CargoSSTO Flat Lining at Low Altitudes

[zarakon]

My problem now is at about 32km. Even with the available jet engines (1-3 depending on throttle) + nukes firing I'm losing altitude. I can't turn the behemoth too vertical or it starts flipping, plus I don't think I have enough force with those engines to do a straight vertical push anyway.

Any Suggestions?

I don't have much experience with giant spaceplanes like this, but I think you should try to ascend slower. See if you can level off around 20-25km and build enough speed to go suborbital from there

[Wanderfound]

I don't have much experience with giant spaceplanes like this, but I think you should try to ascend slower. See if you can level off around 20-25km and build enough speed to go suborbital from there

Yah.

Fast climb to 20,000m, slow climb to 30,000m/Mach 4, then pull the nose up and light the rockets.

[dbmorpher]

Just as a suggestion also, you want to stage your engines in the SPH so you know where each of the CoT's are. If you plan on having different engines on at different heights you need to see where the CoT for different groups are. And also think of better rocket engines too.

[WDude4k]

CoT and CoM were still slightly off, but after shifting fuel around for awhile and making subtle adjustments to the engines, everything is lined up. I'm still having issues with getting enough speed and turning out into space.

Is it just a matter of thrust then? More engines!?

[Wanderfound]

CoT and CoM were still slightly off, but after shifting fuel around for awhile and making subtle adjustments to the engines, everything is lined up. I'm still having issues with getting enough speed and turning out into space.

Is it just a matter of thrust then? More engines!?

Not more, better. Eight turbojets are way more than you need. Swap some of 'em (use the four in the central bay if y'wanna maintain the aesthetics) for something that works in vacuum. As mentioned above, I'd suggest Aerospikes or RAPIERs.

[O-Doc]

You need to put a truckload more elevons on dat tail.

[MarvinKitFox]

What sort of horizontal speed do you achieve?

With 36 ram intakes, 8 turbojet engines, you should start to starve at about 34000. And a horizontal speed of 1850m/s

From there, orbit is just 500-600m/s further.

[WDude4k]

Getting up to 1200 m/s is probably the fastest I've got, without adding in rocket engines. Also I'm not sure how I feel about rapiers because they just eat fuel. I did try replacing a few jet engines with rapiers, but I don't think it did much for me.

Edit: Just from some testing, it takes 175x4 thrust at a 45 degree angle to keep my ship from diving back to earth at 20km (going about 600-650m/s). About 130x4 thrust at 45 degrees to keep me up at 25km (going about 750-800m/s). At 25km I'm lacking the air to keep the engines at the required thrust. That is 4 super sonic jets at about 75% thrust, maybe 70%. I can maintain 120 out of the 4 engines at that height, but it's just not quite enough.

Is it worth adding a few extra intakes that I toggle at high altitudes?

Edited July 31, 2014 by WDude4k

[Wanderfound]

Getting up to 1200 m/s is probably the fastest I've got, without adding in rocket engines. Also I'm not sure how I feel about rapiers because they just eat fuel. I did try replacing a few jet engines with rapiers, but I don't think it did much for me.

RAPIERs are almost as fuel efficient as turbojets while they're in air breathing mode. Don't let them switch to closed cycle until you want them to. Either turn off the auto mode switching (make an action group for it instead) or shut down the RAPIERs entirely after 25,000m and don't turn them back on until the turbojets start to die and you pull the nose up for the final burn.

The trick to maximising your horizontal speed is to keep to low climb rates and minimal angles of attack until it's time for that final burn (when you pull the nose up to 45 degrees and start burning oxidiser).

Get up into the low friction air over 20,000m and then level off to build speed before going higher. By the time you pass 30,000m your climb rate should be down to about 10m/s. Don't pull the nose up until you've extracted as much speed from the air-breathers as possible.

[Wanderfound]

Getting up to 1200 m/s is probably the fastest I've got, without adding in rocket engines. Also I'm not sure how I feel about rapiers because they just eat fuel. I did try replacing a few jet engines with rapiers, but I don't think it did much for me.

Edit: Just from some testing, it takes 175x4 thrust at a 45 degree angle to keep my ship from diving back to earth at 20km (going about 600-650m/s). About 130x4 thrust at 45 degrees to keep me up at 25km (going about 750-800m/s). At 25km I'm lacking the air to keep the engines at the required thrust. That is 4 super sonic jets at about 75% thrust, maybe 70%. I can maintain 120 out of the 4 engines at that height, but it's just not quite enough.

Is it worth adding a few extra intakes that I toggle at high altitudes?

If you need to rely on power to keep you up, it's because the plane is aerodynamically flawed. You want to reduce drag as much as possible. Less intakes, not more.

Fixing the aero and drag issues will allow you to reduce your angle of attack, which in turn will increase your speed and allow the intakes to function more efficiently. This in turn will allow you to get higher before you start using oxidiser.

[WDude4k]

Could the excess drag be caused by the fact my ship is built out of wing parts? Would that cause more drag than say if I built it out of 2x2 panels?

Edit: Nevermind about that ^

I decided to just drop the nukes and outer engines, and drain most of the fuel. This left me with the fuel directly in the center of the plane, and the 4 engines in the cargo hold. I was able to run up to 28km and get about 1200m/s.

Once confirming that the main body is sound, i tried adding back on the 4 nukes and about half of the fuel. Now I'm back to having trouble getting above 10km quickly. It will climb beyond that, but who wants to fly for 5 hours getting to 30km.

Edited August 1, 2014 by WDude4k

[Wanderfound]

If you can crack Mach 4 and 30,000m, you should be able to reach orbit.

[WDude4k]

Ugh. Maybe I'll stick to smaller SSTO's for awhile. I can do those just fine.

[Wanderfound]

Ugh. Maybe I'll stick to smaller SSTO's for awhile. I can do those just fine.

Have a play with this and see if it gives you any design inspiration: http://forum.kerbalspaceprogram.com/threads/88628-Kerbodyne-D7-Heavy-X5-a-heavy-lift-SSTO-spaceplane

[MarvinKitFox]

Getting up to 1200 m/s is probably the fastest I've got, without adding in rocket engines. Also I'm not sure how I feel about rapiers because they just eat fuel. I did try replacing a few jet engines with rapiers, but I don't think it did much for me.

Edit: Just from some testing, it takes 175x4 thrust at a 45 degree angle to keep my ship from diving back to earth at 20km (going about 600-650m/s). About 130x4 thrust at 45 degrees to keep me up at 25km (going about 750-800m/s). At 25km I'm lacking the air to keep the engines at the required thrust. That is 4 super sonic jets at about 75% thrust, maybe 70%. I can maintain 120 out of the 4 engines at that height, but it's just not quite enough.

Is it worth adding a few extra intakes that I toggle at high altitudes?

You want enough intakes to reach >1500m/s

Between 20km and 30km, you should be fighting to keep the nose *down*, as you speed up past red-hot. Angle of flight should be between 0 and 5 degrees noseup, only!

Climbing out through 30km, you should be >1200m/s, and still accelerating & climbing despite your engines throttling down.

At ~36km, you kill turbojets and go rocket.

and p.s.

I did advise 48 ram intakes. Try that.

For a heavy flyer with weak rocket, i need 6 intakes per turbojet to achieve enough atmospheric speed.

But most important is that you are supposed to be thrusting horisontally, NOT up, in the 20-30km range.

Ideally, you want to stay at 30-35km, until you become unable to sustain enough downlift to prevent ascending, *then* nose up and go rocket.

Edited August 1, 2014 by MarvinKitFox

[allmhuran]

I can't turn the behemoth too vertical or it starts flipping

Your jet fuel is mounted towards the front of the aircraft, moving the centre of mass forward. As the fuel is used up the CoM will shift backwards towards the CoL. If the CoM moves too close to the CoL (or worse yet, behind it) the plane will become extremely unstable, especially when the nose is not pointing along the velocity vector (ie, chevron and prograde marker are a long way apart on the navball).

[WDude4k]

Your jet fuel is mounted towards the front of the aircraft, moving the centre of mass forward. As the fuel is used up the CoM will shift backwards towards the CoL. If the CoM moves too close to the CoL (or worse yet, behind it) the plane will become extremely unstable, especially when the nose is not pointing along the velocity vector (ie, chevron and prograde marker are a long way apart on the navball).

Oh hey, I was watching that over 9000 video the other day. Was cool watching the tech tree get unlocked in 1 go with only essentially the 4 basic stock parts.

Aside from that I'm going to be working on a different design before I try and get an airplane into space. I figure If I can get some other big craft up without worrying about the ascetics of the c-17, then maybe I'll have some ideas about it.

Thanks for all the help guys.