Space Fortress can't reach orbit, run out of ideas

[drtricky]

I'm building a flying fortress that's supposed to be able to defend LKO. https://kerbalx.com/drtricky/ABS-17-Nebula

As of current I can't achieve LKO despite several hours of tweaks and attempts. Given it has over 300 parts and requires a mod I'd understand if no one downloaded it. So I'm just here to ask: Can anyone give me any general advice/tips for large SSTOs like mine?

Edited March 8, 2020 by drtricky

[AHHans]

You either have not enough thrust or not enough fuel.

Considering that you have 28 RAPIERs I guess that you run out of fuel. How much vacuum dV do you have with a fully loaded craft?

You also don't need shock-cone intakes and pre-coolers. One shock-cone intake on its own can supply enough intake air for four RAPIERs. Replacing the pre-coolers with simple fuel tanks (Lf only or LfOx) gives you more fuel for the weight.

Edited March 8, 2020 by AHHans

[drtricky]

9 hours ago, AHHans said:

You either have not enough thrust or not enough fuel.

Considering that you have 28 RAPIERs I guess that you run out of fuel. How much vacuum dV do you have with a fully loaded craft?

If I did it correctly it says I have 1885 m/s dV in a vacuum around Kerbin. Another problem is that if I add more fuel tanks then it becomes too heavy to reach space.

Edited March 8, 2020 by drtricky

[Dirkidirk]

30 minutes ago, drtricky said:

If I did it correctly it says I have 1885 m/s dV in a vacuum around Kerbin. Another problem is that if I add more fuel tanks then it becomes too heavy to reach space.

Hmmmm... It looks like you have just too much extra stuff. I recommend that you increase the scale (more engines AND more fuel), or/and use areospikes (in the vacuum, to increase efficiency and delta-V), or just decrease the amount of weapons.

Edited March 8, 2020 by Dirkidirk

[AHHans]

1 hour ago, drtricky said:

If I did it correctly it says I have 1885 m/s dV in a vacuum around Kerbin.

Hmmm... that should be enough.

45 minutes ago, drtricky said:

Another problem is that if I add more fuel tanks then it becomes too heavy to reach space.

Why do you say that? Are you trying to fly that more like a plane or like a rocket?

What does your flight-profile look like? (Which altitude and speed do you aim for when flying the plane?) Is the craft struggling to gain altitude, to get through the sound barrier, to gain speed at supersonic speeds?

The trick with space-plane SSTOs is to get as much out of the air-breathing mode as possible. So fly it like a plane until you have at least 1000 m/s at 20 km altitude before you switch the RAPIERs to closed cycle. My guess is that this should work even with all the (probably high drag) stuff on the surface of the plane.

[bewing]

Engines are heavy, and you have too many. Also, you have no incidence on your wings. You have too many air intakes. You have no canards to provide efficient control. Struts have drag and mass -- you should use autostruts instead. Your nacelles should be mounted to your fuselage and not your wings.

[DeadJohn]

21 hours ago, drtricky said:

If I did it correctly it says I have 1885 m/s dV in a vacuum around Kerbin.

Is that in the VAB/SPH or during actual flight? During flight you might be burning through too much of your liquid fuel in air-breathing mode, leaving you with excess oxidizer and much lower effective vacuum DV.

How fast and high are you able to reach in air-breathing mode? When you toggle to vacuum mode, how much DV and TWR do you have left?

Your screenshot show 6201 m/s. Is that for air-breathing mode using *all* of your liquid fuel?

To get a better estimate of SSTO performance, try the following steps in VAB/SPH:

1. Check air-breathing DV and TWR
   1. Toggle off the vacuum tanks (using the green triangles to the right of the fuel bars) to reserve that fuel.
   2. Toggle your engines to air-breathing open mode
   3. Check sea-level DV and TWR
2. Check vacuum DV and TWR
   1. Toggle on your vacuum tanks. (With rapiers, you can usually leave the liquid-only tanks toggled on since that excess liquid won't be usable in vacuum anyway)
   2. Optionally drain part of your liquid fuel to reflect how much will be used in air-breathing mode. If you skip this you'll get a lower, more conservative vacuum estimate.
   3. Toggle your engines to vacuum closed mode
   4. Check vacuum DV and TWR