What is the limiting factor in your SSTOs?

[Levelord]

Ever since the changes to the aero model in KSP 1.0+ I've been getting more involved with SSTO design. Nose cones and having fewer struts are necessary to minimize drag on crafts as much as possible. In addition, those having rear-facing nose cones also contribute to the overall streamlining and drag-reducing design of craft in KSP.

 

I'm an avid designer for small SSTO space planes. After performing extensive testing on drag-reducing designs, the biggest problem that my SSTO crafts experience is overheating because of their very (ironically) streamlined design. Parts will often heat up and explode, leaving a suborbital debris field and an eventual demise at sea.

 

The general idea for SSTO space planes is that they should take the shallowest ascent profile in order to take advantage of the relatively efficient air breathing engines (usually the RAPIER) to gain as much horizontal speed as possible before switching to rocket mode for the final burn to apoapsis. In KSP, at an altitude of around 18,000m - 24,000m rocket parts will overheat and explode if exposed to speeds roughly above 1,400m/s. This will mean that speeds are limited to below 1,400m/s until the craft gets to a high enough altitude (which is beyond air breathing engine's capabilities) and where closed cycle engines take over for thrust.

 

I've been doing some careful design considerations and have seemed to breach the 1,400m/s speed barrier and reached around 1,600m/s on an air breathing system with little to no overheating. I'll need to perform more testing as I have done in my previous threads before sharing my findings with the KSP community. I would also like to mention that I use vanilla parts and vanilla aero. For now, the results appear promising, as it would mean that I would need to carry even less fuel to achieve orbit with SSTOs.

 

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.

Edited February 11, 2016 by Levelord

[Rune]

Hum. That whole maximum velocity thing? It's kind of not at all what I look for in a SSTO. In fact, I usually settle for 1,250m/s at rocket takeover or so. Fact is, from there orbit is really close, as rockets with ~300s Isp go, so the rocket equation will be sweet on you and give you more than enough dV with reasonable fuel fractions. But pushing for higher speeds needs a lot of extra weight in the form of engines, and that seriously hurts your payload ratio. Be it payload to LKO or range what I'm after (which is mostly how much fuel you can lift to LKO, so again a matter of payload), I always try to maximize the on-orbit mass/GLOW coefficient, and going for a higher rocket takeover speed is rarely the answer.

If anything, I usually have troubles because I go with such low TWR to maximize mass ratio, that then I get issues with going transonic or climbing in a reasonable amount of time. Or, you know, climbing in general. The payload ratio to shoot for is around 33% for a reasonably good bird, with about 0.5 TWR on the runway, BTW. Extremely frugal designs can break 50%, tough...

 

Rune. Payload fraction is IMO the best yardstick for SSTOs.

[Choctofliatrio2.0]

My SSTOś limiting factor is the fact that they don't work.

[jonrd463]

Size, basically. I want to send the equivalent of an An-225 to orbit as an SSTO. I don't know why, but it's this obsession I've got with sending the largest cargo carrying vessel up via horizontal launch without shedding tanks or boosters that I can.

[regex]

I don't find any limiting factor to my SSTOs, when I do make them, aside from the obvious payload reductions; they launch like any other rocket.

[Evanitis]

Hardware.

[qromodynmc]

Lack of Delta V.

Best ssto i ever made had around 1000ms deltav left in orbit which is not very useful for me. Maybe I should try using ions but this time I lack patience.

[Nich]

2 hours ago, Rune said:

Hum. That whole maximum velocity thing? It's kind of not at all what I look for in a SSTO. In fact, I usually settle for 1,250m/s at rocket takeover or so. Fact is, from there orbit is really close, as rockets with ~300s Isp go, so the rocket equation will be sweet on you and give you more than enough dV with reasonable fuel fractions. But pushing for higher speeds needs a lot of extra weight in the form of engines, and that seriously hurts your payload ratio. Be it payload to LKO or range what I'm after (which is mostly how much fuel you can lift to LKO, so again a matter of payload), I always try to maximize the on-orbit mass/GLOW coefficient, and going for a higher rocket takeover speed is rarely the answer.

If anything, I usually have troubles because I go with such low TWR to maximize mass ratio, that then I get issues with going transonic or climbing in a reasonable amount of time. Or, you know, climbing in general. The payload ratio to shoot for is around 33% for a reasonably good bird, with about 0.5 TWR on the runway, BTW. Extremely frugal designs can break 50%, tough...

 

Rune. Payload fraction is IMO the best yardstick for SSTOs.

You know I am leaning away from payload fraction I came up with a challenge to reduce funds on recoverables.  I figure SSTO spaceplanes are going to dominate but I am interested to see if over powered designs over lifted designs use less fuel?

Yes I apologize for the shameless plug   

 

[XeroCoolUSN]

me

[Raptor9]

My observations in my own personal build style is if you have low TWR, you need a good overall lift/weight ratio; or vice versa.  Example: if you have a spaceplane with relatively low total lift rating, you will spend most of your flight with the nose at a high AoA.  This increases the drag on the spaceplane since the entire underbelly is impacting the relative wind.  This may not be an issue if you have a high TWR, overcoming the drag with enough raw thrust to accelerate through the atmosphere.  On the flipside, if you have a low TWR, you will want a lot of lift (and CoM not too far forward of course) to ensure the nose is as close to the flight path vector, low AoA and less drag.

The other significant factor for me is ascent profile.  I usually put the throttle to full and adjust ascent angle to what works best through a series of tests.  Too shallow and my spaceplanes overheat before getting into the thin upper atmosphere where speed isn't an issue;  too steep and my airbreathing engines flameout before I get enough speed, requiring a much longer rocket burn.

[Just Jim]

So far, my only limit seems to be size, and my imagination, I guess. 
I've gotten smaller Mk-1's into orbit like this tourist shuttle into orbit easy enough:

However, I've only come close with a Mk-2... but may finally have an efficiantly working one soon.
And Mk-3?  Forget about it.... so far they've all exploded.
Some quite comically, so at least there was that. 

 

Edited February 12, 2016 by Just Jim

[Mad Rocket Scientist]

First I build an SSTO with too low TWR.  Then I make it more powerful.  Then I run out of fuel on the way to orbit.  Then I add more fuel.  This continues in an upwards spiral until I stop on the SSTO.

[AbacusWizard]

I built some great little spaceplanes in older versions (including a teeny one with an external pilot seat), but the new aerodynamics have made it more difficult for me. The main problem I've had is just not quite having enough ∆v to get to orbit. I can generally get going pretty fast (around 1200 m/s or so?) by the time the air gets too thin for the jet engines, but then my rockets never seem to have quite enough fuel to to get the rest of the way to 2300 m/s. I guess I should just be starting with bigger fuel tanks in general.

 

I did finally get one really sweet spaceplane to work in the current version, though. It started out as an attempt to make an SSTO fuel tanker, but then I realized I could just as easily put a Mk3 cockpit in the front. No cargo capacity, but it carries four intrepid kerbalnauts, and moving crew to and from orbit is mostly what I use spaceplanes for anyway. It has just enough ∆v to get to low orbit, wait for my orbital crew transport skiff to rendezvous with it, transfer crew, and de-orbit. Also it looks awesome. Four double-whiplash jet engine pods mounted on the corners; four aerospike rocket engines on a Mk3 thrust plate on the back. Good times.

 

[Kilmeister]

I generally have no problem getting my SSTOs to orbit with sufficient delta v.  The challenge for me usually lies in the reentry.  I tend to build single wing configs.... so they fly fine at supersonic speeds.... but tend to be a pain to handle upon rentry and eventual landing. have lost my fair share of them while doing the "Safe" part; atmospheric flight.

[PanzerAce]

I usually have excessive in-atmo thrust, and so have to find the angle at which I leave the atmosphere before my SSTO burns up. if we had a way to dump excess heat (which I'm rough-sketching the mod for) reasonably while in atmo, then I could build even better ones....

[Temstar]

36 minutes ago, PanzerAce said:

if we had a way to dump excess heat (which I'm rough-sketching the mod for) reasonably while in atmo, then I could build even better ones....

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

[DrWeirdLust]

Hello fellow Kerbonauts, I am new here. I have played about 120 hours since I bought the game a month ago and I voted for inefficient Ascent path;

I am crazy addicted to this game... I mean this real space program.  I have spent the last week or more going through about 60 evolutions of my ssto design. I keep fine tuning the Ascent profile, and removing any extra weight. I run 4 Whiplash engines and one dart, with 9800 Dv at takeoff and about 1300 dv for my dart. I pop the dart around 18-20k meters and wait for the thrust in my whiplash's to hit 11.k Kn before toggling them off. In Atmo it has a 4-5 TWR, and at around 25k meters the craft has a .7 TWR running on the dart alone. To begin I pull to 40 degrees pitch until 15k, from 15k to 22k 30 degrees, 22-50k 25-20 degrees, 60k 10 degrees, wait for my Apo to hit 75 and lower the pitch to 0-(-10) to circularize.  My delta v is always at about 7 m/s when I cut the fuel to the dart. Which leaves me short for re-entry so if I don't meet the Perfect ascent profile I just scrub the mission. The real limitation for me is my understanding. I have gone from an understanding of basic algebra to calculating Delta V and TWR, and I can say without a doubt the biggest thing I have learned is to get and let Kerbal Engineer do the math.  I assume because I get to an 80km orbit it works, if anyone has .02 cents to spare about anything you read, I'd love to hear it.

 

"Scott Manley told me to"

[Kertech]

Why SSTO when the mission is the best bit! 

 

The only time I've used one is as a crew shuttle to get the ill fated King Edward the second (smashed into the space potato after a glitch) manned for the mission

[RizzoTheRat]

13 hours ago, Rune said:

Payload fraction is IMO the best yardstick for SSTOs

Personally I don't really see payload fraction as that important, the key driver is the funds/kg of payload to orbit.  In general the two are going to be fairly closely linked though.

I've only recently started on SSTO's and so far the largest I've managed gets 5 Mk2 cargo bays to orbit, 2 double length and a single one.

My experiments in Mk3 cargo bay SSTO designs are on hold since I realised by base components won't quite fit in one, but was really struggling to get something to work, I had one that I think should make it to orbit but not with the payload weight I wanted, and due to poor flying the unladen test aircraft exploded due to going too fast too low.  I think my next attempt might have to be with jets and rockets rather than Rapiers though.

They're a fun diversion but the real world time to launch and land the things means I'm probably always going to favour rockets though.

 

 

Edited February 12, 2016 by RizzoTheRat

[Evanitis]

15 minutes ago, Kertech said:

Why SSTO when the mission is the best bit!

To each of their own. I totally love do design and launch stuff. I love the thrill of every kind of landings too. But I get bored on missions that requires more than a few burns. My KSP folders are littered with saves of missions that I might continue one day. When I finish one, it's usually done in multiple short sessions that are days apart.

Not that I couldn't assemble big,  efficient and complex interplanetary crafts by multiple SSTO launches.. I just don't. When I'm in a mood to explore, I rather just use single launch by a standard multi-staged rocket.

I find SSTOs more complicated to design, thus doing so results in great and prolonged fun. And I'd love to make huuuuge spaceplanes, but my Husttle is pushing the limitations of my hardware enough.

[Temstar]

47 minutes ago, Kertech said:

Why SSTO when the mission is the best bit! 

Then there are weirdos like me, largely shun by the daywalkers, who find big hulking engines and their giant exhaust plums mesmerising and the little engines on payloads underwhelming (unless we're talking half a dozen nukes slowly turning cherry red with a 10 minute burn). I design lifters to loft tanks of LFO up to orbit for no reason other than the fact that to me, big rockets and SSTOs are reasons enough for their own existence.

[Curveball Anders]

Mine are only limited by my poor imagination and lack of competence

 

[Rune]

11 hours ago, Nich said:

You know I am leaning away from payload fraction I came up with a challenge to reduce funds on recoverables.  I figure SSTO spaceplanes are going to dominate but I am interested to see if over powered designs over lifted designs use less fuel?

Yes I apologize for the shameless plug   

 

 

1 hour ago, RizzoTheRat said:

Personally I don't really see payload fraction as that important, the key driver is the funds/kg of payload to orbit.  In general the two are going to be fairly closely linked though.

I've only recently started on SSTO's and so far the largest I've managed gets 5 Mk2 cargo bays to orbit, 2 double length and a single one.

My experiments in Mk3 cargo bay SSTO designs are on hold since I realised by base components won't quite fit in one, but was really struggling to get something to work, I had one that I think should make it to orbit but not with the payload weight I wanted, and due to poor flying the unladen test aircraft exploded due to going too fast too low.  I think my next attempt might have to be with jets and rockets rather than Rapiers though.

They're a fun diversion but the real world time to launch and land the things means I'm probably always going to favour rockets though.

 

 

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.

[BLUESTREAK]

Runway length. The waters of Booster Bay are littered with shattered spaceplane parts, and salty from the tears of broken Kerbonauts.

 

Perhaps I'm building just a little bit too big...

[Evanitis]

13 minutes ago, BLUESTREAK said:

Perhaps I'm building just a little bit too big...

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