SSTO Space Planes - 23km and 1400m/s .... now what?

[mellojoe]

When I try to make larger SSTO planes, I hit a wall right around 23,000m and about 1400 m/s. Often I can get near 1600 m/s, but thats about it. I run out of air, switch to rockets, and then use all my delta-V just getting into orbit.

Do I just need to air-hog more? What do I need to be doing?

Here's an example craft:

It was on its way back down, but you get the idea.

All those intakes on top? About 12 or 14 of them clipped together to try to air-hog as much as possible.

((edit))

Here's a video of my current project and my current flight plan:

Here's my flight path:

• Mostly straight up until about 15,000m
  
• Level off and fly straight trying to max out speeds
  
• Slowly let plane climb of own accord
  
• 23,000meters and around 1400 m/s engines start getting air starved and slowly throttle back
  
• When speeds stops increasing, aim upward to gain altitude
  
• When engines finally cut out, switch to rockets

Am I missing something?

((edit))

Craft File Here:

https://www.dropbox.com/s/tqw80lrq2ukoltc/Mk2%20%20Plane%20v4.craft

It uses PorkJet's Mk2 Spaceplane Plus parts, which can be found here:

http://forum.kerbalspaceprogram.com/threads/57662-PorkWorks-dev-thread-Inflatable-habs-and-more%21-Current-project-MK2-stock-expansion

And it uses the combination RCS block thrusters from B9, which can be found here:

https://www.dropbox.com/s/ztdlwtlxy7inhgx/Control_RCS_Blocks.zip

And instead of sticking on a thousand air-intakes, I modified the scramjet air intake to be the equivilent of about 15 ram air intakes. This was done just for aesthetics. These parts were too pretty to part clip a million ram-air intakes.

https://www.dropbox.com/s/0wntxv6i9rm66r7/part.cfg

Edited April 25, 2014 by mellojoe

[Jouni]

Add more intakes or more turbojets. Either one helps, so it's mostly a matter of personal preference.

[Sirine]

All of them can fly to space...try study them.

http://forum.kerbalspaceprogram.com/threads/71015-BSC-Aeris-4a-AND-THE-WINNER-IS

[cantab]

Are you using the circular intakes? I've read that they can actually be counterproductive, costing more in drag than you gain in air. Stick to the rams and maybe the radials.

And to save fuel on the rockets, close the intakes when you fire them, that reduces drag.

[mellojoe]

Updated first post with a video.

[Sirine]

http://forum.kerbalspaceprogram.com/threads/25111-FRAK-1-From-Zokesia-Skunkworks

[Jouni]

The problem seems to be that you throttle down too early, and the remaining turbojets can't push the plane through the thick lower atmosphere fast enough. With six turbojets in a 40-tonne plane, even one ram intake per turbojet is enough to reach 32-33 km without throttling down. You just have to keep the climb rate below 50 m/s after around 25 km. Moderate reentry effects between roughly 20 km and 30 km are a good indicator that you're not climbing too fast for that intake ratio.

I don't really have any experience on how planes with more intakes behave, but at least they shouldn't be any worse.

[FREEFALL1984]

I like to go for a centrally located turbojet as well as the others and have the rockets on the outside of it, once I hit 1400m/s at 22km I disable all but the central turbojet so you dont get screwed when one of them flames out, then I activate my rockets, the boost in speed and lower intake demand lets the central turbojet run until about 30km which helps the rockets get you through the thick part of the atmosphere, also be ready to disable the intakes as soon as you flame out, Also once I hit this alt and speed I tend to lift my nose up to 45 degrees with my rockets firing, this usually lets you get apoapsis to about 60km before the final turbo jet cuts out. meaning you'll have about half a tank for each rocket to pop the apo above 75km and circularize

[KerbMav]

Is it just stock +B9? I would like the craft file for studying.

[Kasuha]

Air drag is still too high at 23 km, so what you need is to get higher and then faster.

Three RAM intakes per active engine are good for comfortable SSTO, it can then cruise at 35 km and achieve almost orbital speed.

And you need about one engine per 10-15 tons of total mass.

Personally I recommend finding flight path where you won't starve your jets of air and run them all as long as possible. Then switch them off (or leave just one running) and start rocket engines.

[KerikBalm]

"When I try to make larger SSTO planes, I hit a wall right around 23,000m and about 1400 m/s"

Moar Fuel? That is plenty high and fast enough to make it into orbit with a rather large payload.

I don't part clip (and as a result, I don't really air hog either, since it's hard to find non part clipping places to put intakes) or use mods (well, not many mods), I get about the numbers you get.

All stock, no part clipping, less than 2 ram air intakes per jet, 2 radial intakes per jet:

It uses the new liquid fuel boosters (I was trying to keep part count down), hauled a ~53-54 ton payload into orbit, with 45 tons of rocket fuel left over (so it used less than 3 orange tanks to hauul about 100 tons to orbit, I should be able to increase the payload by 45 tons, and lead 45 tons of rocket fuel less into the SSTO/put empty tanks into the payload and top them off with the 45 remaining tons in the SSTO)

This earlier one I made pre-23.5 is a bit more air hoggy (and a lot higher part count), with a smaller size payload bay

Its nearly stock, the intakes were modded to have intakes for both "IntakeAir" and IntakeAtmosphere" - but the intake atmosphere intakes were closed, its a resource for an electric fan engine that was not on the craft (i've since split those into two modules, because its annoying to have to go close all the atmo intakes to reduce drag)

So, I think if you just carry extra rocket fuel, your performance on jets is fine.

But I'd add a few things: you have your outer jet pair running last, you should make it the inner pair so if a flameout happens, there is less torque

It seems a waste to run rockets at low throttle because you have to keep your jets at low throttle - although this is hard to avoid with jets... maybe you can be running your outer jets, and set the thrust limter on the inner jets, then activate the inner jets and shut down the outer jets, so you can run your rockets at higher throttle without flaming out your jets.

I also find LV-Ns to suck for getting your apoapsis out into space, their TWR is too low. I'm somewhat skeptical even on the aerospikes (390 vs 370 isn't a big change), maybe have just 1 centrally mounted where your two aerospikes are, and then where the LV-Ns were, add LV-T30s?

Fire the LV-T30s to get your apoapsis up, then just use the LV-N to counteract drag and circularize?

I don't use LV-Ns at all in the atmosphere, but thats more for RP reasons

If it gets to orbit, why do you care how much fuel it has left? what is it used for? crew transfer? or do you want this to be a Single stage to Duna?

I don't get the obsession with SSTOs with high deltaV's, all I care about now is the payload for a SSTO

I've abandoned my single stage to anywhere but orbit designs in favor of orbital rendevous. If I want "fully reusable to Duna", I have a SSTO lifter that launches the interplanetary ship to orbit, and hten lands back at Kerbin.

The interplanetary stage drops a lander to Duna, which is SSTO on duna, refuels the lander when it comes back up, and leaves it at duna, returns to kerbin orbit for refueling by another SSTO.

[mellojoe]

Updated first post with .CRAFT file.

[arq]

23km and 1400m/s is plenty to get to space. What rockets are you using? Don't use aerospikes, their too heavy and even at 15km you're basically at vacuum ISP, so the low-altitude efficiency is wasted. LV-T30s work great. If you are carrying any LV-N, use them too (but LV-N's alone won't be enough to get you to orbit unless you airhog pretty hard).

When you fire the rockets, pitch up to 45deg until you get apo above 50km, then you can moderate that a bit.

[mellojoe]

The problem seems to be that you throttle down too early, and the remaining turbojets can't push the plane through the thick lower atmosphere fast enough. With six turbojets in a 40-tonne plane, even one ram intake per turbojet is enough to reach 32-33 km without throttling down. You just have to keep the climb rate below 50 m/s after around 25 km. Moderate reentry effects between roughly 20 km and 30 km are a good indicator that you're not climbing too fast for that intake ratio.

At 5:45 in the video, just before I throttle down, you can see the plane lurch sideways. That is my cue that one engine is flaming out. I have to throttle down at this point. Is there a way to prevent these flameouts?

At that point in the video I'm at 25,000m, 1500 m/s, and air intake at 0.11.

[mellojoe]

I like to go for a centrally located turbojet as well as the others and have the rockets on the outside of it, once I hit 1400m/s at 22km I disable all but the central turbojet so you dont get screwed when one of them flames out,

At about 7:05, you can see my technique for babysitting twin engines for flameout prevention.

If you Alt-Right Click both engines, you can watch the thrust each is providing. One will start to lose thrust relative to the other. This is my cue to back off the throttle to prevent flameout. A couple SAS units to correct any slight torque. Once you identify which engine flames out, it will always be the same engine everytime that flames out first.

[Claw]

@mellojoe

FYI: The last engine you place is the first one to flame out. If placing via symmetry, the symmetric mirrored engine is considered last.

[How2FoldSoup]

@mellojoe

FYI: The last engine you place is the first one to flame out. If placing via symmetry, the symmetric mirrored engine is considered last.

Just lurking on this thread but this is very useful information. Thank you so much Claw!

I don't have any answers for op I don't play with stock aerodynamics sorry :S

[mellojoe]

@mellojoe

FYI: The last engine you place is the first one to flame out. If placing via symmetry, the symmetric mirrored engine is considered last.

GOOD TIP! I knew about the 'last engine placed' trick. I'm usually trying to build things with odd numbers of engines and ensure I place the middle one last. But I didn't know about teh mirrored engine being the one to go. THANKS!

[Kasuha]

As part of the Affordable Space Program challenge I was flying the monster below which has 10 jet engines. I found that I can prevent any stability problems using correct ascent profile. I had no uneven thrust problems at all (might have been diminished by total number of engines) and when engines started flaming out, they already had so low thrust that the ship's SAS had no problem to handle it. But if I tried to ascend too fast, flameouts would put me to spin and completely out of control.

Note that I was at full throttle all the time till engines started flaming out.

[federally]

"When I try to make larger SSTO planes, I hit a wall right around 23,000m and about 1400 m/s"

Moar Fuel? That is plenty high and fast enough to make it into orbit with a rather large payload.

I don't part clip (and as a result, I don't really air hog either, since it's hard to find non part clipping places to put intakes) or use mods (well, not many mods), I get about the numbers you get.

All stock, no part clipping, less than 2 ram air intakes per jet, 2 radial intakes per jet:

https://scontent-b.xx.fbcdn.net/hphotos-ash4/t31.0-8/p600x600/664358_10102621081633223_3244008254824963639_o.jpg

https://fbcdn-sphotos-b-a.akamaihd.net/hphotos-ak-prn1/t31.0-8/p600x600/10255485_10102621081638213_163568888516558254_o.jpg

It uses the new liquid fuel boosters (I was trying to keep part count down), hauled a ~53-54 ton payload into orbit, with 45 tons of rocket fuel left over (so it used less than 3 orange tanks to hauul about 100 tons to orbit, I should be able to increase the payload by 45 tons, and lead 45 tons of rocket fuel less into the SSTO/put empty tanks into the payload and top them off with the 45 remaining tons in the SSTO)

This earlier one I made pre-23.5 is a bit more air hoggy (and a lot higher part count), with a smaller size payload bay

https://scontent-b.xx.fbcdn.net/hphotos-frc3/t31.0-8/p600x600/1781709_10102522555220943_38363739_o.jpg

https://scontent-b.xx.fbcdn.net/hphotos-frc1/t31.0-8/p600x600/1540475_10102522555076233_1164310068_o.jpg

https://scontent-b.xx.fbcdn.net/hphotos-ash3/t31.0-8/p600x600/1961831_10102522555101183_621013543_o.jpg

Its nearly stock, the intakes were modded to have intakes for both "IntakeAir" and IntakeAtmosphere" - but the intake atmosphere intakes were closed, its a resource for an electric fan engine that was not on the craft (i've since split those into two modules, because its annoying to have to go close all the atmo intakes to reduce drag)

So, I think if you just carry extra rocket fuel, your performance on jets is fine.

But I'd add a few things: you have your outer jet pair running last, you should make it the inner pair so if a flameout happens, there is less torque

It seems a waste to run rockets at low throttle because you have to keep your jets at low throttle - although this is hard to avoid with jets... maybe you can be running your outer jets, and set the thrust limter on the inner jets, then activate the inner jets and shut down the outer jets, so you can run your rockets at higher throttle without flaming out your jets.

I also find LV-Ns to suck for getting your apoapsis out into space, their TWR is too low. I'm somewhat skeptical even on the aerospikes (390 vs 370 isn't a big change), maybe have just 1 centrally mounted where your two aerospikes are, and then where the LV-Ns were, add LV-T30s?

Fire the LV-T30s to get your apoapsis up, then just use the LV-N to counteract drag and circularize?

I don't use LV-Ns at all in the atmosphere, but thats more for RP reasons

If it gets to orbit, why do you care how much fuel it has left? what is it used for? crew transfer? or do you want this to be a Single stage to Duna?

I don't get the obsession with SSTOs with high deltaV's, all I care about now is the payload for a SSTO

I've abandoned my single stage to anywhere but orbit designs in favor of orbital rendevous. If I want "fully reusable to Duna", I have a SSTO lifter that launches the interplanetary ship to orbit, and hten lands back at Kerbin.

The interplanetary stage drops a lander to Duna, which is SSTO on duna, refuels the lander when it comes back up, and leaves it at duna, returns to kerbin orbit for refueling by another SSTO.

Holy crap you need Procedural Wings. It would make that thing look so awesome if it didn't have to be such a patch work

[Monger]

When you can't get higher than 23 km, you have a problem with thrust. Because the atmosphere gets thinner and thinner, your lift from your wings is not helping you anymore, but still adds weight. At this point, bigger wings start to become a problem rather than a solution. What you need is thrust, easy as that. If you have to throttle down your engines early on, then you have to hog more air.

Air breathing engines are perfectly capable of getting to 2000m/s at 35km height. A this point, you really need only a little nudge to get into orbit.

[mellojoe]

It must be my ascent profile then. Can you look at the vid I posted to see whatnI'm doing wrong?

[Claw]

It must be my ascent profile then. Can you look at the vid I posted to see whatnI'm doing wrong?

Well, I usually avoid space plane threads with mod parts since I run stock, but I think you're using stock aero. I watched your video and there were a couple things I noted. (Your craft looks pretty sweet by the way. )

1,400 m/s at 23km isn't bad, considering terminal velocity at that altitude is less than 1,000 m/s. 40t jets are a challenge to get to orbit on TurboJets only.

You're leveling off a bit early for the acceleration build up. I usually wait till between 20km and 25km because the air really thins out and the thrust curve is reaching its maximum. You also want to aim for sort of a smooth transition where you don't level out completely, but not climbing too fast. More of a parabolic arc up to altitude. I find for heavier aircraft it's a bit better to have an upward prograde as you're nearing 30km+ than to be going faster with a level prograde. This makes pushing the AP higher with rockets a bit easier, so you're not experiencing a lot of drag thrusting rockets level at 28km. Also, when you have to pitch way up like that for the rockets, the air intakes become much less effective.

My typical airspeed gates:

10km, 200 m/s

15km, 350 m/s

20km, 650-700 m/s

24km, 1000 m/s

From here, you have to play it out a bit based on weight, but usually...

30km, 1600 m/s

32km, 1800 m/s

Also, with modified intakes you have and the weight of your plane, I think you're better off NOT shutting down motors as you climb. If you watch them closely, you're better off throttling all of them down together and keeping all 6 on. With this build you'll probably have more overall thrust that way but you'll have to pay more attention to flameouts. When building, place your inboard TurboJets last so they flame out first (and cause less flat spins). Also, turning on your NERVAS a little early with this isn't so painful because of the reduced throttle. But the airspeed boost from the rockets can help keep your intake air up a bit longer.

Anyway, that's my 2 cents. Everyone has their own techniques, so I'm sure someone will disagree.

[O-Doc]

Your ascent up to 23km is reasonably solid. Instead of turning off your first couple of turbos, stay at that altitude and maximise your speed. Once your surface acceleration drops below 0.5m/s/s fire up your nukes to push faster and higher at full throttle. You are running heavy so your TWR needs extra oomph to keep that intake air level high. The good news is, you're setup on nukes so you won't burn a huge amount of fuel when pushing up to 35km. This is basically what I do on high dV SSTOs.

Also, at 35km you should be in level flight and gaining altitude from speed(because you are falling off the side of Kerbin). Only angle up if you don't have speed, which you should have plenty of.

You've got yourself a solid design there. I'm guessing you'll have well over 3km/s dV@LKO, even more once you ditch all those extra RCS tanks and the aerospikes.

[Redshift OTF]

Referring to OP, you want about 4 air intakes per jet engine to have a smooth ride into orbit. 6 makes it a joy.