Optimal TWR for Hybrid SSTO

[kinnison]

I have been fiddling around with stock SSTOs. using the MK2 parts I have built a high Delta-V (.3 TWR 4k NERVA in orbit) Cargo plane around 35 tons. for me I find it too difficult to ascend below a TWR of 3. I seems to need more "oomf" to get past the drag issues. and can consistently with relative ease get close to an orbital speed of 2k at 30000meters before switching to rockets.

But i have heard people going as lot as 1.5 or 2 TWR

is a TWR of 3 too much?

Edited December 5, 2014 by kinnison

[Taki117]

For aircraft you should be OK, but for rockets you don't want to exceed terminal velocity, at that point dV losses to air resistance are equal to those of gravity, which results in a net smaller dV loss overall.

[Pecan]

For SSTO rockets - and especially with FAR - a launch TWR of 1.2 seems best. TWR of 3 would be more than you need in almost any circumstances. For a plane that can do 2km/s+ at 30km don't bother to start flattening-out below 20km and do most of your horizontal acceleration at 25km-28km.

[capi3101]

I have been fiddling around with stock SSTOs. using the MK2 parts I have built a high Delta-V (.3 TWR 4k NERVA in orbit) Cargo plane around 35 tons. for me I find it too difficult to ascend below a TWR of 3. I seems to need more "oomf" to get past the drag issues. and can consistently with relative ease get close to an orbital speed of 2k at 30000meters before switching to rockets.

But i have heard people going as lot as 1.5 or 2 TWR

is a TWR of 3 too much?

Plane SSTO, eh? TWR=3.0 does seem a tad excessive in my experience unless you're trying to go for a speed record. It might be you've got less an issue of thrust and more an issue of lift. The less lift you've got, the more thrust you need to compensate for it in general. The general rule is ~2.3 tonnes per Delta Wing / Wing Connector A/B, which works out to 1.15 tonnes per unit of lift; a 1:1 ratio is easier to remember. So for your 35 tonne plane, you want a total of 35 units of lift, roughly the amount provided by 18 (i.e 9 pairs) of Delta Wings/Wing Connectors or the equivalent.

Combine THAT with a TWR of 3 and you'll wind up with a plane that'll climb like a bat out of hell...in fact, it'd probably climb too fast (as in climb at 45 degrees to 10k and then flatten out to 5 degrees, and expect to still go upwards at 100+ m/s). Especially if you stick all your high drag parts (i.e. intakes) up at the front instead of amidships and aft. I had a design like that the other day; thing went into a flat spin and the engines still had sufficient thrust to operate. A few up at the front are okay, just don't over do it.

[Jimbimbibble]

You can manage a TWR less than 1 actually. Very few real planes have TWR larger than 1. You just can't expect to fly it like a rocket. Climbing at 20-30 degrees to 10km then flattening to 10 degrees is more reasonable.

[kinnison]

since i am using stock i tend to have problems in the lift department. So i overcompensate with more engines.

thanks for the feedback

[numerobis]

TWR of 0.5 on liftoff is doable; TWR 1 is easy mode. *But* this is the actual TWR; remember that the turbojet gets only 50% thrust at rest. Is your TWR of 3 based on the nameplate thrust, or on the actual thrust?

With TWR below 0.5, you need so much lift that it gets hard to land again.

[GoSlash27]

kinnison,

Running stock, the rules of thumb are pretty well established.

You want 1 turbojet for every 15 tonnes of aircraft. If you have this and you're having difficulties, it's either because

1) You're not using optimal parts somewhere else

2) You're not using the correct number of parts somewhere else,

or

3) You're not flying an acceptable profile.

There's lots of info out there on how to build efficient and effective stock SSTOs and how to fly them for maximum effectiveness.

Just give a holler if you want some guidance!

Best,

-Slashy

[Pecan]

TWR of 0.5 on liftoff is doable; TWR 1 is easy mode. *But* this is the actual TWR; remember that the turbojet gets only 50% thrust at rest. Is your TWR of 3 based on the nameplate thrust, or on the actual thrust?...

Adding to that -

If you have TWR >1 you can do a vertical launch! With TWR 3 you should more or less be able to do whatever you want and 'lift' from wings is irrelevant.

Remember - wings and 'lift' are only useful for landing. If you 'plane requires them for takeoff you almost certainly don't have enough thrust for speed at altitude. If you have them for 'pretty', just ditch their mass and get important dV. Really - they only count on the way down.

[kinnison]

down to 25 tons. The lifting force is well behind CoG. and i get "stuck" at about 30k when the Twin Turbojets start flaming out. best i have gotten is around 1700m/s orbital speed before I cannot overcome atmospheric drag. I just don't see how I can get more with less. Unless i don't use NERVAs and instead use aerospikes or something else to push be into LKO

[kinnison]

TWR of 0.5 on liftoff is doable; TWR 1 is easy mode. *But* this is the actual TWR; remember that the turbojet gets only 50% thrust at rest. Is your TWR of 3 based on the nameplate thrust, or on the actual thrust?

With TWR below 0.5, you need so much lift that it gets hard to land again.

Using Mechjeb and that is my initial TWR

[GoSlash27]

down to 25 tons. The lifting force is well behind CoG. and i get "stuck" at about 30k when the Twin Turbojets start flaming out. best i have gotten is around 1700m/s orbital speed before I cannot overcome atmospheric drag. I just don't see how I can get more with less. Unless i don't use NERVAs and instead use aerospikes or something else to push be into LKO

25 tonnes is reasonable for 2 turbojets. 35 was not. You don't want your lift too far behind the CoM or you won't be able to generate enough torque to keep the nose up at high speed.

What kind/ how many intakes are you using to feed those engines? How many/ what kind of wings?

You should not be flaming out at 30K at over 1,700 m/sec unless you're not properly pairing your intakes to engines.

Best,

-Slashy

Edited December 6, 2014 by GoSlash27

[numerobis]

MOAR INTAKES.

A couple extra radials will do you wonders.

[kinnison]

6 Rams intakes, i don't think the intakes are the issue.

my ascent profile must suck. I usually level out around 20k and just try to keep the craft accelerating.

[Wanderfound]

TWR of 3 is massively excessive. The whole point of wings is that they permit <1 TWR. But you have to fly it like a spaceplane, not like a rocket.

[GoSlash27]

6 Rams intakes, i don't think the intakes are the issue.

my ascent profile must suck. I usually level out around 20k and just try to keep the craft accelerating.

Yeah, that's way more intakes than you need.

And you've got enough engine to do the job. No mention of the wings, but I'm thinking your problem is your ascent profile. How fast are you going when you hit 20KM altitude?

[GoSlash27]

An example of a typical spaceplane launch profile:

http://s52.photobucket.com/user/GoSlash27/slideshow/KSP/SP%20profile

[capi3101]

Launch profile definitely seems like the issue. Just to verify, can we take a look at some of the designs you've got so far? Pics or craft files, doesn't matter which.

[Pecan]

Launch profile definitely seems like the issue. Just to verify, can we take a look at some of the designs you've got so far? Pics or craft files, doesn't matter which.

This = please post the .craft file so we can all play with it.

I'm not a spaceplane fan but Wanderfound definitely is and others are top-tech flight-engineers. As you've seen from all of us; TWR 3 is much more than you need for anything, so we can't see what the problem is. Nevertheless, as numerobis suggested (and he's numbers man, the clue's in the name!) the figure reported by MJ is not accurate for 'at rest' jets. Even so you should have a TWR at least 1.5 and should be able to do a vertical launch without problems).

[capi3101]

This = please post the .craft file so we can all play with it.

Yeah, pretty much...

[maltesh]

down to 25 tons. The lifting force is well behind CoG. and i get "stuck" at about 30k when the Twin Turbojets start flaming out. best i have gotten is around 1700m/s orbital speed before I cannot overcome atmospheric drag. I just don't see how I can get more with less. Unless i don't use NERVAs and instead use aerospikes or something else to push be into LKO

Offhand question: Although the jet in question is stock...do you have B9 Aerospace installed? It includes a modulemanager config that nerfs the stock jets.

[numerobis]

as numerobis suggested (and he's numbers man, the clue's in the name!) the figure reported by MJ is not accurate for 'at rest' jets.

In the SPH/VAB, I don't remember; I think it's wrong there. But once out of the SPH/VAB, it's mostly accurate (that was the last patch I sent in to MechJeb).