urbanf@yj.data.nokia.fi (Urban Fredriksson) (03/14/91)
From: urbanf@yj.data.nokia.fi (Urban Fredriksson)
Using 20 min cruise fuel in 30 afterburner (F-14) really
sounds like waste, but maybe not in all cases.
A Viggen on full afterburner consumes 13,5 kg/s, still,
if you want to go to 10 000 m from standstill, you actually
save fuel, because it takes MUCH less time.
| Urban Fredriksson | I do NOT speak |"Who tilts at windmills? |
| Stockholm, Sweden | for my employer! | A coward?" |
| Reply to: urban@kista.relay.nokia.fi | I'm off the net in April - May |stevenp@decwrl.pa.dec.com (Steven Philipson) (03/28/91)
From: stevenp@decwrl.pa.dec.com (Steven Philipson) urbanf@yj.data.nokia.fi (Urban Fredriksson) writes; > A Viggen on full afterburner consumes 13,5 kg/s, still, > if you want to go to 10 000 m from standstill, you actually > save fuel, because it takes MUCH less time. Sorry, but you still use more fuel with afterburner (AB). The rate of fuel consumption increases by more than the climb rate. Performance figures for jet fighters are a little hard to come by, but some data is available. The 1980 issue of _Jane's All the World's Aircraft_ contains specific fuel consumption on the GE JT-79-GE-15 engine, which is what powers serveral versions of the F-4 Phantom II. The ratio of thrust with AB to thrust without AB is 1.55 to 1. The ratio of specific fuel consumption (pounds of fuel per hour per pound of thrust) is 2.55 to 1. Thus with AB, the fuel consumption rate is 3.95 times higher than at military power. In order to use less fuel, the aircraft would have to climb over four times faster with AB. The next question is how much faster will the airplane climb with AB than without? I'll spare you the math and simply quote Hurt from _Aerodynamics for Naval Aviators_: "a 50 percent increase in thrust by use of an afterburner may cause an increase in climb of approximately 100 percent." Well, we've got about a 50 percent increase in thrust here, but the fuel flow is up by 300 percent. Thus twice as much fuel is used to get to a given altitude with AB. I know of no aircraft for which fuel used to climb to a given altitude is lower with AB than without. I'd be very interested to see data on any aircraft for which AB climb is more efficient in terms of fuel consumed. -- Steve (the certified flying fanatic) stevenp@decwrl.dec.com