SSTO questions

[MathmoRichard]

Hi, I feel completely new to spaceplanes. After 6 months on the game I never even tried to do anything useful. Today I decided to try and finally make the leap. I read the tutorial on the first page of the tutorials section, and got into orbit on my 5th attempt (+ Mun flyby just), but I have a load of mechanics type questions now.

First, this is my success/failure:

No matter what I do, those RAM intakes keep overheating, I was lucky not to lose them all. Is this bad flying or design or what? Maybe I can solve the ascent but the decent is hopeless, then burn up every time. How were they not made for this?

The second question is: how am I supposed to know how much air the intakes will give? The engine stats say 5.354 units of air per second for each engine. But then the RAM intakes say 1.0, what does this mean? It can't be 1 unit per second or all the tutorials would be wrong because we'd need 5 per engine (!!!!). Also in flight, when I right click an intake, it says 0 units on the green bar, but the engines are still functioning (in fact thrust is increasing). Confusion is rife. This is the main question of the thread, I'm the kind of person who can probably just get it if actually given the numbers.

Next problem was actually designing the thing to have 4 intakes, the above is so ugly, and the radial intakes aren't good enough to just have 2 RAMs.

Then it's the flight plan, I've read how to do it and it seems hit and miss when I try, sometimes I can't get up to speed, sometimes I burn up. Most frustratingly the RAPIERs like to switch to rocket power at 15000 or so, which is earlier than the tutorial says, and sometimes it goes at just 13000 - I know I can do it manually, but surely this means that the engines had lost most of their power quite early because I'm guessing that's how they choose when to switch.

Angle of attack is a big puzzle.

As I said before, the thing I just can't avoid is overheating, how to I do reentry???

Finally, the general structure, how much wing do I need? That is one of my biggest problems, I find it impossible to judge if I've got enough lift. Thrust you can judge by speed etc. I don't know how to work that out.

And what proportion of LF to LFO should I use? One tutorial said half the entire weight of the craft should be LFO!?

My theory so far is to have the CoL 10% behind the CoM with fuel roughly evenly dispersed and ideally the cargo and variable mass parts should go as close as possible to the centres. This good? I've just be guessing as best I can.

Thanks in advance.

Edited May 29, 2015 by MathmoRichard

[Red Iron Crown]

Use airbrakes to ease the reentry process, it makes it much easier to slow quickly enough that you don't overheat. You can also pitch well above prograde in a "cobra" reentry, the additional drag will slow you quicker and the intakes won't take all the brunt.

The "1.0" in the intake description refers to the intake area, it's not the air produced but instead a factor in it (the other factors being speed, angle of attack and air density). One intake per engine is plenty in my experience, if you want more you can use the inline intakes that hold fuel, like the precooler.

For wing area, with a Mk2 fuselage you need very little as the body lifts as well, your design looks to have about the right amount.

For LF:O ratio, I find having about 20% more units of LF than O to cover the airbreathing portion of the ascent to orbit works well. I think you could eliminate one of the large fuselage LF tanks and still make orbit, or replace it with an LFO tank if you want to go further in space once you get there.

Your CoM/CoL plan is sound. Try emptying all the tanks in the VAB and see how that affects the CoM to make sure the relationship doesn't change drastically (or use a mod that shows the dry CoM automatically, RCS Build Aid).

[Mister Dilsby]

Everything Red says is right on. I've found airbrakes to be the SSTO savior. The other thing you can do in the VAB is set your elevons to deploy when you hit the brakes (add their 'deploy' action to the Brake control group). That will make them work like flaps and slow you down even more. Not recommended for any surfaces that yaw though

Next make sure your entry is shallow enough. I start my burn over the mountains about 70-80 degrees west from KSC (can't remember exactly, there is a spot on the ground I reference) and just burn until my periapsis=0. That drops me relatively near KSC and most importantly makes me enter at a shallow enough angle that the upper atmosphere has time to slow me down before the middle atmosphere starts overheating the ship.

Good luck!

[cybersol]

I've had good success with precoolers right before the ram intakes. They are basically intakes with a little LF, but they also have an emissive constant of 0.95, which is similar to other radiators like solar panels. Ironically, they work much better behind the intakes than before the engines.

Intake area is the only thing you really need to know about intakes. Turbojets seem to be happy with 0.01 per engine, but to milk rapiers all the way I use more like 0.016 per rapier. Adding extra intakes is definitely awkward, especially since you don't want to add to much drag from radial attachments. Bicouplers, tailcone/intake combos, and precoolers seem to be the best I've found so far.

You should control the rapiers manually, as you can get 500-800 more m/s speed out of them that way. Switch to closed cycle mode right as your speed stops getting faster.

For reentry, cobra (belly first) is typically all I need now. I don't even have to use airbrakes, but I do recommend them if you are having trouble.

Basically in 1.0.2 stock you need almost no wings (1 lift/5-7 tons), they are just for looks and only slightly for landing:

You definitely have plenty of lift if you can take off by the end of the runway.

LF versus LFO I always tune at the very end. In the craft above they ended up almost equal (numerically, not in their normal .81/1 ratio), other times you need 0-10% more LF depending mostly on drag.

[MathmoRichard]

Thanks everyone for your replies!

Use airbrakes to ease the reentry process, it makes it much easier to slow quickly enough that you don't overheat. You can also pitch well above prograde in a "cobra" reentry, the additional drag will slow you quicker and the intakes won't take all the brunt.

The "1.0" in the intake description refers to the intake area, it's not the air produced but instead a factor in it (the other factors being speed, angle of attack and air density). One intake per engine is plenty in my experience, if you want more you can use the inline intakes that hold fuel, like the precooler.

For wing area, with a Mk2 fuselage you need very little as the body lifts as well, your design looks to have about the right amount.

For LF:O ratio, I find having about 20% more units of LF than O to cover the airbreathing portion of the ascent to orbit works well. I think you could eliminate one of the large fuselage LF tanks and still make orbit, or replace it with an LFO tank if you want to go further in space once you get there.

Your CoM/CoL plan is sound. Try emptying all the tanks in the VAB and see how that affects the CoM to make sure the relationship doesn't change drastically (or use a mod that shows the dry CoM automatically, RCS Build Aid).

Ahhh so for intake air/sec it would be ÃÂAvcos(θ) and according to http://wiki.kerbalspaceprogram.com/wiki/Atmosphere à= 1.223e^(-h/H) where h is the altitude, but what is the scale height H??

Do you think my plane is too big for just taking up a docking port and cargo bay then? Or I could put more in.

Everything Red says is right on. I've found airbrakes to be the SSTO savior. The other thing you can do in the VAB is set your elevons to deploy when you hit the brakes (add their 'deploy' action to the Brake control group). That will make them work like flaps and slow you down even more. Not recommended for any surfaces that yaw though

Next make sure your entry is shallow enough. I start my burn over the mountains about 70-80 degrees west from KSC (can't remember exactly, there is a spot on the ground I reference) and just burn until my periapsis=0. That drops me relatively near KSC and most importantly makes me enter at a shallow enough angle that the upper atmosphere has time to slow me down before the middle atmosphere starts overheating the ship.

Good luck!

I just tried aerobrake and it did help, although I don't really need them if I have flaps! Didn't know about that, that's brilliant.

I also had problems taking off on the run way, I tried pointing the craft up (by sinking the back wheels), but it didn't help much.

I've had good success with precoolers right before the ram intakes. They are basically intakes with a little LF, but they also have an emissive constant of 0.95, which is similar to other radiators like solar panels. Ironically, they work much better behind the intakes than before the engines.

Intake area is the only thing you really need to know about intakes. Turbojets seem to be happy with 0.01 per engine, but to milk rapiers all the way I use more like 0.016 per rapier. Adding extra intakes is definitely awkward, especially since you don't want to add to much drag from radial attachments. Bicouplers, tailcone/intake combos, and precoolers seem to be the best I've found so far.

You should control the rapiers manually, as you can get 500-800 more m/s speed out of them that way. Switch to closed cycle mode right as your speed stops getting faster.

For reentry, cobra (belly first) is typically all I need now. I don't even have to use airbrakes, but I do recommend them if you are having trouble.

Basically in 1.0.2 stock you need almost no wings (1 lift/5-7 tons), they are just for looks and only slightly for landing:

http://i.imgur.com/h0d0OEN.jpg

You definitely have plenty of lift if you can take off by the end of the runway.

LF versus LFO I always tune at the very end. In the craft above they ended up almost equal (numerically, not in their normal .81/1 ratio), other times you need 0-10% more LF depending mostly on drag.

Proceeders are the single most difficult to understand part for me. What do they do exactly? I would have thought they take existing air from other intakes and cool it or something :S So are you saying they are just another intake with some fuel for some reason which will help with heat issues?

I struggle coming in belly first because my CoL is so far back, it wants to come in forwards.

Ironically, they work much better behind the intakes than before the engines.

Why? that makes no sense if they are just part heat sinks, maybe they really do cool the air?

I'll control the RAPIERs manually and report back soon.

Thanks again

[Red Iron Crown]

Ahhh so for intake air/sec it would be ÃÂAvcos(θ) and according to http://wiki.kerbalspaceprogram.com/wiki/Atmosphere à= 1.223e^(-h/H) where h is the altitude, but what is the scale height H??

Do you think my plane is too big for just taking up a docking port and cargo bay then? Or I could put more in.

I believe that wiki page is a bit outdated now, air density is calculated in a more complex way since 1.0 involving temperature and altitude. I don't know the details of that calculation, sorry.

As for plane size, is there anything in the payload bay? I test mine with full ore containers as they're dense and easy to balance. I suspect you're carrying more fuel than is strictly necessary, for comparison have a look at this plane I made for ferrying small payloads to LKO (some cheaterpants part clipping used to make the intakes a bit more attractive looking).

Proceeders are the single most difficult to understand part for me. What do they do exactly? I would have thought they take existing air from other intakes and cool it or something :S So are you saying they are just another intake with some fuel for some reason which will help with heat issues?

I struggle coming in belly first because my CoL is so far back, it wants to come in forwards.

Why? that makes no sense if they are just part heat sinks, maybe they really do cool the air?

I'll control the RAPIERs manually and report back soon.

Thanks again

As far as I can tell the precooler is just an inline intake + tank, it doesn't have any special properties for managing heat (or if it does they're not very noticeable).

[cybersol]

Precoolers are like engine nacelles (0.005 intake area and 40 liquid fuel), but they have "emissiveConstant = 0.95" instead of "emissiveConstant = 0.6". That is the same emissive constant as other good radiator parts like wings and solar panels.

Reading why the heat model works the way it does is like reading tea leaves. However, I think the precoolers are best near the intakes because they are getting small overall of amounts of heat but the intakes still tend to explode, so the nearby heat emission helps them not explode so much. Meanwhile, the engines generate much more heat internally and seem to be better when in closer contact with a larger fuel tank to act as a thermal sink.

It wanting to come in forward but you leaning into the wind is what makes the cobra entry bleed off speed.

Edited May 30, 2015 by cybersol

[MathmoRichard]

So I would love to give more updates, but this happens so much I can't do anything!!

It just falls apart whenever I put it on the runway now

[cybersol]

Yeah, the Mk3 joints are made out of silly putty. Struts are always more of an art than science, but there seem to be a few weak points on every large plane. The back landing gears need to transmit the load through to the main body of the plane. Plus the cargo needs to be secure to both ends of the cargo bay. My strut plan on planes is currently:

1) Stitch together the outer edges of the wings if using the component wings

2) Struts to transfer load from the back gears to the body

3) Struts to secure the cargo

4) A couple of 2x symmetric struts may still be needed in key places to hold the body together. Only put them there if they are needed though, because they will add stress to other joints.

Hammer Orange SSTO Album (with an ascent guide)