Single-Turbojet SSTO Spaceplane

[juzeris]

Excellent heavyweight Falconer, inspired me to make another entry. Nowhere near as big as yours, but it has it's own charm, I'd think.

Thus presenting to you the Spinoctyl, type J. Clocks in at 25.1t wet on the runway, turbojet and twin nuke powered, has plenty of mileage provided you can pump that fuel around... Yes, we have no idea how much delta V it has precisely.

Also here's the photos from our demonstration flight, where Jeb took it to orbit, and then safely landed it.... well... somewhere. Is that even Kerbin?

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Engineer's note: I (personally) don't subscribe to the "precooler cools thing" belief, but it's an excellent intake which is completely sufficient to power one turbojet. And then you can mount a proper nose cone instead of having a blunt intake screaming through the air like a warhammer.

And to embed albums from imgur, just type imgur in brackets, then the 5 symbol code from the url, and then close the tag, so, for example, if I'm embeding http://imgur.com/a/8YhVP, then it goes like this:

[IMGUR]8YhVP[/imgUR]

Edited June 6, 2015 by juzeris

[NikkyD]

The precooler only has a very high value of heat dissipation, so it "makes heat go away" faster than other parts.

normal nacelle

emissiveConstant = 0.6

ram intake

emissiveConstant = 0.7

precooler & shock cone

emissiveConstant = 0.95

emissiveConstant is the emissivity factor in the black-body radiation equation (flux in W = Stefan-Boltzmann constant * emissivity * area * temperatureK^4). It most certainly cannot be >1 (1 is a perfect black body).

Edited June 6, 2015 by NikkyD

[juzeris]

Please note all the below is just my opinion based on my probably flawed understanding of KSP physics.

Yes, the precooler does have an emissive constant of 0.95, which is the same as wings, control surfaces and the shock cone intake, and only slightly higher than mk3 (0.87), mk2 (0.8) parts and engines (0.8, 0.83 for nukes). It's significantly higher than rocket parts (including mk1 LF tank), cockpits and sensitive equipment, but it's probably not gonna pull its weight on a rocket as a radiator, and the thermodynamics benefit is marginal on a spaceplane.

To put it into numbers, given a constant conductive energy flux into the part and the formula above, a precooler emits heat as if its temperature is (0.8/0.95)^(1/4)=~4.4% higher than an engine of the same temperature. Compared to the usual part threshold of 2000, that's 1915, which is almost completely red on the temperature gauge. The actual benefit will be even smaller due to the fact that conduction is not immediate and it will be at a lower temperature than the heating part.

I personally have exactly one plane which stabilizes in terms of temperature at above 1900 and it's engineered and flown to do specifically that, and scenarios where a part goes above 1900 but survives are rare in my life. It's either way too much heat or manageable heat. And these are the only times where it would make a difference. Also, for short term heating, a part with a high thermal mass (for example a full fuel tank) will probably absorb more heat which you can dissipate later than the precooler will radiate away due to the higher emissivity, though I don't have the numbers to back that up, just my intuition. Your mileage may vary.

[NikkyD]

The OLD story, lack of parts in stock KSP. If you build a very small, minimalistic plane, you dont have pure liquid fuel tanks. The standard fuselage tank holds 150 units, way too much. Using a FLT100 without oxi is inefficient. A nacelle now gives intake air and holds 40 units of liquid fuel for a weight that is just within acceptable limits... guess that is its use. If you play sandbox, the cost doesnt matter so i use the precooler as i need a light weight item that holds liquid fuel only.

I am sure there are ppl who manage to overheat certain engines and they might benefit from it. Esp. LVN requires liquid fuel now, so sticking and LVN behind a precooler may make sense even tho i doubt it.

On a personal note:

is it a new bug or feature that closing air intakes no longer changes their drag value ?! I don't understand how an Intake can generate so much drag anyway, its streamlining most of the air through to the engine so where does it pile up ?!

[Falconer]

Tried a quick experiment NikkyD. Took off he precoolers and replaced them with Mk1 fuselage LF tanks... and some other minor tweaks to fix stability pushing the mass up to around 48t.

The nukes got hotter and instead of running them at 100% power or off like on the original launches where they'd stabilize at about 80% overheat). I had to throttle them back to about 50% while making the final push up to orbital altitudes.

There may be a piloting dependence in there too... less time spent oscillating as you wrestle to keep the nose pointed in the right direction.. and more time thrusting and accelerating straight forward. Things seem to cool off while the craft is wobbling.

If someone has that mod which allows stupidly high launch platforms... might be interesting to bolt a nuke to the launch stability platform at various altitudes... then see how long it takes it to overheat and explode when bolted to the Mk1 liquid fuselage with the much lower heat dissipation figures than the Mk2/Mk3 fuselages. Or mounted directly to a precooler inlet. In space though the experiment would be fairly straightforward... attach nuke to a big fuel tank in orbit... start burning out of system at full throttle... see how long it takes to get fireworks based on which part it's attached to.

Another question though would also be the drag figure on the precooler... since it's an inlet it would have higher drag and also self-generate more heat at speed possibly eliminating it's benefits as a cooling system. Thought I saw someone mention that inlet drag may not change when closed in 1.0.

[Tarmenius]

is it a new bug or feature that closing air intakes no longer changes their drag value ?! I don't understand how an Intake can generate so much drag anyway, its streamlining most of the air through to the engine so where does it pile up ?!

From what I understand, it piles up in the engine itself. Or, it would in real life; I doubt it's actually modeled that way in KSP. Come to think of it, closing the intakes should probably generate more drag as you could imagine the air entering the intake only to hit a flat surface inside.

[Wanderfound]

From what I understand, it piles up in the engine itself. Or, it would in real life; I doubt it's actually modeled that way in KSP. Come to think of it, closing the intakes should probably generate more drag as you could imagine the air entering the intake only to hit a flat surface inside.

...except that the obvious way to close an intake is to push the shock cone or intake ramp all the way forwards. You end up with a fairly streamlined surface.

But, yeah, intakes are draggy because the air is slowed, pressurised and fed to the engine; it doesn't just breeze straight through.

[Rune]

Rune,

Wow! I'll have to try that! Thanks for the tip

Best,

-Slashy

Ok, now I think I have to thank you, if only for the push to keep trying. Yesterday I cracked the single engine RAPIER SSTO! And now I have a nice point of comparison, on the low TWR end of the table, and built by me so I know it inside and out. Turns out, the key was intake drag, I used a couple precoolers "in-line-but-looking-side-to-side" (attached through nodes so they wouldn't create drag, but offset heavily, basically) and went trough the Mach barrier without issues on a 15mT design. Subsequently, that meant a rocket takeover at about 1,300m/s and >500m/s on orbit, on a similar design as the ~18mT one that is here.

What I've learned? All in all, the RAPIER gets the higher payload fraction (same payload, 3mT lighter on the runway). BUT. The turbojet+LV-909 design gets the higher vacuum delta-v! The higher isp of the dedicated vacuum engines and the higher transonic thrust of the turbojet more than make up for the increase in powerplant weight, so you might get a fatter SSTO, but it will be a longer-ranged one, especially if you are planning a mission profile with refueling in mind.

So now I have a design rule: if it is meant to go to LKO, and no further, then RAPIERs are the way to go, with Turbojets added to save big designs that are just a tiny bit starved for thrust. However, for long-range stuff that isn't transported by other stuff (high kerbin orbits, Mun/Minmus flybys), then the turbojets will probably get a higher range when paired with the right in-space powerplant. And, you know, the all-liquid SSTO. I have one of those, obviously, but I still think I have a long way to go before I max out dV. I suspect a RAPIER hybrid design will get better results there on account of the nuke's low thrust (actually, I know, because people have posted such things).

Rune. The forums are a great learning tool!

[selfish_meme]

...except that the obvious way to close an intake is to push the shock cone or intake ramp all the way forwards. You end up with a fairly streamlined surface.

But, yeah, intakes are draggy because the air is slowed, pressurised and fed to the engine; it doesn't just breeze straight through.

I was watching my intake (shock cone) drag today at around 1200m/s and it was at 2.0, I closed the intake and it went to 0.3?

[georgTF]

That should be leftover code from previous versions of KSP that handled intake drag differently. There should be no effect to closing intakes in 1.0/1.0.2

I'm also keeping an eye on the thread hoping to see new solutions. Currently also trying to get this twin turbo/4 nuke into orbit. It's almost there, but in athmosphere flight and wing configurations have been problematic. It wont be easy to refuel it either, but it should have lots of liquid tanks and 240 thrust with minimum deadweight. It can't actually ascend with full tanks, but that's neither here nor there.

I did build a tourist ferry by redesigning the previously posted Double Trouble design and changing it to a twin turbojet. It can carry 5 tourists to a Minmus landing + Mun orbit after refuelling in LKO. Can also land on the Mun if refuelled on Minmus.

No longer single-turbojet craft admittedly, but spun off from ideas cropped up in this thread. Two jets are needed once the mass of the craft grows too large i'm affraid.

[NikkyD]

I was watching my intake (shock cone) drag today at around 1200m/s and it was at 2.0, I closed the intake and it went to 0.3?

That's what it once was. You have to turn on the aero values in ALT+F12 menu. There you can see that intakes now always have the same amount of drag... which is total crap