SSTO running out of fuel really early..

[FlexT_ape]

So, my SSTO is running out of fuel early. This is my first SSTO so I don't know if I'm doing anything wrong. I've watched lots of videos about piloting SSTO's and I've followed all instructions but I run out at like 33.3 kilometers

SCREENSHOTS: https://imgur.com/a/0CYLa7x

[Gordon Fecyk]

Since you have Kerbal Engineer installed and handy, you can try this approach:

First, in the Spaceplane Hangar, turn the Rapiers to Closed Cycle mode and design the craft so it has about 2500 m/s (Kerbin orbital speed plus some room to manouvre) and a thrust-to-weight ratio of just above one. Leave enough room for liquid fuel only. Set KER to atmospheric mode and altitude about 15 km up or so. You want to be going faster than 1 km/s surface speed and at this altitude before going closed cycle.

Second, switch the Rapiers back to air-breathing mode, and design the air-breathing part to get to that altitude and speed on air-breathing mode only, so that when you get to over 1 km/s you can cut over to closed cycle and climb the rest of the way up. Add liquid fuel only tanks such that you don't use any liquid fuel from your rocket fuel tanks. Ideally you want to have an exact match of LF and OX when you cut over to closed cycle, or maybe have a slight excess of LF so you can fly around a bit after re-entry.

The idea is to fly up as high and fast as you can on air breathing mode, then you're already 1 km/s or even more speed-wise when you cut over to closed cycle. By having enough closed cycle delta-v at that altitude and speed, you actually only need to speed up by an additional 1300 m/s but you will be fighting atmospheric drag while doing that. By having a TWR slightly more than one, you can overcome that drag and just point prograde until you get to space.

Those Mk2 parts are very draggy when your angle of attack (angle away from Prograde) is more than a couple of degrees, so you're going to find yourself cruising at low altitude while you pick up speed, before you try to start climbing. 

I have designed SSTO heavy lifters with a TWR of only 0.5 or so, but those were designed for Ferram Aerospace's drag model and I relied a lot on hypersonic lift.

(Do you have a craft file we can inspect? Just in case there's things missed by just trying to reconstruct it from the images?)

Edited August 25, 2018 by Gordon Fecyk

[Gordon Fecyk]

Let's see if this improves things:

This craft should be a close match to yours in terms of cargo capacity and power, but it gets to low Kerbin orbit with delta-v to spare. The dV displayed in Kerbal Engineer is after unloading a 4 tonne craft from the Mk2 cargo bay.

The big changes:

• Much shorter body but with the same cargo room.
• Replace several Mk2 tanks with Mk1 tanks on the side, for less drag.
• Replace tail plane with Big-S wing strakes, to get more liquid fuel loaded and still have a tail plane.
• Remove 2-1 adapters and attached second pair of Rapiers to Mk1 tanks, for even less drag.

This design still needs work, as after unloading the cargo centre of mass drifts behind centre of lift, making re-entry rather interesting.

[adsii1970]

26 minutes ago, Gordon Fecyk said:

This design still needs work, as after unloading the cargo centre of mass drifts behind centre of lift, making re-entry rather interesting.

This can be easily remedied by adding the Big-Swing strakes to your craft empty. When you're about to do your deorbit burn, transfer excess fuel from your tanks to the empty strakes. It does help to move the CoM a little towards the front of the craft.

[Dman979]

Moved to Gameplay Questions.

[Laie]

14 hours ago, FlexT_ape said:

So, my SSTO is running out of fuel early. [...] SCREENSHOTS: https://imgur.com/a/0CYLa7x

Oh boy.... the vessel looks as if you've carefully read every advice about avoiding drag, and then deliberately did the opposite. Mk2 parts, quadcoupler, lots of radial stuff... the huge solar panel mounted sideways is an especially nice touch.

That said, with four rapiers you should still be able to brute-force your way to orbit. I suspect that real problem, in your case, is the ascent profile. Or, on second glance, maybe you're really not carrying enough fuel. The last picture shows you running out after four minutes of flight time, which *is* early.

For starters, I'd suggest you try it with a smaller and simpler vessel that can only make orbit and come back, without carrying any payload or fulfilling any kind of mission. This makes for short test cycles; any lessons learned can then easily be applied to bigger craft that are actually good for something.

[bewing]

I think the above posts have missed the main point. To get where you are with the fuel you have left -- you have to have gone the entire way in "closed cycle" mode.

Rapier engines have two modes, as mentioned above. "Closed cycle" mode turns your engines into rather poor quality rocket engines. "Airbreathing" mode turns your engines into jet engines. In airbreathing mode, your engines use only liquid fuel -- and use it moderately efficiently. You can switch between modes by clicking a button in the context menu, but the smarter way is by using an action group, because that will switch all four engines at the same time. The engines will also automatically switch from airbreathing to closed cycle mode at somewhere around 29km altitude, but that is probably later than you want for this design.

Laie's point about drag is also valid. To be truly functional, a spaceplane needs to be very carefully optimized for drag -- and your design needs some drag reduction work.

But back to the engines: what's the point of having the two modes? To get to space, you can't go the whole way with jet engines. They run out of air halfway up. So you need rocket engines too. Rapier engines give you both kinds of engines in one package. But that's how you have to use them. Put them in airbreathing mode to start with, as said above. Fly halfway to space and build up a lot of speed using your liquid fuel. Then switch to rocket mode, burn your oxidizer and most of the rest of your LF, and get the rest of the way to space.