Havin a hard time here with SSTO Rocket

[Xyphos]

I'm trying to come up with an SSTO rocket with a 225 ton payload that would be able to land on minmus, refuel and return (or go interplanetary)
initial design was an SSTO spaceplane but the payload required too many air-breathing engines to make it a viable solution
so I opted for a rocket instead. but it seems that no matter how many fuel tanks and engines I add, I just can't get enough Delta-V to get the thing into orbit.
any advise?

[Corona688]

"SSTO" and "rocket" are not a match made in heaven.

"SSTO" and "225 ton payload" are not a match made in heaven either.

"SSTO", "225 ton payload", and "rocket" all together will be a very kerbal kind of rocket.

[Xyphos]

well that explains it then.

[Alpha 360]

Use rapier engines by the dozen and hope for the best

[Xyphos]

10 minutes ago, Alpha 360 said:

Use rapier engines by the dozen and hope for the best

only if it were so simple.
more rapier engines = more fuel consumed = more fuel required = more weight = ineffective results.
it won't even get to sub-orbit, it'll just hang at 350 m/s and refuse to climb past 6000 meters.
even if you dive to sea-level, and manage to pitch back up, it'll lose all that velocity to the angular drag.

Edited July 3, 2016 by Xyphos

[Green Baron]

Way back in .90 or so i tried that as well and gave up. Even with proper staging and asparagus it might be cheaper to split the flight in two (should funds matter). Though i don't know where the break-even between single and multiple launch is.

Also upsizing a rocket doesn't necessarily increase dV, i've noticed :-)

 

oh, ninja'd

Edit: why won't you split that thing in several functional parts, like lander, isru-unit and a drive-stage for the interplanetary shots ? On your way back and out there you don't have to accelerate the whole behemoth, just the lander with kerbals and science.

Edited July 3, 2016 by Green Baron

[Van Disaster]

You probably could if you built a bunch of Rapiers around a big rocket engine, but really 225t *payload* is madness for one launch if you need SSTO. I'm wondering what on earth you're trying to launch, 225t for a lander+miner seems excessive. Either way, building it in orbit seems more sensible.

[Bill Phil]

The biggest fuel tanks have the worst mass ratios.

[Tex_NL]

9 hours ago, Corona688 said:

"SSTO" and "rocket" are not a match made in heaven.

"SSTO" and "225 ton payload" are not a match made in heaven either.

"SSTO", "225 ton payload", and "rocket" all together will be a very kerbal kind of rocket.

SSTO and rocket is easy. Very easy in fact. They ARE a match made in heaven.
SSTO and 225 ton is just a matter of scaling it up.
SSTO, 225 ton and rocket is starting to get Kerbal but still very much within the realm of possibility.

Edited July 3, 2016 by Tex_NL

[Norcalplanner]

To get a 225 ton payload into orbit with an SSTO rocket, it'll need to be huge. In my experience, a single Mammoth SSTO can lift about 36 tons to LKO, so you're looking at 8 of them for the payload you're talking about. Since I last did this a few versions ago, I have no idea what reentry heating would be like these days with an unshielded rocket. Another thing to consider would be to go with a recoverable core and disposable SRBs - adding some cheap thrust at liftoff will greatly reduce your gravity losses and allow the rest of the lifter to be smaller and cheaper.

[Vaporized Steel]

reaction to this thread...

W..H..A..T..T..T.........H,.u..H!? S e R i O u S l Y!?!?

Talking about seriously, I am sure it is possible.

Mind the part count though and the very low FPS on that monstrosity, it will definitely not be economical, but a accomplishment nonetheless.

Have you any pictures of "Attempts" at this, showing us your in a general direction. Because if so, i want to know how...

Edited July 3, 2016 by Vaporized Steel

[magnemoe]

Is the 225 ton dry mass or is it rocket fully tanked on Minmus? 
Also why require it to go to Minmus, that is 1.2 km/s extra, you can always refuel in LKO, in that case you can send it anywhere. 
However unless you want to landing it in kerbin again I would not have that as an specification. 

An 225 ton payload SSTO should be posible putting a 7 3.75 meter cores together with mammoth engines below,
Have an inflatable heat shields on top should mostly protect the stack going down nose first and flipping around before landing. 
 

[Blaarkies]

20 hours ago, Bill Phil said:

The biggest fuel tanks have the worst mass ratios.

Fuel Tank Wiki
If you are talking about MK3(and the flying type plane parts) then yes, they have a ration of +-8 instead of 9.
All basic rocket fuel tanks have a w/d ration of 9, except for adapters and other weird parts.

[Blaarkies]

20 hours ago, Tex_NL said:

SSTO and rocket is easy. Very easy in fact. They ARE a match made in heaven.
SSTO and 225 ton is just a matter of scaling it up.
SSTO, 225 ton and rocket is starting to get Kerbal but still very much within the realm of possibility.

I agree with @Corona688.
Many players still use asperigus staging just because it is faster(dant have to worry about landing it again), and don't have to prototype the design for hours until it is finaly successful. If SSTO rockets are easy, then why is everyone so excited to pull off their own version of a Falcon 9 landing?

My SSTO rockets are around 110t rocket for 14t payloads. Scaling up by this factor give a launch mass of 1992t. With stock parts, that is a massive rocket no matter how you look at it. Just because it gets dwarfed by some youtube video, doesn't make it a tiny quick build rocket

[Madrias]

I'm not a master of SSTO's, nor any good at rockets, but, I'd be thinking of just getting it into orbit first, then using a docking port and a second launch fuel tanker to refill it before sending it to Minmus.  It'd be a little less delta-V requirement, and once you're in orbit, it's just a matter of picking a destination.

As for that payload...  Is there any way you could possibly send that thing up empty?  It'd make it much easier - every bit of weight you can keep out of the payload is going to make the climb to orbit easier.  Once you're in orbit, then send up your fuel tankers to refill your payload.

And as for SSTO...  Does it really have to be, or is this a challenge where you have to SSTO and you're determined to put the most amount of mass in orbit?

[Netherman555]

Yeah i'm terrible at SSTO's as well...

What I would suggest is first trying to figure out how to lower payload weight.  If you can't do that, try to construct it in space is because there's no way you're getting that off the ground.

Also use rapiers to get you into space but also use nuclear engines to supplement them.  That should be enough if planned well.

[cantab]

Rocket SSTO is 100% doable in KSP. (It's 100% doable in real life too). In order to pack as much fuel as possible in the stage you want a high TWR engine and a low overall liftoff TWR. With a 200+ ton payload look at the Twin Boar, Mammoth, and Vector. Do not expect good payload fractions.

In career mode, unless you can land it afterwards, it's probably not worth it. Re-entry with a large light booster is generally fairly easy, but touchdown is likely to require either chute spam or a precisely timed Falcon 9 style burn. Both those options eat into your already slim payload fraction.

As for the idea of orbital assembly, I can say from experience that a high part count single launch is vastly less painful than a high part count docking.

[Warzouz]

On 3/7/2016 at 7:29 AM, Xyphos said:

I'm trying to come up with an SSTO rocket with a 225 ton payload that would be able to land on minmus, refuel and return (or go interplanetary)
initial design was an SSTO spaceplane but the payload required too many air-breathing engines to make it a viable solution
so I opted for a rocket instead. but it seems that no matter how many fuel tanks and engines I add, I just can't get enough Delta-V to get the thing into orbit.
any advise?

You'll find you happiness there, even it was designed for 1.0.4 (reentry my be rough in 1.1.x)

There is a lifter for 600T. More seriously, you can adjust the 200T lifter to carry your 225T to LKO.

EDIT : as I was looking at my table, you should need 5 mammoth engines to lift off very easily (should be rated for 250T). Then add the needed amount of fuel. Ultra easy. Using 12 to 14 mainsails should be good too (might be more expensive though).

Edited July 4, 2016 by Warzouz

[Warzouz]

50 minutes ago, cantab said:

Rocket SSTO is 100% doable in KSP. (It's 100% doable in real life too). In order to pack as much fuel as possible in the stage you want a high TWR engine and a low overall liftoff TWR. With a 200+ ton payload look at the Twin Boar, Mammoth, and Vector. Do not expect good payload fractions.

SSTO rockets are easy in KSP because you only need 3200m/s to LKO. I'm not sure that's really possible using that to real life LEO except for marginal payloads.

For example, for fun, I tried to transform Saturn V into a SSTO rocket. Totally ignoring that TWR would drastically going down, the rocket would have to be 5 time bigger (14000T instead of 3000T) without taking into account the new engines you should add to lift it (so you would need more fuel and more engines and more fuel and more engines and more fuel)... Sure Saturn V wasn't only LEO, but you get the point.

SSTO rocket (and even space planes) works nicely when low orbit dV is reasonable. The difference between a Kerbal SSTO rocket and a regular rocket is not that much (20 to 25% more massive ?) so it's manageable by current engines, just adding one more. IRL (for Sarturn V, it's 5 times heavier and it wouldn't even lift off until nearly all fuel would be depleted....)

That also explains why SSTO rocket are mainly a post release designs. Before, LKO was 4500m/s, SSTO rockets would have been much heavier and expensive compared to regular rocket (or even asparagus)

 

As for payload fraction in KSP, SSTO do worse than staged rockets. I think that best payload fraction is around 25%. My SSTo rockets are around 16 to 19% (which is not that bad is you consider that I keep fuel and parts for the recovery)

Finally, I totally agree with your final statement about docking into orbit. To do that, you need more parts which will moslty stay on you final ship. Only do that if you like doing rendez-vous and docking.

 

EDIT : this topic should be moved in "Gameplay questions" forum

Edited July 4, 2016 by Warzouz

[Bill Phil]

4 hours ago, Blaarkies said:

Fuel Tank Wiki
If you are talking about MK3(and the flying type plane parts) then yes, they have a ration of +-8 instead of 9.
All basic rocket fuel tanks have a w/d ration of 9, except for adapters and other weird parts.

Huh. The 3.75 meter tanks were much worse a few versions ago...

[cantab]

Yeah, that got fixed a version or two back.

[septemberWaves]

What exactly is this 225t payload and why do you think you need an SSTO rocket for it? I have seen some truly gargantuan rockets in this game (most of which are something to do with Eve) but if you're really lifting that sort of mass to anywhere then it's a lot better just to use a lot of asparagus staging and a big non-recoverable launcher. Even in reality, it is often (currently) cheaper to use an expendable launcher that's effective than to use a reusable one that is significantly larger for the same payload. Recoverability is nice, but it's not always practical - especially when you're talking about getting 225 tonnes to Minmus with an SSTO.

[Snark]

Moving to Gameplay Questions.

If you're willing to go for mods... SpaceY can be handy.  It has 5m parts and some very tall tanks. It also has some very high-thrust engines.

Reason why this is significant:

You do have one thing going for you, in going so big:  you have the opportunity to drastically cut two major loss factors for launch from Kerbin:  specifically, gravity loss and aerodynamic drag.

Gravity loss is a huge dV sink.  The way to reduce it is, 1. go faster sooner, so you spend less time on the vertical climb, and 2. tip over to a more horizontal trajectory sooner, also so you spend less time on the vertical climb.  The way to accomplish both of those is to have a higher TWR.

However, there's a practical limit imposed by aerodynamic drag.  If you go a lot faster than terminal velocity, you waste scads of dV on drag, and end up losing more than you gain.

Really big ships, if they're aerodynamic, have an advantage here.  They suffer less drag, proportional to their size; aero losses aren't as significant.  Therefore they can get away with a higher TWR and a sharper gravity turn.

However, you only get that benefit if you get big by getting tall-- expanding out horizontally in a massive asparagus won't help you.  And the stock parts put a practical limit on how tall and skinny you can get; your rocket turns into a floppy uncontrollable noodle.

That's where SpaceY can come in really handy:  big, tall parts that let you make a massive ship that's extremely tall, and skinny in proportion to its height, but still stays pretty stiff.  I haven't tried using it to make a big vertical-takeoff SSTO, but I bet it would work really well.

One other thing to consider:  I'm not sure what your reason for wanting SSTO is.  If it's a "moral" or RP thing-- i.e. "I want it to be SSTO because I think the idea of SSTO is cool and I just like it that way-- then please ignore the following.    However, if going SSTO is for economic reasons, i.e. you want to save money on recovery costs and the like, and you're willing to be flexible if there's a cheap reason to do so-- then SpaceY also gives you another option.  It has some really big 3.75m and extremely effective SRBs which are (relatively speaking) pretty cheap.  So if you're willing to go with "SRB off the pad, and then one stage to orbit", you could put one of those on the bottom, and have a 225-ton 3.75m stack sitting on top of it powered by a Rhino.  That would work really well.