B9 SSTO Help

[Taki117]

So, I'm having a bit of trouble getting a spaceplane into orbit. I have plenty of LF, just not enough Oxidizer, I can get it up to about 30km on Jets, but my Apoapsis isn't much higher (Maybe 32km) Is this normal? Also any advice would be greatly appreciated.

Mods used: B9, FAR, Kerbal Joint Reinforcement.

Tech Level: Tier 7 Completed, Nothing in Tier 8 unlocked.

Craft:

Craft File: https://docs.google.com/file/d/0BzOU_F85Qj0YNXc0Y3FLS0Nzd1E/edit

Launch Profile:

1. Take off and ascend at 35 deg.

2. At about 30km switch to rockets, pitch to 45 deg.

3. Raise Apopapsis to about 70km.

4. Circularize (As much as fuel will alow)

Edited March 1, 2014 by Taki117

[Armchair Rocket Scientist]

So, I'm having a bit of trouble getting a spaceplane into orbit. I have plenty of LF, just not enough Oxidizer, I can get it up to about 30km on Jets, but my Apoapsis isn't much higher (Maybe 32km) Is this normal? Also any advice would be greatly appreciated.

Mods used: B9, FAR, Kerbal Joint Reinforcement.

Tech Level: Tier 7 Completed, Nothing in Tier 8 unlocked.

Craft:

http://i.imgur.com/p8VupdH.png

http://i.imgur.com/4mQkJtK.png

http://i.imgur.com/q3OiZVt.png

Craft File: https://docs.google.com/file/d/0BzOU_F85Qj0YNXc0Y3FLS0Nzd1E/edit

Launch Profile:

1. Take off and ascend at 35 deg.

2. At about 30km switch to rockets, pitch to 45 deg.

3. Raise Apopapsis to about 70km.

4. Circularize (As much as fuel will alow)

All right, I can see several possible issues.

1. The B9 turbojets have a higher air-fuel ratio than stock, and from what I've heard they also have a worse thrust-velocity curve. I don't know how using FAR affects this, but it's possible that's making it harder to reach high speeds on jets.

2. 35 degrees is a pretty steep ascent profile. In general, you want to flatten out your trajectory at high altitudes and build up as much horizontal speed as possible on the jets. How fast are you going when you switch to rockets?

3. You appear to be using the big white radial engines. These are garbage. Change them for 2 LV-T45s, or if you can control your vehicle fine without thrust vectoring LV-T30s or aerospikes. You'll be getting a 50-70s boost in ISP (at 30 km engines are pretty much at vacuum isp) for at most an extra 0.3 tons.

[Taki117]

All right, I can see several possible issues.

1. The B9 turbojets have a higher air-fuel ratio than stock, and from what I've heard they also have a worse thrust-velocity curve. I don't know how using FAR affects this, but it's possible that's making it harder to reach high speeds on jets.

Possible, but I'm trying to do this without stock. This is more of a proof of concept. I figure if I can do it with this then I can do it with a much more limited selection of parts.

2. 35 degrees is a pretty steep ascent profile. In general, you want to flatten out your trajectory at high altitudes and build up as much horizontal speed as possible on the jets. How fast are you going when you switch to rockets?

Generally I'm at about 1.5-1.8 km/s at an altitude of about 30km. Define high-altitude? I start losing lift due to thinning atmosphere about 10km, (Hence the steep ascent profile, to try and get my apoapsis as high as possible as fast as possible) If I pitch to less than about 20 deg I start loosing altitude.

3. You appear to be using the big white radial engines. These are garbage. Change them for 2 LV-T45s, or if you can control your vehicle fine without thrust vectoring LV-T30s or aerospikes. You'll be getting a 50-70s boost in ISP (at 30 km engines are pretty much at vacuum isp) for at most an extra 0.3 tons.

I'll try that and see if it helps.

Replaced with an aerospike, got up to 1.2 km/s at about 22km before I had to activate the Aerospike final Apoapsis 60km.

Edited February 28, 2014 by Taki117
Update

[How2FoldSoup]

Your flight profile is pretty important with sstos. When I fly mine I try to gain altitude up to about ~15-20km asap and then level out to gain horizontal velocity. When your engines begin to flame out is when you want to switch to rocket power. I agree with Armchair Rocket Scientist about the use of rocket engines too. If you're using b9 you could also stick a sabre M on the back too. You shouldn't be losing too much altitude from having a somewhat low AoA. You might also need more lift. If you're not opposed to it and want to build sstos more often I suggest downloading the procedural wings mod. It's by far one of my favorite mods because it simplifies so many things and lets you customize your wings how you want them.

Oh I just double checked your ssto and found a little of your problem. Your jets are all on the top of your wings causing your CoT to be above your CoM. This will push your nose down no matter what you do. You should put 1 pair of engines on top of the wing and 1 on bottom, or whatever makes you happy as long as the CoT is going through your CoM

[Hodo]

Ok, several minor problems with your craft and flight profile.

1- your CoT is slightly higher than the CoM which is pushing the nose down under max thrust.

2- you have 1 intake per jet engine, which is limiting your overall altitude. I suggest adding another set of intakes somewhere on the aircraft.

3- lower your climb profile. I usually climb at 35-45deg till 12-15km alt then bring it down to 10-15deg to gain as much speed as I can till 25-30km altitude. Those D30s will die off around mach 3-3.5 which means you are losing most of your thrust at that point. You will need another 1400m/s roughly to get to orbital velocity.

4-The rockets you are using while cool, are horribly inefficient. I would suggest switching over to a pair of the standard boosters.

My goal when I fly my SSTOs is to gain as much speed as I can before I have to kill the jet engines. In FAR that is around mach 3-4 if I am using jets, around mach 5-6 if I am using SABREs or RAPIERs.

Other than that it is a decent looking first SSTO.

I suggest checking out a few of the craft in my thread of SSTOs that I have made.

http://forum.kerbalspaceprogram.com/threads/56945-Hodo-Creations-SSTO-craft-designs-and-files-for-those-who-want-them

Unfortunately I won't be making anymore anytime soon as I am going for a more realism setup, which means I need to relearn some skills but give me a few weeks and I will be back at it with SSTOs for a RSS sized Kerbin and using Real Fuels with the AYJ Jet engine plugin that sets the jets to be more realistic. So far I am happy I made an aircraft that goes Mach 2.

[Taki117]

Little update. It seems no matter what I've tried I can't get passed an Apoapsis of about 60km. What should my TWR be for the rocket stage of the ascent? (Because that seems to be where the issue is)

[WhiteKnuckle]

Little update. It seems no matter what I've tried I can't get passed an Apoapsis of about 60km. What should my TWR be for the rocket stage of the ascent? (Because that seems to be where the issue is)

I don't think you're getting enough speed out of your jet stage, you should pretty much just be using rockets to circularise your orbit.

Try leveling off at 20km and just tacking on speed, once you get to 2200-2400m/s your apoapsis should be well outside the atmo and you can just coast up.

[Hodo]

Little update. It seems no matter what I've tried I can't get passed an Apoapsis of about 60km. What should my TWR be for the rocket stage of the ascent? (Because that seems to be where the issue is)

I like my rocket stage to be TWR of at least .8-1.5:1 ratio. The higher the ratio the less amount of time I will spend in that stage.

[Taki117]

Sigh, Troubles still abound.

Current Iteration:

I have a couple more ideas on how to change it, but I still can't get it into space.

[Hodo]

Ok you are hauling WAY to much liquid fuel with you. Change out those tanks with LFO tanks so you can take as much oxidizer as you can. And you are still going WAY to slow before you change over to your rocket engines. You need to be going at least mach 4 before you change over to your rocket. You can get there below 25km alt pretty easy with those engines, but they drop off real fast after 3.5. So I would look at changing them out with the stock turbojet engines.

[LordOfTheOrbits]

You are taking way too much liquid fuel and way too less oxidizer with you. Try replacing most of the liquid fuel tanks with LFO tanks. I usually have 4 times as many LFO tanks as pure liquid fuel tanks in my SSTOs.

[Taki117]

I would like to thank everyone who helped me get into space with my first operation SSTO!

Still needs a name (And a few other bits, like RCS ports, and I'm going to have to rework the solar panels) but I did it thanks to the wonderful community here!

[Th_]

Congrats on your first SSTO! I had to try about 5 different iterations before I got one into a stable orbit. For a name for a first of anything, name it after something you would remember, like a favorite author's last name or something of the sort. Just so ya remember it

[Hodo]

One last suggestion and this will help you out with piloting your future SSTOs.

Change the information your KER shows you. A lot of the information you have on there now is pointless.

Look at a few of the pictures in this album of one of my SSTO space fighter planes for the information I use for piloting.

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