Two tenths of a ton...

[RocketBlam]

So, one of my personal KSP quests has been to build a single-engine SSTO to visit Laythe. I've built two and four engine Laythe landers, but they're so heavy, they need a big, expensive transfer stage. As just a check to make sure it can do it, I create one that can SSTO on Kerbin, then I launch a transfer stage later, and a refueler.

I first designed this in Sandbox, and made sure it worked, and it did. Then I transferred it over to my career mode, made a few minor changes, and tried to launch it. But while I could get it into orbit in Sandbox, I just couldn't manage it in Career, despite several tries.

First I thought it must be that Jed was a 5-star pilot in Sandbox, but only a 2-star in my career, so the better piloting must have been modeled somehow in fuel usage, but no, it doesn't seem that it is. When I transferred this (slightly modified!) craft back to Sandbox, it no longer could make it there either.

In frustration, I checked the two versions, and found that the new one was a mere .3 tons heavier than the old one. That's how close this was, for a 9.5 ton vehicle. I got rid of a couple of intakes, and a reaction wheel, and a bit of fuel, getting rid of .2 tons, and launched again, and lo and behold, it made it into orbit - barely. 14 m/s of fuel left.

That's all it finally took... two tenths of a ton. Two percent of the launch weight.

[TimePeriod]

Spacecraft exchange.

or

Mission Reports.

Edited March 28, 2016 by TimePeriod

[RocketBlam]

It's not really either, I'm not sharing the craft, and the mission isn't done. It's more of just an observation.

[Nich]

Considering 2% of launch weight is 6% or more of payload depending on payload fraction this make sense

[bewing]

I think if you switch to the micro landing leg you will still be able to land just fine, and you will save more weight.

If you switch to a 3-wing design (with two in an upward V and the 3rd pointing straight down, and moving the juno to that 3rd wing) -- that might save you a fair bit more, while still being flyable. But you'd lose some lift, so that's iffy, I admit.

But you could get back your fuel and your reaction wheel.

Edited March 28, 2016 by bewing

[Brainlord Mesomorph]

I would point out that the horizontal stabilizers/fins  are probably causing more drag than you need. If they were half that size it might save some fuel.

Edited March 28, 2016 by Brainlord Mesomorph

[bewing]

Well, I'm not sure it could fly (would be stable) in Kerbin's atmosphere as an SSTO with less canard. -- If I built it, I'd probably put about 4 times as much.

[Renegrade]

Damn, 14m/s left in orbit is reaaaaallly cutting it close.  I like to have at least 100 :S

[RocketBlam]

10 hours ago, bewing said:

I think if you switch to the micro landing leg you will still be able to land just fine, and you will save more weight.

If you switch to a 3-wing design (with two in an upward V and the 3rd pointing straight down, and moving the juno to that 3rd wing) -- that might save you a fair bit more, while still being flyable. But you'd lose some lift, so that's iffy, I admit.

I don't think I'd trust the micro legs. Remember it lands fully fueled, so it's pretty heavy. Even with these legs I like to use some jet thrust to soften the landing.

The three-wing design is certainly worth looking into.

[bewing]

Heh. Those micro legs are stronger than they look.  My tourist bus is 9 tons completely dry, and I land it on 3 micro legs (w/ 2 radial parachutes, suspension locked) at about 11 m/s.

[RocketBlam]

They're strong enough I guess, but they compress a lot under weight. Also, at higher landing speeds, the vehicles tend to bounce, and on a tall vehicle like this, bouncing usually puts your craft on its side.

[bewing]

Yeah, that's why I lock the suspension, and turn on SAS after the parachutes are open. There are always tradeoffs.