What is the limiting factor in your SSTOs?

[Nich]

39 minutes ago, Rune said:

 

You are kind of right, you both... and yet wrong in the final implications. With 100% recovery on the runway, the cost of hardware can be taken out of the equation, so the only metric when figuring out the √/kg of a SSTO is the cost of fuel... and therefore payload fraction. The higher it is, the more payload and less fuel that your SSTO is (as a fraction), so the better √/kg you get to orbit. So yeah, maximize payload ratio, and the cost will go down a low as it can. By the same metric, a rocket, however reusable it is, won't get such a high payload fraction, and therefore will always spend mor money in fuel, even if it is cheaper to put on the pad.

 

Rune. Rocketry is all about ratios.

I am going to have to disagree.  If you hit 55% payload fraction where 28% is fuel and 27% is structural it will be more expensive then a 50% payload fraction where you use 20% fuel and 30% structural.  Having more atm thrust will get you to altitude quicker for less fuel and accelerate you quicker using less fuel.  Same applies with vacuum engines a higher TWR mean less time in atm and less drag and more oberth.

To be honest I suspect that the difference is less then 2%  I might start playing around with Nefarmous Payload fraction winner and see if A I can get it to orbit and B if I can squeeze any more funds/t out

[RizzoTheRat]

57 minutes ago, Rune said:

 

You are kind of right, you both... and yet wrong in the final implications. With 100% recovery on the runway, the cost of hardware can be taken out of the equation, so the only metric when figuring out the √/kg of a SSTO is the cost of fuel... and therefore payload fraction. The higher it is, the more payload and less fuel that your SSTO is (as a fraction), so the better √/kg you get to orbit. So yeah, maximize payload ratio, and the cost will go down a low as it can. By the same metric, a rocket, however reusable it is, won't get such a high payload fraction, and therefore will always spend mor money in fuel, even if it is cheaper to put on the pad.

 

Rune. Rocketry is all about ratios.

Yeah,  I was taking payload fraction to mean percentage of the total craft weight,  which is why I'm not bothered about it,  if you're meaning payload to fuel ratio then that's exactly it.

[Wolfos31]

Despite playing KSP since .18 I have only just started to make use of SSTOs. And even then it's only in sandbox because I need every advantage I can get to make the darn things work. Part of my problem is definitely the ascent profile, I don't pay close attention to the steps as I've seen some people do. I don't note pitch angles or speed really. I just sort of point up and go until my jets flame out and then I switch over. My other problem is payload fraction/delta-v on orbit. 

Typically I can just barely manage a low orbit and that's not even very useful as everything I launch with a rocket typically ends up in 100km or higher orbit. My latest success is a Mk3 spaceplane that can carry a modest 9 tons to orbit and land safely. But the plane itself I believe weighs over 40 tons, I'd have to double check. 

[Rune]

53 minutes ago, Nich said:

I am going to have to disagree.  If you hit 55% payload fraction where 28% is fuel and 27% is structural it will be more expensive then a 50% payload fraction where you use 20% fuel and 30% structural.  Having more atm thrust will get you to altitude quicker for less fuel and accelerate you quicker using less fuel.  Same applies with vacuum engines a higher TWR mean less time in atm and less drag and more oberth.

To be honest I suspect that the difference is less then 2%  I might start playing around with Nefarmous Payload fraction winner and see if A I can get it to orbit and B if I can squeeze any more funds/t out

High TWR designs burn more fuel to make the last km/s to orbit on account of that higher engine weight, tough, and given the Isp/dVs involved, I have an unconfirmed hunch it will always mass more. The designs on the payload fraction challenge should speak for themselves, with the first places crowded by low TWR RAPIER-powered birds with >50% payload ratio. I don't care what fuel/engine percentage you have there, if you lift up more weight than you have in the rest of the plane, that's gotta be the cheapest thing ever. As you say, the difference will be minimal among them. But hey, more data points can't hurt, so feel free to prove me wrong with some experimenting!

42 minutes ago, RizzoTheRat said:

Yeah,  I was taking payload fraction to mean percentage of the total craft weight,  which is why I'm not bothered about it,  if you're meaning payload to fuel ratio then that's exactly it.

The difference is pretty much academic, methinks. In some corner cases, yeah, a low TWR SSTO design might burn a bit more LF per kg of payload than another with slightly higher TWR. But airbreathing SSTOs vs rockets, with recovery included? No freaking way the rockets come even close in √/kg.

 

Rune. And that is why I lift everything I can in SSTOs and my fund meter only ticks upwards even though I do very few contract missions these days.

Edited February 12, 2016 by Rune

[RizzoTheRat]

Complete agreement there, I was only meaning comparing different SSTO planes, not comparing a spaceplane to a rocket.

The real time it takes it the main reason against them for me.

[BLUESTREAK]

1 hour ago, Evanitis said:

Nonsense! Just add moar boosters. And more wings. And even moar boosters. I might sound like joking, but I'm not.

Ah, I can get them off the ground using detachable SRBs. But then, it rather stops being a SSTO craft! More lift is called for, but then bigger wings result in needing bigger engines and the whole thing spirals upwards in mass and ends up smashing into the sea at 250m/s. If they work, my SSTOs are a bit like the original Rolls Royce HOTOL concept - "Great, if you wanted a payload of 7 tons. And if you wanted those 7 tons to be its own hydraulic systems" (Alan Bond - I'm paraphrasing, because I can't find the original quote...). 

[Evanitis]

29 minutes ago, BLUESTREAK said:

Ah, I can get them off the ground using detachable SRBs. But then, it rather stops being a SSTO craft!

Sure, I didn't mean more detachable boosters.

29 minutes ago, BLUESTREAK said:

More lift is called for, but then bigger wings result in needing bigger engines and the whole thing spirals upwards in mass and ends up smashing into the sea at 250m/s.

You forgot to add fuel - possibly to counterbalance the engines (while keeping in mind that the CoM shouldn't move much or at all when the tanks run dry). Payload stays the same, and the trinity of wings-engines-fuel eventually reaches the required amount and ratio. Than one can start the fun-part: balancing.

Though, as I said earlier, the limiting factor for me is hardware. My heavy lifter in SSTO mode is tested for 42t, probably could do more, and has a ~300t total weight. It causes... bearable lag on my system, but I definitely don't want to fly anything that produces an even lower performance than that. If your post refers to a considerably larger construction than that, disregard my comment. Maybe runway length is indeed the final frontier. Well, unless we start using the huge green field besides that.

Edited February 12, 2016 by Evanitis

[eddiew]

Insufficient altitude and efficiency with air-breathing engines.

I remember the old days of turbojetting along at 32km at mach 5... Now you're lucky to get past 20, and you have to pack more LFO.

[Temstar]

42 ton payload with a 300 ton SSTO spaceplane (14% payload fraction) is pretty bad - you can hit that payload fraction with a single stage, no lifting surface rocket. In fact yours truly shows it's possible to build a SSTO, 100% recovery rocket with more than 15% payload fraction.

I'm no expert at SSTO and my work horse cargo SSTO that I use in my career manages 34 ton of payload for a maximum take off weight of over 130 tons for 26% payload fraction, and that's a career craft with all the trimmings like RCS, docking port and so on. The extremely efficient crafts  in payload fraction challenge is getting more than 50% payload fraction.

 

 

[Spartwo]

-Too heavy for jets to lift it to altitude(20 tons or more)

-Too light for sufficient fuel to brute force it out(7 tons or less)

[PanzerAce]

13 hours ago, Temstar said:

Regenerative cooling unit that provide active cooling by consuming some LF?

Basically, yeah, regenerative, but rather than having a seperate LF drain, I'm thinking of using a function to decrease isp as a function of how much heat it's dumping. Since, unless I'm way off base, that's going to be the most "accurate" way of building in a trade-off

[BLUESTREAK]

5 hours ago, Evanitis said:

...Well, unless we start using the huge green field besides that.

Hang on... If I just turn the thing around in the hanger, it'll point the wrong way on the runway, then I have km after km of nice green fields to rumble across. Once I'm airborne, all I have to do is turn prograde and I'm good...

[Evanitis]

7 minutes ago, BLUESTREAK said:

If I just turn the thing around in the hanger, it'll point the wrong way on the runway, then I have km after km of nice green fields to rumble across.

After a tiny bit of a bump. But that bump looks very scary from the perspective of a huge plane. It can be managed though.

I'd also consider the nice ramp the launchpad has. It's longer and the space is tight, but much safer.

Edited February 12, 2016 by Evanitis

[jonrd463]

Regarding size, I always point to this as my crowning achievement. I built it when the B9 pack with the HX parts first came out. Of course, my first thought was "I gotta make a space plane out of those!" This is absolutely useless, as it carries no payload. It's only purpose is to 1. Fly to space in one stage, and 2. Be frikkin' HUGE. It succeeded in both counts. I call it the S.S. MOTHER OF GOD.

[BLUESTREAK]

7 minutes ago, jonrd463 said:

Regarding size, I always point to this as my crowning achievement. I built it when the B9 pack with the HX parts first came out. Of course, my first thought was "I gotta make a space plane out of those!" This is absolutely useless, as it carries no payload. It's only purpose is to 1. Fly to space in one stage, and 2. Be frikkin' HUGE. It succeeded in both counts. I call it the S.S. MOTHER OF GOD.

Ye Gads! Look at the size of that thing!

 

...I do like the little canards at the front, they look like they were almost added as an afterthought!

[jonrd463]

4 minutes ago, BLUESTREAK said:

Ye Gads! Look at the size of that thing!

 

...I do like the little canards at the front, they look like they were almost added as an afterthought!

Heh, I can't remember why I added them. I think it was because the previous flight test showed that I needed just a hair more pitch authority to take off, and those pushed it just over the edge.

[AbacusWizard]

55 minutes ago, jonrd463 said:

Regarding size, I always point to this as my crowning achievement. I built it when the B9 pack with the HX parts first came out. Of course, my first thought was "I gotta make a space plane out of those!" This is absolutely useless, as it carries no payload. It's only purpose is to 1. Fly to space in one stage, and 2. Be frikkin' HUGE. It succeeded in both counts. I call it the S.S. MOTHER OF GOD.

First thought: "Eh, it's big, but not ALL that big."

Second thought: "Wait, is that a Mk3 cockpit??"

[Hodo]

I have been building SSTOs in FAR for a couple of years now.  And I can say my biggest factor in my SSTO designs is I tend to design them to do to much.  So I have a wide range of SSTOs.  But unfortunately I havent made an SSTO that I have been happy with that is a full utility craft.  I have fighters that are SSTOs that work after a year since design, I have science SSTOs that work.  But no long range science SSTOs.

 

I have this which is coming up on a 6 months old.  I use this little thing more than I care to admit.

Edited February 12, 2016 by Hodo

[Frank_G]

Its the heat due to high velocity in my SSTO designs. You can easily bring a vessel to 1.500 m/s, even with a TWR around 0.6... Exceeding 1.500 m/s at an altitude around 21.000 m makes your SSTO going popcorn for sure. So at the moment i am forced to switch the engine cycle from airbreathing to closed at around 1.470 m/s and enter a more steep ascend, to avoid this.

[tempsgk]

I would like to build more SSTOs, but the most pressing limiting factor is the LAG.  

[1greywind]

Bad mass/thrust ratio of LV-Ns. I'm used to old 2.5 nukes, new ones are too heavy

[Guest]

Basically, everything is a limiting factor.

I started playing when KSP was in 0.23, and I still cannot SSTO.

[Fearless Son]

The slow, gradual build up of horizontal velocity is something that seems much harder to do in the new version compared to the old.  But on the contrary, getting up to high speed lower down seems a lot easier... but then, overheating is now a thing that can destroy you if you try to build up too much speed down there.  

It seems to me that the most practical way to do it is to go for a spaceplane with lots of atmospheric thrust, and go for a very aggressive ascent, one that will allow it to power its way up on a suborbital trajectory on jets alone at a sharp angle.  Then once it gets to the upper atmosphere, you start adding a bunch of horizontal thrust with rocket engines, preferably ones with a good balance of thrust and ISP for your craft.  

If you try to go for a gradual, shallow build up, you tend to run out of thrust faster than you escape atmospheric drag, and you run out of fuel trying.  At least that has been my experience.  

[Geschosskopf]

On 2/11/2016 at 2:26 PM, Levelord said:

In the meantime I would like to ask the KSP community what their limiting factors were when they designed their SSTOs. I'm interested to see if more people experienced overheating as their biggest limiting factor.

Honestly, the real limit on my SSTOs is the fact that to my way of thinking, they're just an alternative means of getting some payload to orbit, and not an end in and of themselves.  What I enjoy most in KSP is mucking about on other planets and thus view getting to orbit as a necessary evil that is best breezed through as quickly as possible so I can devote the bulk of my limited play time to the stuff I prefer doing.  Therefore, I begrudge all the time it takes not only to design and tweak spaceplanes but then actually use them, compared to the ease, simplicity, and speed of rockets.  Besides, most of the stuff I send to other planets is too big to fit in cargo bays anyway, and working around this with spaceplanes wastes even more time with multiple launches and repeated dockings.

All that said, I do find spaceplanes very cool so make a few now and again.  But I keep them fairly small (thus easier to make airworthy) and use them only on special occasions, like bringing down the crew of some massive, unlandable interplanetary ship that's just returned.   This is somehow more fitting than sending up capsules to splash down.  Otherwise, I lack the incentive to do anything else with them, so this is my real limitation.

 

Edited February 13, 2016 by Geschosskopf

[nosirrbro]

For whatever reason, the style of SSTO I make never has a high enough TWR. They have lots of fuel, but often just re-enter the atmosphere (Or possibly never even left it) before I can use it all up and get to orbit.