Scale-O-Rama jet engines

[finsrin]

Points and question about changing scale of jet engines.
I find a 1/144 jet engine that "looks right" on a kit bash to 1/72.  Is smaller as 1/72 so has less thrust.
Being smaller in all three dimensions for kitbashing purposes I consider it not half as powerful.
I consider it to have .5 x .5 x .5 (.125) the thrust it had as 1/144. 
A 1/144 10,000 lbt engine now models a 1,250lbt engine on a 1/72 model.  Though, a bit of rounding off ratings is nice.
You engine-engineer folks have comments on correctness or incorrectness of this approach ?
Thanks
Bill

[elmayerle]

For the scaleorama you're doing, it would be more a function of cross-sectional area than length, more of .5 x .5 or .250.  For a smaller leap, figure that a 1/72 F110 would replace a 1/48 F404/F414.  The cross-sectional area is a driver in mass flow,  which directly affects how much thrust you can get.  Feel free to ask further if I've left you confused.

[finsrin]

Makes sense.
Like it as a rule of thumb approach for styrene technology jet engines.
Slowy Jet has scale-o-rama engines.  1/100 became 1/72.  Should check numbers to see what I claimed there.
Much appreciated - Thanks.

[Weaver]

There are lots of different factors that can affect it. For an empirical method, find a real life engine of equivalent technology level to your model that's the same size as your model engine post-scaleorama. That is to say, if your scalorama'd engine is 1960s tech and comes out at 36" diameter and 8 feet long, find a real engine of those dimensions and take your thrust figure from there.

[finsrin]

As an example case for me and others.
Monogram made a 1/121 B-58 and Revell made a 1/94 B-58.
B-58 engine is a J-79.  Found that J-79-GE-17 is rated:  11,905 lb dry; 17,835 lb with afterburner.
Transfered to a 1/72 model what are approx ratings for Monogram and Revell engines ?
Thanks
Bill

[deathjester]

There are lots of different factors that can affect it. For an empirical method, find a real life engine of equivalent technology level to your model that's the same size as your model engine post-scaleorama. That is to say, if your scalorama'd engine is 1960s tech and comes out at 36" diameter and 8 feet long, find a real engine of those dimensions and take your thrust figure from there.

That sounds roughly like a J-85 sorta size, doesn't it?

[Old Wombat]

The circular intake area of the engine is the biggest factor & is calculated as (Pi)r²,
thus your engine volume calculation kinda becomes L x (Pi)r²

If you halve r (r/2), your calculation becomes L/2 x (Pi)(r/2)² or L/2 x ((Pi))r²)/4
or (L x (Pi)r²)/8, which is actually considerably less than 1/4 of the original.

(I think I got that right, my maths is a little rusty )



Guy

[Weaver]

As an example case for me and others.
Monogram made a 1/121 B-58 and Revell made a 1/94 B-58.
B-58 engine is a J-79.  Found that J-79-GE-17 is rated:  11,905 lb dry; 17,835 lb with afterburner.
Transfered to a 1/72 model what are approx ratings for Monogram and Revell engines ?
Thanks
Bill

Okay so by the empirical method:

A J-79 is 5.3m long and 1m wide. Bear in mind that all J-79s were afterburning, which adds considerable length to the engine. A CJ-805 (civilian J-79 without afterburner) is just under 3m long (without hush kit or thrust reverser)

The Revell engine in 1/72nd would be just over 4m long and 0.76m wide. A fair equivalent would be the Bristol Orpheus. This was 0.82m wide but of simpler technology, so the scaled J-79 should be a fair match for it. The basic Orpheus was 1.9m long, but there was an afterburning version whose length I can't find, but which you'd expect to be in the 3.5 to 4.5m range. This engine put out 6,800 lb dry and 8,170 lb in afterburner, and since it's development was never finished, you could reasonably project that later versions would do better than that.


The Monogram engine in 1/72nd would be 3.15m long and 0.6m wide. The Turmansky RD-9 is probably the nearest thing dimensionally at 0.67m wide, and that put out 6,600 lb dry and 8, 300 lb with afterburner.

You can see from this that the real world equivalents of both scaled engines overlap considerably in thrust values, which just goes to show that thrust is only approximately equivalent to engine size anyway: there are lots of other factors that can affect it. In particular, afterburners can range in power from those that only add a modest amount of thrust to those that nearly double it.

[Old Wombat]

I was just figuring on a down-scale of the same engine, which would probably result in a mere 1/8 (+/- a little bit for friction, etc.) of the original thrust. A totally differnt engine, with more modern engineering... Well, you might even get equal thrust.



Guy

[elmayerle]

As an example case for me and others.
Monogram made a 1/121 B-58 and Revell made a 1/94 B-58.
B-58 engine is a J-79.  Found that J-79-GE-17 is rated:  11,905 lb dry; 17,835 lb with afterburner.
Transfered to a 1/72 model what are approx ratings for Monogram and Revell engines ?
Thanks
Bill

Well, taking a quick and dirty cut at it using intake area and assuming same generation engine technology, the 1/94 engine would be (72/94)**2 the thrust of the same engine in 1/72, or roughly 7980 dry and 10,460 in full burner.  The 1/121 engine would be (72/121)**2 or roughly 4200 dry and 6300 in full burner.