Duna SSTO; How much fuel do I need?

[Mr. Peabody]

I've been working on SSTO spacecraft recently with the goal of building one that can reach Duna and back. This is what I came up with:

This is an older screenshot. Since it was taken I have removed the drag inducing ailerons and made a few small adjustments like more batteries.

It can reach LKO with 1,200 units of liquid fuel remaining for the nuclear engine. So my question is, is that enough fuel to reach Duna and get back or do I need to make more adjustments to the design? If someone could let me know I would be appreciative and it would spare me having to experiment myself and save me some precious time.

[Foxster]

It's more useful to talk about the craft's dV than its amount of fuel. 

I suggest installing a mod that will tell you this, like MechJeb or KER. 

[Kryxal]

Are you just looking to get to Duna and orbit, or land there?  Also, are you making use of that monoprop?

[Spricigo]

As @Foxster pointed is more useful to talk  about deltaV.

In any case, I' think that craft, with 1200unit of LF in LKO,  can reach Duna and maybe get back to orbit from the surface, at that point it will be stranded.

 

 

 

[Zhetaan]

Echoing @Foxster, delta-V is going to say a lot more about this craft's capability than the amount of fuel in the tank.  Lacking a dV readout from a mod, if you can tell what the mass of the vehicle is (and the amount of fuel in it when you report the mass), then we can calculate the dV from that.

I would strongly suggest that you get rid of the monopropellant and attitude jets in favour of reaction wheels for attitude control.  If this vessel's job is to go to Duna, then you will have better chances if you focus on just doing that.  If you put the monopropellant where it's needed, such as in a station, keep the docking port for space-based refuelling or crew rotation capability, and dock with a station the mass-conscious way (with a station-based tug), then you may find it will help you shave away mass that you really don't need to carry with you.

[Mr. Peabody]

2 hours ago, Kryxal said:

Are you just looking to get to Duna and orbit, or land there?  Also, are you making use of that monoprop?

I took this SSTO to Minmus and back, so i was using the mono propellant at the time the screenshot was taken.

18 minutes ago, Spricigo said:

As @Foxster pointed is more useful to talk  about deltaV.

In any case, I' think that craft, with 1200unit of LF in LKO,  can reach Duna and maybe get back to orbit from the surface, at that point it will be stranded.

 

 

 

Thanks. I do have mechjeb, so next time I am in the game I can find the deltaV.

[AeroGav]

3 hours ago, The Dunatian said:

I've been working on SSTO spacecraft recently with the goal of building one that can reach Duna and back.  

It can reach LKO with 1,200 units of liquid fuel remaining for the nuclear engine. So my question is, is that enough fuel to reach Duna and get back or do I need to make more adjustments to the design? If someone could let me know I would be appreciative and it would spare me having to experiment myself and save me some precious time.

That screenshot is so dark i can barely make anything out ?  How about sharing the craft file?

I have a spreadsheet that can calculate your delta V so long as you input the mass of the craft (from the info tab in the M (manuver node plotting) screen)  and the units of fuel remaining on fuel guage

https://www.dropbox.com/s/g9htaev3jv323c5/delta v.xlsx?dl=0

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Since it was taken I have removed the drag inducing ailerons and made a few small adjustments like more batteries.

Wings and control surfaces produce hardly any drag at all, when they are at a small angle to the wind.   I have enabled the aero data display in the ALT F12 menu and you can see that those stowable solar panels, even when stowed, produce 40% of the drag of the Big S wing (the second largest wing part in game).   Most of the drag is from fuselage.    Rather than eliminating control surfaces,  right click on them and reduce the authority slider, so they deflect at smaller angles and you can control your craft to fly at smaller angles relative to prograde.  As regards pitch,  you can reduce drag a lot by making the craft well balanced full vs empty so it requires less control input to hold the nose up.

 

 

[Mr. Peabody]

1 hour ago, AeroGav said:

That screenshot is so dark i can barely make anything out ?  How about sharing the craft file?

I have a spreadsheet that can calculate your delta V so long as you input the mass of the craft (from the info tab in the M (manuver node plotting) screen)  and the units of fuel remaining on fuel guage

https://www.dropbox.com/s/g9htaev3jv323c5/delta v.xlsx?dl=0

Wings and control surfaces produce hardly any drag at all, when they are at a small angle to the wind.   I have enabled the aero data display in the ALT F12 menu and you can see that those stowable solar panels, even when stowed, produce 40% of the drag of the Big S wing (the second largest wing part in game).   Most of the drag is from fuselage.    Rather than eliminating control surfaces,  right click on them and reduce the authority slider, so they deflect at smaller angles and you can control your craft to fly at smaller angles relative to prograde.  As regards pitch,  you can reduce drag a lot by making the craft well balanced full vs empty so it requires less control input to hold the nose up.

 

 

Thanks for the tips. I will try that with the ailerons. I never really wanted to remove them anyway. (They make the plane look a lot better) The SSTO is pretty well balanced already. 

[bewing]

To get to Duna and back with an SSTO, you need a pretty highly optimized spaceplane. Just on the face of it, yours just doesn't look optimized enough. Too much pretty stuff on it. So I'd bet Spricigo is right -- you may be able to get to Duna, and there's a slim chance you can even get back to LDO. But I'd bet you can't get any further than that. Heavy cockpit, extra aero surfaces, RCS blocks, very draggy solar panels, deadweight RAPIERs.

[Geschosskopf]

On 10/31/2017 at 11:22 AM, The Dunatian said:

It can reach LKO with 1,200 units of liquid fuel remaining for the nuclear engine. So my question is, is that enough fuel to reach Duna and get back or do I need to make more adjustments to the design? If someone could let me know I would be appreciative and it would spare me having to experiment myself and save me some precious time.

This type of craft at Duna functions essentially as a pure rocket.  The wings are too small for the mass it has to provide any noticeable lift in Duna's thin air; all they do is provide stabilization.  The plus side is, Duna's air is so thin that LV-Ns work fine at the surface.   But what you need is enough LV-N dV to reach the surface and, if you don't refuel once you're down, get back to orbit again.  That's shown on the dV maps.

The main problem you'll have is landing (takeoff is not an issue).  You have a choice.   You can either come down tail-first like a rocket lander or you can come in horizontally like a plane.  Tail-first, you slow to a hover just above the ground, then cut the engine, rotate 90^, and drop onto your wheels.  Not a problem in Duna's low gravity, but you need a TWR at Duna > 1.

Horizontal landing is more complicated.  Because of your tiny, Kerbin-friendly wings, you'll have to maintain huge horizontal speed right down to the ground at Duna to avoid getting too much vertical speed from lack of lift.  Neither air- nor wheel brakes work worth a damn at Duna due to thin air and low gravity, so the only way to stop once down is with serious retro trust.  And the only places you can land are at the bottoms of a couple of wide depressions which are the only places with any lengthy sections of RELATIVELY flat terrain on the whole planet.

If you want to land horizontally anywhere else, you'll need mondo retro thrust to kill your horizontal speed so you can stop in the short spaces of flat terrain available out in the dune fields that are 80% of Duna's surface.  But getting that slow means your wings are doing nothing at all, so you'll also need some good VTOL engines on the bottom to hold you up at low speed.

Or, you can just have a bunch of parachutes, and maybe some VTOL engines on the bottom if you don't have quite enough chutes, and land that way, avoiding all the problems noted above.

Edited November 4, 2017 by Geschosskopf

[AeroGav]

20 hours ago, Geschosskopf said:

 You can either come down tail-first like a rocket lander or you can come in horizontally like a plane.  Tail-first, you slow to a hover just above the ground, then cut the engine, rotate 90^, and drop onto your wheels.  Not a problem in Duna's low gravity, but you need a TWR at Duna > 1.

Having the TWR is easy enough but control can be tough,  plane's are aerodynamically stable going forwards and really aren't happy going backwards for a tail sitting landing.  The transition could be tricky,  and since you'll need to do it quite high up, could use a fair bit of fuel.

 

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Horizontal landing is more complicated.  Because of your tiny, Kerbin-friendly wings, you'll have to maintain huge horizontal speed right down to the ground at Duna to avoid getting too much vertical speed from lack of lift. 

 

Not so much Kerbin-friendly as Kerbin-sufficient or Kerbin-adequate.      There is actually very little kerbin performance penalty to adding more wing area,  and I would recommend upgrading to Big S wing and strake parts wherever possible. 

 Something like my STOL challenge airplane  can land on Duna with wing lift alone but that's extreme.  https://kerbalx.com/AeroGav/stol

Lift jets are probably easier to work with , but thanks to low gravity all you really need is one Vernier motor per 7 tons of mass or so to bring your landing speed right down.   The more wing area, and the slower the pure aerodynamic stall speed,  the less oxidizer the landing will need.

I don't think stopping distance is particularly an issue - you have a whole planet to stop with after all,  and to be honest, no part is much less hilly than any other.    Just make sure you're landing at a low altitude biome where the speeds are merely scary, as opposed to ludicrous.     However,  a braking parachute helps to keep the craft going prograde when it inevitably hits uneven terrain during the rollout.  The drag keeps the plane going prograde better than reaction wheels , nosewheel steering and desperate rudder applications can. 

Most  of my landings come to grief this way - one wheel hits a bump, plane gets sideways, wingtip digs in, usually crew survive but airplane in pieces.

 Here's some footage of Val testing out the crashworthiness of one of my designs

 

[Geschosskopf]

Just now, AeroGav said:

Having the TWR is easy enough but control can be tough,  plane's are aerodynamically stable going forwards and really aren't happy going backwards for a tail sitting landing.  The transition could be tricky,  and since you'll need to do it quite high up, could use a fair bit of fuel.

Or you can just parachute the thing to the landing.

 

Just now, AeroGav said:

Not so much Kerbin-friendly as Kerbin-sufficient or Kerbin-adequate.      There is actually very little kerbin performance penalty to adding more wing area,  and I would recommend upgrading to Big S wing and strake parts wherever possible. 

Actually flying with wings on Duna isn't all that hard, you just need a very low wing loading and some sort of engine that doesn't need free-range O2.  The real problems are:

1. Getting anything that can actually fly on Duna off Kerbin's surface.  The wing area needed for the requisite low wing loading tend to big rather too large for launching as a spaceplane or shuttle.  The best way I've found to handle this is the back-asswards SSTO rocket where the Duna plane (or some even multiple of them) are the tail fins of what is in effect a gigantic skycrane lifting off Kerbin.
2. Slowing down enough to land in the dune fields where "runways" are only 1-2km AT MOST.  Again, you can avoid this problem with sufficient chutes but if you want to actually fly the thing down, you need a stall speed ON DUNA at an altitude there of 2-3km of about 30m/s, AND retrothrust on top of that because airbrakes and wheel brakes don't really do anything there.

 

Just now, AeroGav said:

 I don't think stopping distance is particularly an issue - you have a whole planet to stop with after all,  and to be honest, no part is much less hilly than any other. 

You must play with a very low terrain detail setting.  On high terrain detail, the longest stretch of anything remotely approaching reasonably flat ground on about 80% of Duna's surface is about 1km, 2km in rare instances.  These occasional flat spots are surrounded by steep dunes on all sides.  If you can't stop in time, you do a face-plant on the next upslope.  The only exceptions are in the bigger depressions, where you might have 20km of relatively flat ground.

 

Just now, AeroGav said:

 Just make sure you're landing at a low altitude biome where the speeds are merely scary, as opposed to ludicrous.

But then you're stuck seeing only about 10% of Duna.  If you can land (and take off again) from anywhere except the steepest mountains, you can actually enjoy the scenery.

Granted, flying on Duna since the Great Atmosphere Change is WAY easier than it was with the so-called "Soup-o-Sphere".  Back in the old days, it was actually a hard thing to do.  See this old thread, especially the last page denoting the differences then and now.  Nowadays, going to Duna without the expectation of flying all over it is just as ludicrous as going to Laythe without an airplane.  But you STILL need an on-Duna stall speed of about 30m/s at 2-3km and lots of retrothrust.  It's just that these stats are a LOT easier to achieve these days.

[AeroGav]

3 minutes ago, Geschosskopf said:

You must play with a very low terrain detail setting.  On high terrain detail, the longest stretch of anything remotely approaching reasonably flat ground on about 80% of Duna's surface is about 1km, 2km in rare instances.  These occasional flat spots are surrounded by steep dunes on all sides.  If you can't stop in time, you do a face-plant on the next upslope.  The only exceptions are in the bigger depressions, where you might have 20km of relatively flat ground.

Not at all, see my video.  The terrain is so bumpy and any relatively flat bits so short in comparison the distance from which you can see them, and your turning radius, it is pointless even trying to find them.   Just try to control forward speed and rate of descent, keep the wings level and try to keep her going straight after touchdown.   The terrain's gonna do what it's gonna do - it's like the level 1 runway.

6 minutes ago, Geschosskopf said:

Getting anything that can actually fly on Duna off Kerbin's surface.  The wing area needed for the requisite low wing loading tend to big rather too large for launching as a spaceplane or shuttle.  The best way I've found to handle this is the back-asswards SSTO rocket where the Duna plane (or some even multiple of them) are the tail fins of what is in effect a gigantic skycrane lifting off Kerbin.

The large wings (like on the craft in my video) would certainly be a problem for launching dreamchaser style !  

They do add mass to the transfer burns too, but most delta v is used getting off kerbin and  with nuke engines and jets and all the other junk a ssto has to drag there and back it gets lost in the noise somewhat.     If the wings have incidence it can be a problem having too much lift on kerbin, you bounce out of the atmosphere before you've hit the top speed of your jet engine.    Once you're on closed cycle thrust, it actually helps with your lift:drag ratio in the upper atmosphere.    Granted , you only really notice that on an oxidizer-less design that relies on nukes alone above 25km.   With Dual RAPIERs the OP will have over 6 times the thrust of my craft.

Best thing for a horizontal landings, i'm sure we can all agree

• generous (but not ridiculous) wing area
• vernier lift engines -  can go in service bays when not needed
• drag chute to keep it going straight on rollout
• good landing gear design that makes the airplane stable as an automobile.  Understeer i think they call it.       I suck at this.  I cannnot build a rover that can drive from one end of the space centre to the other without killing every last one of my Kerbals.

On an unrelated note, i'd pack a tiny IRSU setup rather than return fuel.  Given the dry mass of a spaceplane's stuff, it's less noticeable than on a capsule mission.   You've got to wait for a transfer window anyway,  being able to refuel gives you flexibility.

[Geschosskopf]

34 minutes ago, AeroGav said:

Not at all, see my video.  The terrain is so bumpy and any relatively flat bits so short in comparison the distance from which you can see them, and your turning radius, it is pointless even trying to find them.   Just try to control forward speed and rate of descent, keep the wings level and try to keep her going straight after touchdown.   The terrain's gonna do what it's gonna do - it's like the level 1 runway.

Hehehe, we differ substantially in our philosophies of approaching the problem, but we're both able to fly on Duna.  Which even today is still a badge of honor, albeit a bit tarnished , even degraded from gold to bronze.  I salute you, fellow Duna aviator

My approach is, if you've got a stall speed on Duna of 30m/s at 2-3km ASL, you can circle around the vicinity of where you want to land, pick your spot, and land there STOL-wise.  You still need retrothrust to stop in the available space, of course, but if you can do this, you OWN Duna, because these days such a plane can also usually circumnavigate Duna 2-3 times on internal fuel at supercruise bumping the bottom edge of space,  If it also had even a Spark LFO engine, needed only for circularization in vacuum, it could serve indefinitely as a Duna SSTO to anywhere to or from Duna's surface, provided you had a mining base on the surface.

Of course, such a plane is in no way capable of SSTOing off Kerbin's surface, which is what the OP is asking about.  An SSTO spaceplane that can reach space from Kerbin is essentially just a rocket on Duna, and penalized by carrying both useless air-breathing engines and having too little wing to be of any real use on Duna.  An SSTO spaceplane that can reach space from Duna AND still actually fly around there as an airplane cannot, by any stretch of the imagination, SSTO from Kerbin.

So my advice to the OP remains the same:  if you want to take off from the KSC runway, land on Duna, and return, you need to treat in-atmosphere portion of the mission at Duna as rocketry, not airplanetry.  If landing horizontally doesn't work, just cover the top with chutes and/or cover the bottom with Vernors, or some point of balance in between, and land that way.  Once down, you can take off horizontally on LV-Ns, but your wings do little, if any, good.

 

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Best thing for a horizontal landings, i'm sure we can all agree

• generous (but not ridiculous) wing area
• vernier lift engines -  can go in service bays when not needed
• drag chute to keep it going straight on rollout
• good landing gear design that makes the airplane stable as an automobile.  Understeer i think they call it.       I suck at this.  I cannnot build a rover that can drive from one end of the space centre to the other without killing every last one of my Kerbals.

if you have sufficient wing area for STOL performance in the dune fields, you need no chutes or Vernors.  But you still need retrothrust because even a touchdown speed of 30m/s at 2-3km altitude on Duna still means that neither air- nor wheel brakes work enough to save you in the space available.  So the real way to cut down on landing run is to use full canopy, not droge, chutes.  Of course, you'll then need an engineer skilled enough to repack them repeatedly.

A lot of torgue is also required, to keep you from flipping when the mondo landing gear hit bumps.  I find that a tail-dragger configuration works better than tricycle.

 

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On an unrelated note, i'd pack a tiny IRSU setup rather than return fuel.  Given the dry mass of a spaceplane's stuff, it's less noticeable than on a capsule mission.   You've got to wait for a transfer window anyway,  being able to refuel gives you flexibility.

If you use electric props from some mod, you can circumnavigate Duna a couple times on a small LFO tank and fuel cells, via supercruise..  And this powerplant can get you into space, after which you need just a nudge of an LFO rocket engine to stay there.