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Fire-Dragon-DoL

Can't get this SSTO in orbit even if it reaches 2200 m/s at 35000 m!

Question

As the title says, I managed to reach 2200 m/s at 35000, but I can't really push it in orbit. I managed to reach (roughly) 40000m with the nuclear engine but at that point I start falling back on earth, with speed decreasing.

Please notice that I'm not using FAR and I have some experience (I played 300 hours, so I'm not noob but far from expert considering the ~2000 hours people played this game).

The issue is obviously that LV-N doesn't have enough thrust to push me outside og Kerbal gravity. I would love to hear a few suggestions.

Also, I would like not to disintegrate the design, it's the first time I managed to get a nicely-designed SSTO with a lot of delta V (6200), I'm aiming for a trip to laythe, land, come back to kerbin.

Any good suggestion? I need to push this guy in LKO!

2014-12-27_01-12-40.jpg2014-12-27_01-17-17.jpg

Delta V with turbojets:

2014-12-27_01-19-57.jpg

Delta V with LV-N:

2014-12-27_01-21-22.jpg

Edited by Fire-Dragon-DoL

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First issue:

The staging was backwards and it launched using the LV-N.

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At ~38 000m I reached ~1900 m/s (surface speed), but I can't go any higher (LV-N can't increase my vertical speed) and Turbojets don't work anymore (no air, so I shut them down and close intakes). Can't get into orbit from there in any way, I even tried with a 30° angle from 20000 at max speed to get some vertical speed, but I really can't go higher than that. LV-N don't bring me out of atmosphere. My next bet is adding more intakes and trying to get out through jets.

Hang on a sec before you do that...

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Hang on a sec before you do that...

You've definitely got an asymmetric thrust deal going on here.

Let me see if I can fix it...

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Okay, that's what your problem was; asymmetric flameout due to o2 starvation. (how on Kerbin did you manage to fly that thing so fast with that going on??)

Please check out my tutorial here

http://forum.kerbalspaceprogram.com/threads/102182-So-you-want-to-build-a-space-plane

Look at the sections regarding multiengine spaceplanes and launch profile.

I'll get you the craft file back shortly.

Best,

-Slashy

Edited by GoSlash27

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FireDragon_zps6c8878cb.jpg

http://wikisend.com/download/731806/FireDragon.craft

You'll want to reset the action groups the way you want them, but it's good to go otherwise.

You'll probably want to get rid of those giant swept wings as vertical stabilizers. Also, you don't really need active rudders.

Best,

-Slashy

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http://i52.photobucket.com/albums/g13/GoSlash27/FireDragon_zps6c8878cb.jpg

http://wikisend.com/download/731806/FireDragon.craft

You'll want to reset the action groups the way you want them, but it's good to go otherwise.

You'll probably want to get rid of those giant swept wings as vertical stabilizers. Also, you don't really need active rudders.

Best,

-Slashy

Can you give me a brief overview of whatever you changed (if anything?)? I need to understand if I'm lacking pilot skills or engineering skills (or both). I tried comparing the two craft files with a diff tool, but whatever I found was missing mechjeb part and very minor coordinate changes on various parts, but probably was mostly an error related to floating point numbers, so those coordinates probably "didn't change" at all.

By the way, thanks a lot for helping me so far ;)

I'm reading the tutorial right now.

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Fire Dragon,

No problem!

All I had to do to fix it was remove the intakes and engines, then reinstall them without symmetry enabled as I show in the tutorial.

First I placed the left intakes, then the left turbojet, then the right intakes, and finally the right turbojet.

I removed the references to MechJeb (had to to open it) and set up an action group on button 1 to toggle the intakes and turbojets.

You probably want to apply this fix to your original model in order to keep MJ and your action groups how you want them.

Good luck!,

-Slashy

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Fire Dragon,

No problem!

All I had to do to fix it was remove the intakes and engines, then reinstall them without symmetry enabled as I show in the tutorial.

First I placed the left intakes, then the left turbojet, then the right intakes, and finally the right turbojet.

I removed the references to MechJeb (had to to open it) and set up an action group on button 1 to toggle the intakes and turbojets.

You probably want to apply this fix to your original model in order to keep MJ and your action groups how you want them.

Good luck!,

-Slashy

One question, how do you manage to align intakes without mirror?

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One question, how do you manage to align intakes without mirror?

By being very, very careful. Also, you can use markers like straight lines on the textures to help

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If you end up needing just a bit more thrust to push yourself into space, the Mk55's may be a good thing to have, since Squad buffed them up in the last update.

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By being very, very careful. Also, you can use markers like straight lines on the textures to help

^ What arma said.

Placing ram intakes and engines is easy (they only attach in one spot). Radial intakes... it's just a matter of lining them up according to the local landmarks.

And they don't have to be perfect. A little misalignment there won't throw you noticeably out of balance.

You just don't want to use symmetry when placing engines or intakes on a multiengine plane or it'll wreck everything*.

So what I do is this:

While setting up the airplane and getting everything tweaked and balanced, I'll just slap it together using symmetry. Once I've got everything where I want it, I will yank the engines and intakes and place them individually.

*Except where you have a single subassembly that contains an engine and all of it's intakes. You can place that using symmetry without suffering starvation thrust asymmetry effects.

Best,

-Slashy

Edited by GoSlash27

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^ What arma said.

Placing ram intakes and engines is easy (they only attach in one spot). Radial intakes... it's just a matter of lining them up according to the local landmarks.

And they don't have to be perfect. A little misalignment there won't throw you noticeably out of balance.

You just don't want to use symmetry when placing engines or intakes on a multiengine plane or it'll wreck everything*.

So what I do is this:

While setting up the airplane and getting everything tweaked and balanced, I'll just slap it together using symmetry. Once I've got everything where I want it, I will yank the engines and intakes and place them individually.

*Except where you have a single subassembly that contains an engine and all of it's intakes. You can place that using symmetry without suffering starvation thrust asymmetry effects.

Best,

-Slashy

This footnote is a great suggestion.

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I'm trying to follow everything, even your tutorial, but I wonder how you even keep 100 m/s vertical speed at ~30 000, turbojet doesn't push enough because air intake becomes quite low, so I had to lower throttle.

I tried looking for tutorials on youtube, but none of them uses 2 turbojets + 1 LV-N, damn them (and with anything not LV-N it's too easy).

I think reached this point I should aim top... I wonder how, considering as soon as I start climbing up, my jet engines will start loosing thrust and the LV-N can't bring me up outside atmosphere.

2014-12-29_00-23-49.jpg

Edited by Fire-Dragon-DoL

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I'm trying to follow everything, even your tutorial, but I wonder how you even keep 100 m/s vertical speed at ~30 000, turbojet doesn't push enough because air intake becomes quite low, so I had to lower throttle.

I tried looking for tutorials on youtube, but none of them uses 2 turbojets + 1 LV-N, damn them (and with anything not LV-N it's too easy).

I think reached this point I should aim top... I wonder how, considering as soon as I start climbing up, my jet engines will start loosing thrust and the LV-N can't bring me up outside atmosphere.

According to that pic, you're right in the 25-32km "wall". The objective in that portion of the flight is to maintain 20-22* AoA and let speed build on it's own. The faster you go, the more air will be shoved into your intakes.

Eventually, you will begin to climb at a more rapid pace. Just gotta be patient...

Remember: 22* pitch and let the speed build. Also, you don't have to start throttling back until the engines are pretty dim.

If you'd like, I can post pics of what it should look like with your plane.

Best,

-Slashy

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^ And again, those big swept wings as vertical stabilizers are holding you back. You don't need rudders or giant tailplanes. They're just unnecessary weight and drag.

Just a couple little fins back there are fine to keep you pointed the right way.

Best,

-Slashy

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You might just need a few more intakes (lol), but it's certainly possible to do with turbojets (easier for small planes though). Ideally you should be able to keep going past 40km, but probably not for long.

As for Laythe, I'm not sure exactly how much deltaV you need (it's been way too long since I've went there lol) but going by this you need at least 2k for the transfer from Kerbin to jool, plus maybe 10-40m/s to adjust your trajectory to be a Laythe encounter. For the return trip, you need ~2.8k for escape velocity from Laythe (a little less just to get back to Jool's SoI) and then roughly another 1.7k for the transfer from there to Kerbin. So as a rough estimate 6.5km/s?

Looks like this guy was right.

I added intakes while keeping the same plane I had. Weight increased by 0.4, but turbojets worked up to 42~ Km instead of ~32, so I managed to reach MUCH higher speeds (2200 orbital), then I turn on LV-N and the Kerbin influence is much lower, so it manages to push me up and forward, bringing me out of 50 000 where it's void, so I shutdown turbojets and just go up into orbit, nice one, I also have 4800 delta v to spare!

How to mark the topic as answered?

2014-12-29_01-20-34.jpg

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According to that pic, you're right in the 25-32km "wall". The objective in that portion of the flight is to maintain 20-22* AoA and let speed build on it's own. The faster you go, the more air will be shoved into your intakes.

Eventually, you will begin to climb at a more rapid pace. Just gotta be patient...

Remember: 22* pitch and let the speed build. Also, you don't have to start throttling back until the engines are pretty dim.

If you'd like, I can post pics of what it should look like with your plane.

Best,

-Slashy

I'll retry with your technique, but I must admit that adding some intakes actually made the entire manouver much easier (though, I feel a bit like cheating, there is no oxygen at 40 000m <.< )

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How to mark the topic as answered?

There's literally a sticky at the top of the forum that tells you exactly how to do that. Glad to hear adding intakes worked though :D

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No problem. Glad you're on the right track!

To change the thread to "answered", just edit your OP in advanced mode. It'll give you the option to change it.

Best,

-Slashy

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Well, it required some optimization and also slash taught me some very important stuff about wings and how to fly SSTO (must admit, I was adding a lot of intakes so it was much easier, never noticed it was so hard to fly them up outside atmosphere).

You should definitely make a youtube guide to add to your topic-tutorial Slash ;)

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Ive made 10 (5 of them variants of one design) laythe and back capable SSTOs, and from my experience 2200m/s is plenty for getting to 70km on jets.

What is your ascent profile? Most of my craft use a maximize velocity and punch out approach. I dont have a clue what the technical term for this is, but i use rapiers, so 2200m/s is approximately the hard limit all my craft can do. My standard issue approach to getting to orbit is to find my maximum height i can maintain maximum thrust at. Then the basic profile is blast up to that altitude as fast as i can (if TWR is high enough go straight vertical, if its low use the more traditional way to increase altitude). Then you sit at that altitude neither increasing or decreasing altitude but increasing speed to around 1900 surface (2050 orbit approx prograde). Then once i attain this velocity, i aim at 30degrees above prograde, and gun it until my AP is at 50-60km, then lower the angle to 10 degrees and just blast jets until i can get into orbit. Most of my craft use a hair less then 1 RAM intake/ton, so the optimal height to start maximizing the velocity is at around 29-32km, and most of my craft are capable of attaining from 70km to 100km AP on pure rapiers. After this even 1kn of thrust pushing 10t is enough to throw the craft into a stable orbit before it passes the point of no-return.

your specific craft may need some tweaking, your primary issues is most likely a lack of air intakes. Unless there is something i cant see in the picture, 4 measly cones for 40 TONS! is not gonna let you do anything suborbital. I strongly encourage at a bare minimum 1 RAM intake for 4 tons of craft at a bare minimum. That means that for 40 tons, at a bare minimum 10 intakes. Nukes are fine to enable above 10km, but a single nuke pushing 40 tons, will only start to really increase ur altitude/speed above 45km. if you cannot get above 40km, preferably 50km before switching to nukes, you either need to have more powerful rockets to enable at a lower altitude (rapiers are nice for this), or multiple nukes, although the latter is more counterproductive in general as nukes are extremeley heavy, and will cut your dV by quite a bit.

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