New aero spaceplanes

[Nich]

The new GE90 has a static twr of 6.3:1 which is higher then the rapier and the wheesly. Sadly I have forgotten how velocity effects thrust but I thought thrust was constant for jets and power is constant for piston prop setups. This goes against a rule I remember that a jet engines static thrust is lower then its max thrust and there is some way to convert at a specific speed.

for an ideal engine with no frictional losses

F = (mdot *V)e - (mdot*V)o

mdoto = density*A*Vo

mdote = mdoto(1+1/14.7)

Ve = Vo + (enegry added by fuel burning at stoic)

F = ((density*A*Vo+1/14.7)(Vo + energy)) - density*A*Vo^2

F = density*A*Vo^2 + (1+1/14.7)*density*A*Vo + density*A*Vo*energy +(1+1/14.7)*energy - density*A*Vo^2

F = (1+1/14.7)*density*A*Vo + density*A*Vo*energy + (1+1/14.7)*energy

Assuming level flight density, A and energy are all constants thus F is proportional to 2V

Edited August 5, 2015 by Nich

[GoSlash27]

Nich,

The P&W F-100 gets a higher t/w than both of those plus the Rapier. But that's in zone 5 afterburner, and it's Isp is only 1552s when you do that. That changes things considerably.

Best,

-Slashy

[GoSlash27]

I hear you there, Slashy. The point is very well made, and I have to say you have me convinced.

If they do end up nerfing them further, the storm of emotional outbursts across the forum could be difficult to weather.

Happy landings!

Starhawk,

Yeah, I can well imagine! I don't really want them "nerfed" per se... I think spaceplanes are reasonably accessible as they are. I'd just like to see it rebalanced so that jets make lousy rockets. I think it's possible to have both by simulating afterburners in the throttle curve. At under 90% throttle the thrust is halved but you have over 3 times the current Isp. At over 90% the thrust is as it is now, but the Isp falls to half the current value. I would imagine the Weasley wouldn't have the afterburner, so it would have lower t/w but excellent Isp at all times.

This would make spaceplanes work about the same as they do now, but would make jets less efficient as vertical lifters.

Not that I'm all that wound- up about it. If they never fix this, I'm not going to jump and scream about it.

Best,

-Slashy

[ajburges]

Curious if there is such a challenge in RSS or another mod that models Kerbin at Earth scale. Do airbreathers still outperform rockets in getting to space when they are no longer launching from a "fun sized" planet?

That's the catch. Lifter rockets run at circa 300 s. Getting ½-ⅆyour orbital acceleration with 3000 s engines will always win. OTOH, the margins get much tighter when you can only leverage that efficiency for ⅛ of your orbital acceleration and need to lug that extra mass for the remainder. Still SSTO is of dubious benefit in general once you leave the fun size system. Imagine where our space exploration would be if we could make the Blackbird an SSTO with a few JATO rockets!

Isp:

> The SABER engine peaks at 3500 s and goes to Mach 5.5!

> RR Olympus (Concorde) peaks at 3000 s and reaches Mach 2

> P&W J58 (SR-71) peaks a 1800 s and hits Mach 2.

The RAPIERS seem more "realistically" modeled than some other engines. However, a nerf to Whiplashes would make them as obsolete as Whesleys.

[Nich]

I am pretty sure afterburners add only 5-10% thrust this does double fuel consumption if I remember correctly. Updating the engines to get less ISP when they are overspeed could make sense in addition to reduced thrust that way 1400-1500 only see 800-1000 isp What is mach 3.75 about 1200 m/s ?

Edited August 5, 2015 by Nich

[juanml82]

Keep in mind the Whiplashes are described as a "turboramjet". As in, some Kerbal engineering wonder that doubles as a turbojet at low speeds and as a ramjet at higher speeds. Or a ramjet that works at low speeds.

[GoSlash27]

I am pretty sure afterburners add only 5-10% thrust this does double fuel consumption if I remember correctly. Updating the engines to get less ISP when they are overspeed could make sense in addition to reduced thrust that way 1400-1500 only see 800-1000 isp What is mach 3.75 about 1200 m/s ?

Nich,

Working from memory, afterburners roughly double the thrust, but multiply the fuel consumption by 5.

Best,

-Slashy

[Mic_n]

Keep in mind the Whiplashes are described as a "turboramjet". As in, some Kerbal engineering wonder that doubles as a turbojet at low speeds and as a ramjet at higher speeds. Or a ramjet that works at low speeds.

The J58 used in the SR71 (which is no doubt the inspiration for the engine in question, right down to the new shockwave exhaust graphics) was commonly referred to as a turboramjet. Not technically correct, but quite widely used. Essentially it had a series of ducts and vanes from the intake to exhaust that could redirect varying volumes of air around the compressor turbines. A turbojet at low speeds, and "ramjet-ish" at higher ones, relying on that ram effect to compress air through the turbine bypass.

It's not a Kerbal engineering wonder, it's a SkunkWorks engineering wonder