What's an ideal TWR for spaceplanes?

[Gamer]

I'm getting into spaceplanes, (Yes i've neglected them for 500 hours) and i'm still learning, what's an ideal TWR for spaceplanes? (Maybe a screenshot of the spaceplane)

Thanks in advance.

[Gamer]

Nobody?

[Syd Khaos]

You get no answer because there is not one that will cover all planes.

Anything from 1.8 to 2.6 or so...depending on craft design, accent profile, and other such factors.

I typically shoot for 2 or 2.2 and then go from there if there is issues. However I only use SSTOs for small crew/refit missions in Kerbin orbit. If your building massive cargo SSTOs you will need your baseline TWR a bit higher than that to get off the ground.

[funkynutz]

Originally posted by SydKhaos:

You get no answer because there is not one that will cover all planes.

Anything from 1.8 to 2.6 or so...depending on craft design, accent profile, and other such factors.

I typically shoot for 2 or 2.2 and then go from there if there is issues. However I only use SSTOs for small crew/refit missions in Kerbin orbit. If your building massive cargo SSTOs you will need your baseline TWR a bit higher than that to get off the ground.

Seriously?!?!??! ARGH! Muthaf........

This would explain why most of my SSTO designs fail miserably lol. I've been treating them like rockets with regards to TWR, aiming for around 1.1 for a basic crew transport, and 1.5 for small payload transports... The ones that have worked I'd thought were massively overpowered and innefficient

[Azunai]

hell no. a TWR of 0.7 is fine. anything higher is really just wasteful (more engine mass than you need). i've gotten planes with a runway TWR of 0.3 to orbit successfully, but that's painful and doesn't really pay off.

[CalmLlama]

http://steamcommunity.com/sharedfiles/filedetails/?id=701858258

This plane has about a .9 TWR on takeoff, and i rotate to the ascent angle (about 55 degrees) once i get to 1.1 TWR.

[Rhedd]

Yep. Spaceplanes can have LOWER TWR than rockets, not higher. Yay wings!
I don't have any detailed advice, though, because they aren't really my thing.

[Gamer]

Thanks for the input! I would've put this sooner but I was sleeping, i'm building a cargo SSTO (preferably Mk3) that can just bring something rather small into orbit. Don't get me wrong I did make one Mk3 SSTO that did make it to orbit and deployed a small satallite, but every time I try to build something slightly bigger, it doesn't make it to orbit...
http://steamcommunity.com/sharedfiles/filedetails/?id=715652021

[Azunai]

why use a cockpit on a cargo plane? that cockpit is porbably heavier than the whole payload you're trying to launch, and it's really just dead weight since a simple probe core gets the job (control) done just as well (or better)

feel free to take a look at the KSP screenshots in my steam profile. i don't want to spam your thread with lots of images of my creations, but maybe you can learn a few things about viable SSTO designs.

if you don't make orbit, chances are that you don't have enough fuel on board (especially for the closed cycle "rocket" part of the ascent). or you don't use the airbreathing phase of the ascent properly. ideally, your plane should build up about 1600 m/s *before* you even toggle to the inefficient closed cycle mode. so you really only need about 1000 m/s (possibly even less if you're good at that stuff) worth of "rocket power" to turn that ascent into an orbit

frankly, mk3 parts have too much drag to use them for small cargo. you use the mk3 format because you can put relatively large items (2.5m parts) in the cargo bay. if you just want to deploy some satellite, you're better off NOT using an SSTO at all. getting a small satellite to orbit is much easier with a small, efficient rocket. if you really really want to use an SSTO for such a trivial task, consider using an mk2 design. small stuff (1.25m parts or the tiny 0.625m parts) fits nicely in the mk2 cargo bay and the plane is less "overkill"

use mk3 planes to get large space station modules or large quantities of fuel to orbit.

[Gamer]

Can't thank you guys enough!
http://steamcommunity.com/sharedfiles/filedetails/?id=715744421

And I somehow managed to land back at the KSC and nothing blew up!
http://steamcommunity.com/sharedfiles/filedetails/?id=715744588

[Rhedd]

Awesome! Landing at KSC is the mark of a true champion!

[andylaugel]

Looks like this one peaks at a TWR of 3.7. ;) But then, while easy to fly, this probably is not the best spaceplane.
http://steamcommunity.com/sharedfiles/filedetails/?id=716198894

[Rhedd]

Originally posted by andylaugel:

Looks like this one peaks at a TWR of 3.7. ;) But then, while easy to fly, this probably is not the best spaceplane.
http://steamcommunity.com/sharedfiles/filedetails/?id=716198894

Yeah. See all that fire? That's where most of that 3.7 TWR is going. It's literally being used to set your plane on fire, not push it into orbit.

Any time you see fire like that on ascent, keep in mind that your spending precious fuel to do nothing but make pretty lights. So, not exactly ideal.

[Azunai]

there's a big difference between the TWR at the runway and the TWR in flight. the usual design helper tools (MJ/KER) will only show you the "static" TWR of the engines.

as a genetal rule, you don't want to go too high with that, because a plane has wings and can take off with a low (<1 TWR) and it will build up a lot more thrust during flight.

a runway TWR of ~0.7 results in a peak TWR somewhere around ~2 when the plane gets to it's preferred speed (mach 3.7 for rapier engines) and will slowly fall off afterards as you accelerate further. at that point, you should be out of the dense part of the atmosphere, so the drag you need to overcome is also a lot lower.

in my experience, a plane can still accelerate nicely up to about ~22km altitude even though the displayed TWR at that point can be as low as 0.5 - simply because there isn't much of an atmopshere left that will slow you down.