getting home.

later,

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

i have no doubt that at low speed and altitude the turbofan is better, however as speed passes over 2 mach and altitude increase over 60k the turbofans shows his limitations.
i know about a study to equip the sr-71 with phoenix missile but i don't know if there were for offense or defense.
anyway such a fighter in my opinion would change air combat for ever.

Elite 1K Joined: May 26, 2005 - 08:39 PM Posts: 1496

Gums wrote:

Down low, it would be a high-bypass fan with a burner.

Heck yes, can anyone say A-10D? Laughing

Cad wrote:

i know about a study to equip the sr-71 with phoenix missile but i don't know if there were for offense or defense.
anyway such a fighter in my opinion would change air combat for ever.

It was the YF-12 IIRC, and that's not an AIM-54 Phoenix, as the filename indicates it's an AIM-47 Falcon. Hughes used the technology when developing the Phoenix.

The problem as I understand it is when you're going Mach-Many you have a horrid turning radius. So if your first shot misses/is defeated against a slower but maneuverable opponent then your cheeks are flapping in the breeze as you overshoot.

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

turning radius is not a issue since the actual fighters can only intercept sr 71 head on ( by the way the mig-25 failed)
from rear aspect the AIM-120 could catch up with a sr-71 only from 10 miles or closer.

Salute!

Good grief, Cad, why have jet motors at all?

What's your point?

Only things zipping around in the ionosphere can't/don't turn, so clever warriors have come up with other means to counter them. Like SAM's, lip-shots from slower jets that have great radar and great missiles, the secret U.S. death ray, oooops....

Sorry, but the classic dogfights are determined by laws of nature and human instincts. They will be fought mostly at low altitudes, and by humans. The most efficient motors in the arena will be determined by simple physics and aero.

And by the way, "just what, if any, actual experience does Cad have in such matters?"

Sorry, cheap shot.

The face shot air-to-air scenario may be the new thing, but certainly not very romantic. Hell! I want my "colonial Viper" or "X-Wing", dammit.

later,

Good post, Gums Smile

!

Just a couple of tidbits on the YF-12A:

The YF-12A used the same turboramjets as the SR-71. At above about Mach 1.5, the supersonic shockwave from the tip of the intake cone was directed into the throat of the intake by spike postion and was directed into a system of bypass tubes. This air bypassed the core of the engine and was directed into the afterburner, creating most of the thrust at that speed. Gums got it right - high bypass with a burner Wink

.

The YF-12A was intended as an interceptor to bring down bomber formations and carried four of the aforementioned missiles. What wasn't mentioned was that these missiles were to be armed with 10kt (get ready Gums Razz

) NUKULAR warheads Shocked

! No operational launches with nuclear warheads were conducted, but there were quite a few launches with inert warheads - at least one impacted its target after being launched at about 80 or 100 miles, 80K+ feet, and Mach3+ (working from memory here).

Oh yeah, one other thing, the Blackbird family of aircraft did have a huge turning radius Wink

. Check out the SR-71 operating manual for some interesting reading Cool

.

For more info from the people who know, go to www.area51specialprojects.com and click on the link for the YF-12A Smile

.

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

i know that the fighter that i`m thinking about sounds like a X-WING or something similar and probably in the nest 20-30 years such a fighter would not be build.
SAM`S would still be a treat to such a fighter but the reaction time from actual fighters would be very short, note the fact that the mig-25 needed 405 miles lead distance to intercept.

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

the fact that sr-71 does not have turn radius does not mean that the Blackbird is bad jet.
u just CAN`T DO that kind of turns at that speed and altitude.
the problem is to design a jet that has good performance in a wide envelope (speeds and altitudes)
the turbofan has maximum performance is at sea level, as speeds and altitudes increase, performance goes down,
so basically limits your speed and ceiling.

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

Cad wrote:

the fact that sr-71 does not have turn radius does not mean that the Blackbird is bad jet.
u just CAN`T DO that kind of turns at that speed and altitude.
the problem is to design a jet that has good performance in a wide envelope (speeds and altitudes)
the turbofan has maximum performance is at sea level, as speeds and altitudes increase, performance goes down,
so basically limits your speed and ceiling.

Perhaps you meant that a turbojet (hell... any jet) produces the most thrust at sea level. This of course is true, but your speed is much more limited at low altitude than high. Also, fuel economy is terrible at sea level. High thrust = high fuel flow.

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

No contest about the fuel efficiency...
turbofan is much better when it comes to that, also a aircraft could have much better range, payload, loiter time using a turbofan. today turbofans are also light ( the rafale engine weight is 1,977.50 lb).
the fans in the turbofan engine use the energy provided by the turbine to operate but do not power the turbine in the same way that the axial compressor does in the turbojet.
this works fine at low speed BUT as speed increases the power needed to operate the fans increases too...
if u can`t compress the air your not going to get the extra trust.
the ramjet solves this problem but only above ~ 1.5 mach.

Cad,

Sorry if I left the wrong impression - I never meant to say that the Blackbird was a bad jet Wink

. I think it would've been a great interceptor for going against bomber groups as it was intended. After all - more speed = interception farther out. Take shots from long range (no question if it's an enemy bomber formation Smile

) and head back - no tight turning involved.

Active Member Joined: Mar 14, 2007 - 11:44 PM Posts: 155

British Phantoms were fitted with the larger and more powerful Rolls-Royce Spey turbofan engines with 20,515 pound-force (91.25 kN) of afterburning thrust each for improved takeoff performance, and many of the subsystems were replaced with British-manufactured equivalents. The larger engines required more air which necessitated 20 percent larger air intakes and compromised top speed and high altitude performance. Range improved, however, due to the turbofans' better fuel efficiency. Bang Head

According to wiki.

Elite 1K Joined: Aug 19, 2004 - 09:19 AM Posts: 1092

Cad,

I'm afraid you're not understanding the advantages of low bypass afterburning turbofans as opposed to afterburning turbojets. A low bypass AB turbofan basically uses the bypass "cold" flow as passive cooling for the AB section so that it can operate at higher temperatures. This results in higher AB temps and, hence, increased exit velocity which equals increased thrust. I'm not sure why you think an low BP AB turbofan can't push an aircraft just as fast as a turbojet. It can probably push it faster. They both stop being efficient as you approach Mach 3.

Forum: Technology

Is the Turbojet dead ?