Turbojet/fan Efficency

Cold war, Korea, Vietnam, and Desert Storm - up to and including for example the A-10, F-15, Mirage 200, MiG-29, and F-18.
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kdub104

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Unread post11 Oct 2019, 21:11

How do we measure efficiency? Pounds of fuel used per mile? A speed or distance calculation?

Take the GEJ79 DASH 19 variant. Thrust is measured at 17,900 lbs on bench I assume but at altitude and speed (25K+ ft and Mach 2+) thrust was theorized by GE engineers to be in excess of 25,000lbs.

Distance covered at Mach 2 is approximately 22 miles a minute.

The -19 F-104 cruised at 70K ft @ M2.0 and burned 100 lbs of fuel per minute. One minute and 100 lbs of fuel covers 22 miles. This is 4.5 lbs per mile.

Looking at a turbofan, how do we measure efficiency? Same parameters? What if we use the F110 and pick a variant: -129 or the PW 100-200/220. I recall reading here on the board the F-16 clean out ranges the F-104G (non DASH 19) with 4 tanks subsonically (the profile wasn't shared i.e. speed and altitude).

What is so fascinating is the J79 turbojet/airframe is just "getting started" at speed and altitude where the turbofan/airframe is running out of steam. I ran some numbers and calculated the -19 F-104 "could" see 500 miles range cruising at 70K ft and M2.0.
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zhangmdev

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Unread post11 Oct 2019, 21:50

thrust specific fuel consumption, TSFC

https://www.grc.nasa.gov/WWW/k-12/airplane/sfc.html

Turbojet is only efficient at high altitude and high speed. The problem is it has to reach those optimal conditions first. At low altitude and low speed, say taxing and takeoff, turbojet has very poor fuel economy. It have to mix and burn fuel with all the air pass through. Turbojets on early jetliners were called "blow torches". A Mach 2 fighter has little use if it has no leg. If the plane mostly cruises at subsonic speed, only goes supersonic occasionally, a turbofan engine is a far better choice.
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sprstdlyscottsmn

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Unread post11 Oct 2019, 21:50

Typically nm/lb is used in the manuals.

It was 73,000ft. M2 at that altitude is 1,153ktas. That makes 19.22nm/min. That makes 0.1922nm/lb That, as a single data point, is a really good fuel burn. That is nearly "clean F-16 at optimum cruise" efficiency. But the F-104 did not just "spawn-in" at that flight condition.

Total point to point distance for that flight was minimum 386nm with no routing concerns. Total fuel used prior to the landing break was 4,400lb. Efficiency of the flight being not worse than 0.0877nm/lb. It burned a lot of gas getting to that efficient point. 1,000lb of fuel just to get from 0.9M to 2.0M in 27nm. 0.027nm/lb for acceleration. Then you have to climb.

I don't know what the reserves were for the F-104, but even compared to other F-104s, the lightweight A models with the -19 motor were hotrods and fuel sparing at the same time.
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kdub104

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Unread post16 Oct 2019, 21:37

I am thinking 1200 lbs was the fuel reserve. Subtract this from the load of 5800lbs and there isn't a whole lot to play with. Tip tanks provide about 2200 lbs and they are Mach 2 rated. Tip tanks give the 104, 8000lbs of "usabIe fuel". From what I have read seems most if not all Mach 2 flights were performed clean. Unable to find one article where tip tanks were taken into the Mach.

Need an Italian f-104S pilot to join the boards.
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Father 104 Driver; "Everything Else Takes Bird Strikes in The Rear"

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