[1.0.4] Mk3 Hypersonic Scramjet update!

[nestor_d]

One of the first updates I had in mind for my Mk3 Hypersonic Systems mod was adding a scramjet engine, and it's finally here! This scramjet will mostly work like a real life one in that it doesn't produce thrust at zero, or even at low speeds. You will need to accelerate to at least mach 1 before it starts to work, and higher if you want it to produce any significant amount of thrust. Once at around mach 5 however, it will give enough acceleration to shoot a plane into space at near orbital velocity at apoapsis. Real life scramjets will not work until around mach 5, however I decided to lower the threshold for a few reasons. The first one is that the speeds needed in KSP are much lower than in real life. Low Kerbin Orbital speed is a little more than 2km/s, while LEO speed is about 8km/s. There aren't many air breathing engines that will get you to mach 5 in KSP so that's another reason to lower the threshold. Last, but not least. It's possible that some scramjets could work like ramjets below mach 5, still no thrust at low speeds, but no need to actually go hypersonic before it works so it's assumed this is one of those. See video below for a quick guide. Also here's the main forum post for the mod with download links: http://forum.kerbalspaceprogram.com/threads/130548-1-0-4-Mk3-Hypersonic-Systems

Also, for those who are into the more technical stuff, here is the velocity curve, with thrust multipliers and actual thrust at different mach numbers. Notice that it's zero until mach 1, grows slowly until mach 2 and then starts growing exponentially peaking at mach 8.8.

Edited August 21, 2015 by nestor_d
Image was wrong

[Wolf Baginski]

Orbital velocity reads at around Mach 7.8 in Kerbal Engineer as you start touching the atmosphere so things could get a mite warm. The altitude maybe is more significant, since it will need air but too much makes the ship explode, though this curve would give you a quick velocity boost with all that thrust at Mach 4 and above.

It does look as though you got the Mach numbers wrong when you labelled the graph.

[nestor_d]

Orbital velocity reads at around Mach 7.8 in Kerbal Engineer as you start touching the atmosphere so things could get a mite warm. The altitude maybe is more significant, since it will need air but too much makes the ship explode, though this curve would give you a quick velocity boost with all that thrust at Mach 4 and above.

It does look as though you got the Mach numbers wrong when you labelled the graph.

Thanks! I hadn't noticed I missed changing the X axis when doing the graph, fixed it now! Well and about the altitude thing yeah, and actually the atmCurve (which I didn't post here) and the intake speed value are made so it can perform for a while in the upper reaches of the atmosphere. Up there you can easily go at mach 7 without exploding. I the video there was some minor explosion, I'm actually not sure exactly what exploded because the ship still looks fine, but yeah, should've probably pitched up to the upper atomosphere faster to avoid overheating

[mariohm1311]

As if we didn't have enough OP engines, now there is a "scramjet"...

[nestor_d]

As if we didn't have enough OP engines, now there is a "scramjet"...

Well yeah, but scramjets are actually a thing in real life. But to try to counter the possible OP-ness of such an engine I gave it a rather low specific impulse as far as air-breathing which is in line with the performance of real scramjets. It's also rather expensive heavy-ish and not very versatile in terms of how it can be used since it's radial attachment, rather large and it can't be used as the only means of propulsion. But of course suggestions are welcome in terms of possible OP-ness

[mariohm1311]

"possible OP-ness"

You do realise that a scramjet has a TWR of 2, no? That's less than what our NERVA has... The "OP-ness" is the one you want.

EDIT: FFS, more than 5000 kN of thrust, come on, I don't think your scramjet weighs 250 tons.

EDIT2: So you lower the threshold speed yet you don't lower the max speed, good one OP.

Edited August 25, 2015 by mariohm1311

[nestor_d]

You do realise that a scramjet has a TWR of 2, no? That's less than what our NERVA has... The "OP-ness" is the one you want.

EDIT: FFS, more than 5000 kN of thrust, come on, I don't think your scramjet weighs 250 tons.

EDIT2: So you lower the threshold speed yet you don't lower the max speed, good one OP.

Well yes, it has a TWR of 2 when it's mounted on a launch vehicle, so it's not the scramjet itself that is so heavy. If you put two of this on a heavy Mk3 plane you will not get much more than a TWR of 4, which seems reasonable. Also, I technically did lower the max speed, which is projected to be up to mach 25, but in any case, given the drop in thrust after mach 8.8 and with altitude, i seriously doubt it's even possible to go past mach 12. And as I said on the original post, it's not that I lowered the threshold speed just because, a proposed feature of scramjet engines is that they will have variable geometry so they can work as ramjets at low supersonic speed and then transition to scramjet at higher speed.

[mariohm1311]

Well yes, it has a TWR of 2 when it's mounted on a launch vehicle, so it's not the scramjet itself that is so heavy.

No, you're wrong. The scramjet by itself only has a TWR of 2. Do you think there's a universal dummy weight to test engines' TWR?

[nestor_d]

No, you're wrong. The scramjet by itself only has a TWR of 2. Do you think there's a universal dummy weight to test engines' TWR?

"One issue is that scramjet engines are predicted to have exceptionally poor thrust-to-weight ratio of around 2, when installed in a launch vehicle." <- from Wikipedia that's what I get from that. In any case, you may be right, but that's how all engines work in KSP if you look at the velCurve for the Whiplash you'll see it has a max thrust of 754, which gives it an astounding TWR of 44. By comparison, the Pratt and Whitney F-119 on the F-22 which has an exceptionally high TWR for air-breathing engines has a TWR of a little more than 9. By that measure, stock engines are also extremely OP