Spaceplane / SSTO oxidizer blues

[lodger]

A few days ago I built my first real SSTO spaceplane (Mk-2 based fuselage). While it does reach LKO (~ 100 Km orbit) and I am able to get the thing down in one piece (albeit not at the same location the plane took off), I just can't reach my orbital station (~ 160 Km orbit) with it. I permanently run out of oxidizer when burning retrograde in order to slow down and rendezvous with my orbital station. Would some of the more exprienced SSTO guys here mind to  give me a few hints at building a fuel efficient SSTO? I'm currently using three five RAPIERs to get to orbit. Would it be better to use two J-X4 Whiplas engines and one LV-N Nerv engine instead, thus eliminating the need for oxidizer at all? My craft file can be found here: http://gallery.ufud.org/test/ASES-SP2-Xkos.craft

In general I wonder how it is possible to build a spaceplane that can do a round-trip through the whole system? I mean, sure you will probably do ISRU and stuff but still the ship itself must be huge, right?

Edited December 9, 2016 by lodger
corrected a typo

[Mastikator]

A NERV nuclear thruster might help.

[steve_v]

8 minutes ago, lodger said:

Would it be better to use two J-X4 Whiplas engines and one LV-N Nerv engine instead, thus eliminating the need for oxidizer at all?

Better is relative, while you may indeed get more DV out of it, nuke spaceplanes are generally considered "hard" on account of the low TWR of the NERV. I'd persevere with the Rapiers before trying that.
On your current problem: The obvious answer is "take more oxidiser (or less LiquidFuel)". The obvious question is: "What are you trying to get to orbit (payload)?"
In any case, Can't say much without seeing it. Pics?

I don't usually play without FAR, but I built this wee thing to go to Laythe during pre-release (stock aero) - with decent piloting it makes 100KM orbit with ~300m/s DV (plenty for rendezvous with the transfer stage) to play with, IIRC:

Only ~2T payload, but about as light a Mk.2 design as I have and pretty fun to fly.

21 minutes ago, lodger said:

In general I wonder how it is possible to build a spaceplane that can to a round-trip through the whole system?

Honestly, dunno. Never tried. IMO Spaceplanes are for getting to orbit or landing on Laythe... Suspect it'd be a fair bit bigger.

[lodger]

Thanks you for you answers so far. I've added the link to my craft file to my original post, images will follow as soon as I get home and can upload them.

[Val]

Moved from KSP Discusion.

[Val]

1 hour ago, lodger said:

Would it be better to use two J-X4 Whiplas engines and one LV-N Nerv engine instead, thus eliminating the need for oxidizer at all?

No. Even if you don't bring Oxidizer it's always better to use a RAPIER instead of a Whiplash. The RAPIER goes much faster and much higher.

Here's a craft for inspiration.

In the standard config it can bring up 7 t payload to LKO (80 km) and have around 400 m/s dV spare. If you replace the forward cargo bay with an LFO section, you can get more than 900 m/s dV out of it with 3.5 t payload in the remaining bay.

 

[lodger]

4 minutes ago, Val said:

No. Even if you don't bring Oxidizer it's always better to use a RAPIER instead of a Whiplash. The RAPIER goes much faster and much higher.

Here's a craft for inspiration.

In the standard config it can bring up 7 t payload to LKO (80 km) and have around 400 m/s dV spare. If you replace the forward cargo bay with an LFO section, you can get more than 900 m/s dV out of it with 3.5 t payload in the remaining bay.

 

Thanks for the input. So I'll stick to LFO and RAPIER engines.

I can see in your screenshot that the tanks behind the air intakes are LF fuselages. Are these Mk0 or Mk1 LF fuselages?

[katateochi]

I just had a look at your craft, the one on the download link has 5 rapiers (your post said three, maybe you've updated it since?). Anyway, I just tried your craft and I was able to reach 100km orbit with ~1300dV spare, so you've got way more than needed to get the job done!  
In terms of efficiency, for a 6 crew craft to reach ~200km you can go a lot smaller (probably half the weight), I think your craft was ~60tons, this one is ~30 tons and can reach 100km orbit with just under 1000dV spare (all tanks are full, no adjustments. It's a bit low on RCS fuel and the thruster balance isn't right yet). I think @Val would say this was overpowered and he's probably right! 

 

 

 

 

[Val]

Quote

I can see in your screenshot that the tanks behind the air intakes are LF fuselages. Are these Mk0 or Mk1 LF fuselages?

Mk1 LF fuselage.

 

 

[lodger]

25 minutes ago, katateochi said:

I just had a look at your craft, the one on the download link has 5 rapiers (your post said three, maybe you've updated it since?). Anyway, I just tried your craft and I was able to reach 100km orbit with ~1300dV spare, so you've got way more than needed to get the job done!  
In terms of efficiency, for a 6 crew craft to reach ~200km you can go a lot smaller (probably half the weight), I think your craft was ~60tons, this one is ~30 tons and can reach 100km orbit with just under 1000dV spare (all tanks are full, no adjustments. It's a bit low on RCS fuel and the thruster balance isn't right yet). I think @Val would say this was overpowered and he's probably right! 

I just read the Ascent Profile @Val provides with his craft and I think I may burn too much fuel while  getting up because during ascent I hold the nose at 20° - 30° pitch. 

 

[Val]

33 minutes ago, katateochi said:

I think @Val would say this was overpowered and he's probably right! 

It has external solar panels and no Angle of Incidence, so it's gonna need the extra power

[bewing]

15 minutes ago, lodger said:

I just read the Ascent Profile @Val provides with his craft and I think I may burn too much fuel while  getting up because during ascent I hold the nose at 20° - 30° pitch. 

 

You have to hold pitches like that with MK1-based SSTOs, because you need the altitude to avoid heatsploding (unless you use tricks and exploits to avoid the heat). But MK2 and MK3 should be flown much closer to prograde for the aerodynamic efficiency of it -- 10 degree AoA at most, I'd say. They can handle the heat.

 

[lodger]

9 minutes ago, bewing said:

You have to hold pitches like that with MK1-based SSTOs, because you need the altitude to avoid heatsploding (unless you use tricks and exploits to avoid the heat). But MK2 and MK3 should be flown much closer to prograde for the aerodynamic efficiency of it -- 10 degree AoA at most, I'd say. They can handle the heat.

 

That makes sense since I started out with a Mk1 based SSTO and while figuring out how to get to orbit I found a document that mentions a 20° - 30° pitch for a similar Mk1 craft. So I probaby got these values totally wrong for my Mk2 design. Thank you for pointing out this important difference between Mk1 and Mk2/3 based SSTOs

Edited November 21, 2016 by lodger

[ForScience6686]

Well you said you can make orbit.   try to make orbit at the moment of rendezvous.   it can be difficult to time it right,  but it can be done.

[Sharpy]

I'm afraid you may be doing *something* considerably wrong if you can't get the rendezvous right.

Can you get to *any* 160km orbit? If you can, then rendezvous is perfectly doable. Just make sure to make your encounter tangent to the target orbit, not crossing it at any considerable angle. Also, if it's inclined, try launching into the right inclination (wait until target's orbit passes over KSC, and the target is somewhere above Booster Bay, then launch towards the target).

[lodger]

37 minutes ago, Sharpy said:

I'm afraid you may be doing *something* considerably wrong if you can't get the rendezvous right.

Can you get to *any* 160km orbit? If you can, then rendezvous is perfectly doable. Just make sure to make your encounter tangent to the target orbit, not crossing it at any considerable angle. Also, if it's inclined, try launching into the right inclination (wait until target's orbit passes over KSC, and the target is somewhere above Booster Bay, then launch towards the target).

Well, it's not like I never did rendezvous manouvers. Quite the opposite. But with *this* particular design I run out of fuel every time I get to the rendezvous point and want to slow down adjust my orbit / dV to match that of the station. Anyway, thanks for the hint on making the encounter tangent to the target orbit. I'll try that one.

Edited November 21, 2016 by lodger

[AeroGav]

6 hours ago, lodger said:

A few days ago I built my first real SSTO spaceplane (Mk-2 based fuselage). While it does reach LKO (~ 100 Km orbit) and I am able to get the thing down in one piece (albeit not at the same location the plane took off), I just can't reach my orbital station (~ 160 Km orbit) with it. I permanently run out of oxidizer when burning retrograde in order to slow down and rendezvous with my orbital station. Would some of the more exprienced SSTO guys here mind to  give me a few hints at building a fuel efficient SSTO? I'm currently using three five RAPIERs to get to orbit. Would it be better to use two J-X4 Whiplas engines and one LV-N Nerv engine instead, thus eliminating the need for oxidizer at all? My craft file can be found here: http://gallery.ufud.org/test/ASES-SP2-Xkos.craft

In general I wonder how it is possible to build a spaceplane that can to a round-trip through the whole system? I mean, sure you will probably do ISRU and stuff but still the ship itself must be huge, right?

Hi Lodger,

I had a quick play with your ship,  this is my version of it -

https://kerbalx.com/AeroGav/ASES-SP2-Xkos-Dual-Nukes

There were a few things causing excessive drag,   it had a lot of radially attached parts (4 way thruster blocks, solar panels) though the main problem as always is the draggy mk2 fuselage.   My version is half the weight, but has more wing area and they are angled up at 3 degres.  This means you can fly it all the way to space with SAS set to prograde, which means the barn door fuselage is at 0 AoA therefore generating the least possible drag.

It only has 1 rapier and 2 nukes.   Gets up to 1500 m/s on the rapier alone - after that things get rather hot.  Firing up the pair of nukes takes it to orbit pretty smartly.  Action group 2 switches the RAPIER to close cycle for good measure, not sure it really needs it tbh.

Also  I switched to Big S wings and strakes, since they hold LF for the nukes.   Better than modular wings in that regard.

Lastly,  I switched to an inline cockpit.   The pointy one looks fab but as it's on the front it gets hot too easily.  Honestly I have less heat problems on mk1 inline cockpits than i do with mk2 pointys.

However, you'd come up with a pretty good basic layout that minimises handling changes full to empty, which saved me a headache.

I'm not sure if nuke SSTO are "hard".  "Hard" comes about because of poor handling characteristics or razor thin delta v margins, or both.   Assuming the designer's done a good job with the handling,  it just boils down to whichever solution - chemical or nuclear - gets you there with the biggest delta v margin to allow for errors.  The deeper into space you go, the better nuke tends to look i think.   quick video of me building a nuke ssto in 9  minutes (with flight to orbit, all sped up x2) -

Spoiler

 

 

Edited November 21, 2016 by AeroGav

[lodger]

Well, I got into orbit and still have plenty of oxidizer and liquid fuel left. I did not change the original design but just set a 10° AoA and followed the ascent profile that @Val provided. This got me quite fast up into space and circularizing took only a fraction of time it did before.  I'm now approaching my orbital station and have 533 LF and 841 Ox left in the tanks (and that's only because I slightly overshot my AP by .. uh ... 100Km). Enough to make it back home multiple times!

Next on the list is an overhaul of the craft, reducing drag and weight. 

Thank you very much, poeple - I really did learn a few things from you. Screenshots will follow once I am able to leave the SPH and upload them  ... 

 

EDIT: here's a screenshot of the beauty, next to my orbital station (really, it's just a closet with a docking adapter) to which an ASES SP-2 (the predecessor) is docked. Spaceward Ho!

Edited November 21, 2016 by lodger
added nice picture of amateur level SSTO