What is the limiting factor in your SSTOs?

[suicidejunkie]

On Saturday, February 13, 2016 at 3:41 PM, nosirrbro said:

I usually keep around 5-10 degrees profile in the jet engines to get the most velocity out of them as possible since they are so damned efficient, then once they begin to flame out I do a hard turn upward to 75-85 degrees with my rocket engines to get me into the vacuum where they are most efficient, but at that point I often fall out of the sky as my craft is too heavy for the engines without any lift from the wings

This sounds exactly like the 'Cobra' maneuver I use to bring my spaceplanes out of orbit... except you've got roughly 10x the atmospheric pressure that low down, so you're braking MUCH more efficiently.

Blowing away all your horizontal speed for some vertical speed allows a safe landing, but isn't good for going to orbit.

[Guest]

My SSTOs are always back-heavy at first. It's not my fault that everything I want on the plane* belongs in the back.

 

*more engines

[panzer1b]

My limiting factor (and to some extent the reason i ended my obsession with SSTO starfighters in general) has to be a combination of mass/payload/range.  Idealy, i would have a 10t SSTO that has around 6000 dV in LKO and does not use ion engines (unless 1.1 improves high part count performance ions are worthless on anything but a scanner probe or ultra lightweight droid).  Actually getting into LKO is a joke since its easier to design a SSTO then a bloody launch stage rocket for a 60t payload (takes quite some engineering to get a capital ship in orbit on a rocket).  My issue is having something that doesnt weigh incredible amounts, and still has reasonable dV left in LKO (and my dV desires basically force nuke engines on you, which are bloody heavy not to mention awkward to place anywhere).  That and i like to carry ~1.5-3t of weapons onboard, which is just impossible while having super high dV, reasonable TWR, and having a vessel that isnt heavy to begin with.

 

I think my designs just cant be done stock:

under 10t, 1.5t payload, at least 0.2TWR in space, and most of all at least 5000dV after you reach LKO (so i can do a laythe roundtrip with it).

If anyone has something remotely similar to that please gimme a link, id die for a 100% stock SSTO that could do all of the above.

Edited March 2, 2016 by panzer1b

[KSK]

I'm voting vessel size but I really mean 'landing a vessel of any size'. I can build a nice Mk3 spaceplane without any problems. Lifts off nice, gets to orbit without any problems, re-enters safely, even has enough fuel left to overshoot the KSC, turn around and line up on the runway.

I just can't land them.   

Doesnt matter how slow I'm going or what my rate of descent is, I touch the ground and explode. Totally kills any fun in using spaceplanes.

[boostme]

Fuel. Always fuel. I just hate using disposable craft for tourist missions, so always wait until I can SSTO them. In a tourist mission in my most recent career mode game I designed a monster MK3 plane with a lander in the cargo bay. I took six tourists on a jaunt to the Mun, and Minmus and back, landing the pilot and three tourists on both moons. I had to refuel at Minmus, but my mining operation there was still very new and I had no way to get the fuel up from the surface. My passengers had to orbit Minmus for weeks while they waited for a fuel lifter to arrive from Kerbin. When I eventually got them home, after re-entry, I ended up gliding about 80km out from the runway with no fuel at all and found that the plane was unflyable with empty tanks. Always make sure your plane can still fly when the tanks are empty before you leave on a month-long journey. Luckily I still had lots of RCS fuel left and was just able to keep the nose up if I kept burning RCS. All the way to the runway it became apparent that it would be a close thing with the RCS fuel. It ran out as I was doing the final landing flare, causing the nose to dip sharply, landing the entire MK3 plane (and lander) on the single nose wheel. Somehow the plane bounced twice before coming to a halt with no damage at all. The bone-dry MK3 plane sitting on the runway after a month long journey was indeed something to behold. It is because of moments like these that I play KSP.

My planes only get names when they complete their second mission successfully, so this successful (barely) MK3 tourist plane will remain forever nameless.

[AeroGav]

I build oxidizer-free SSTOs with Rapier/NERV engine combo and plenty of Big-S delta wings.   Getting up to the Rapier ceiling is the easy part,  what sorts out men from boys is the ability to progress from there to orbit with just the meagre 60kn off each NERV engine.   So long as your thrust exceeds total drag,  you're getting closer to orbit with every passing second.    With the stock aerodynamic model, optimum lift:drag ratio occurs at 2 degrees angle of attack at subsonic speeds , rising to 5 degrees as you approach orbital velocity.     

The problem is that at 30km+,   the air is VERY thin  and flying the optimum angle of attack for lift/drag ratio can result in stuff overheating from being too fast, too low, for too long.    The limiting factor for me, when i add too much cargo,  is that i run into a dilemma - not exceed 5 degrees angle of attack, and blow up.   OR,   pitch up to 10 degrees and be higher up, and cooler, for a given mach number.  But,  my drag is then too much at this increased alpha to gain speed,  and i gradually fall back down into the airbreathing reigime again.

 

[Edax]

1 hour ago, AeroGav said:

I build oxidizer-free SSTOs with Rapier/NERV engine combo and plenty of Big-S delta wings.   Getting up to the Rapier ceiling is the easy part,  what sorts out men from boys is the ability to progress from there to orbit with just the meagre 60kn off each NERV engine.   So long as your thrust exceeds total drag,  you're getting closer to orbit with every passing second.    With the stock aerodynamic model, optimum lift:drag ratio occurs at 2 degrees angle of attack at subsonic speeds , rising to 5 degrees as you approach orbital velocity.     

The problem is that at 30km+,   the air is VERY thin  and flying the optimum angle of attack for lift/drag ratio can result in stuff overheating from being too fast, too low, for too long.    The limiting factor for me, when i add too much cargo,  is that i run into a dilemma - not exceed 5 degrees angle of attack, and blow up.   OR,   pitch up to 10 degrees and be higher up, and cooler, for a given mach number.  But,  my drag is then too much at this increased alpha to gain speed,  and i gradually fall back down into the airbreathing reigime again.

 

Yeah, after the latest version change, I've given up pure-LF SSTO launches, I instead use disposable oxidizer tanks and raipers which I dump when I'm in orbit, so that the remained spaceplane is pure LF-goodness.

[TimePeriod]

Don't fly SSTO's, so I don't see any problems.

[Edax]

18 minutes ago, TimePeriod said:

Don't fly SSTO's, so I don't see any problems.

I'll get right on that.........

[AeroGav]

On 21/03/2016 at 4:56 PM, Edax said:

Yeah, after the latest version change, I've given up pure-LF SSTO launches, I instead use disposable oxidizer tanks and raipers which I dump when I'm in orbit, so that the remained spaceplane is pure LF-goodness.

To be fair, my latest SSTO does OK on a LF only launch,  though this was a test flight with empty cargo bay.    We reached Minmus with 900 Delta V remaining.   A light scientific payload probably wouldn't mess with the launch profile too much, indeed it would have to be light in order to still have the Delta V to make it to Minmus.  If I decided to use this aircraft to fly a denser payload to low kerbin orbit,  I could offload some fuel to keep the overall wing loading down.  I could also fill the oxidizer tanks to capacity (on this flight, they only contained a small amount to power the Vernier lifting engines for Minmus landing) for a small additional boost.

Spoiler

 

26.5km and Mach 4.   The Rapier has lost 2/3 of the thrust it had at 24.5km, which was the altitude at which it no longer made enough thrust to continue climbing.   For this reason, adding another Rapier to gain a kilometre or two of airbreathing ceiling doesn't seem worth the penalty of another 2tons dead weight out of the atmosphere.

Spoiler

 

Mach 4 at 28km.   Unfortunately we've run out of lift and will be stuck at this altitude for a little while.    Notice that in the last 1.5km altitude our Rapier output halved again.

Spoiler

 

Mach 5 and 29km.  For the past minute we stagnated at this altitude while the heat built up.   86.6% of Critical temperature, this is the hottest the airframe got in this mission.   When she finally starts climbing again and the temperature trends down,  I breathe a big sigh of relief.

Spoiler

 

 

Mach 5.5 @ 38km.   Going up..

 

Spoiler

 

Nearing 42km and Mach 6

 

Spoiler

 

30 seconds later we're at Mach 6.5 and 45km, but the real thing to notice is our Apoapse, which has shot from 45km to 61.4km.   In another 10 seconds it's out of the atmosphere completely - time for MECO

 

 

Edited March 23, 2016 by AeroGav
spoiler tags added for pictures

[Octa]

1. The Enginner is not qualified enough to design a proper SSTO to start with
2. The Pilot cannot fly it correctly

So basically all of the above. I tend to overengineer things.

[Zoidos]

The biggest limiting factors in my Spaceplance missions are my piloting and my engineering skills. Everything else is fine.

[11of10]

A combination of available time to play, aerodynamics, and the fact that I have some oversized cargo that is too big even for MK3 cargo bays, and yet needs to be recovered after deployment on another planets. A problem I haven't solved yet in over a year. 

Also, I never managed to get anything larger than MK2 into orbit. Just can't get it to work, no matter what.

MK2s work beautifully, but MK3 are a nightmare, it seems.

[Octa]

11 hours ago, Zoidos said:

The biggest limiting factors in my Spaceplance missions are my piloting and my engineering skills. Everything else is fine.

At least the parts work and do their job.

[I_Killed_Jeb]

5 hours ago, 11of10 said:

A combination of available time to play, aerodynamics, and the fact that I have some oversized cargo that is too big even for MK3 cargo bays, and yet needs to be recovered after deployment on another planets. A problem I haven't solved yet in over a year. 

Also, I never managed to get anything larger than MK2 into orbit. Just can't get it to work, no matter what.

MK2s work beautifully, but MK3 are a nightmare, it seems.

Something you can consider is this - take your favorite MK2 design, and write down payload (all non fuel parts, incl. cabin), fuel, lift, and number of engines. Build up your Mk3 plane core, just the cabin and payload. Then scale up the wing, fuel and engines a proportionate amount. Works quite well with some tweaks here and there.

[theend3r]

Reentry? No matter the angle they usually end up pretty bad. Or they did before I learned that the best type of reentry is the most chaotic/Kerbal one: spin as much as you can in all directions so your craft loses a lot of speed and all the heat isn't concentrated only on one part. I've had 95% success rate after that.

Spoiler

 

 

 

Edited March 24, 2016 by theend3r

[zeekkay]

Biggest obstacle for me is usually habitation part placement and keeping the craft balanced. I like all the habitation elements being connected. (Command module, crew cabins, docking port, etc)

Other is just pilot skill... always seem to end up cruising for awhile to get to KSC, or end up having to turn around.

I rarely make mk3 SSTO's. Big rockets are more fun.

 

Tip on finding out how much fuel you need:

1) Slap a prototype of your ship together, including the engines, expected payload, etc. Include fuel for rockets, but NOT fuel for your jet engines. (Not that shape of ship that important, it's the weight)

2) Use kerbal engineer and add/remmove fuel tanks as necessary, until you like the dV readings from your rockets. (Make sure to disable jet engine thrust so you only get dV from rockets)

3) Add some extra fuel for jet engines and begin laying out your craft.

Edited March 24, 2016 by zeekkay
typo