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digger1213

Mining SSTOs

Question

I have struggled for quite a long time to create a mining SSTO that could get itself to Laythe, and fly around... until I can be bothered to bring the Kerbals back. The process has resulted in many failures, all looking relatively like this one:

4inuAr7.jpg

I usually run into one of three or so problems.

1: Not enough TWR to reach orbit.

2: Not enough DV to reach orbit.

3. Flops around on the runway until it blows up. (Or fails to take off...)

I usually find it too heavy to take off, so I add more wings. I then find it too heavy for the jet engines to effectively carry, struggling to make orbit, or unable to get fast enough.

If I do manage to ever get it into orbit, I do not have enough TWR to go anywhere.

I have been inspired by the SSTO in this Reddit post but even when replicating it as accurately as possible I find it impossible to takeoff the runway, or otherwise orbital TWR is less than 0.1 which I find unacceptable (yet still without enough Delta-V to get anywhere). Should I try using the smaller refinery and drills? Is there anything I can do to try and improve my designs?
Thanks.

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Utilize Big-S delta wings that are both wings and fuel tanks.

Put all those surface amount stuff into cargo bay if possible - they produce a lot of drag, they really do.

Too heavy to take off is not a problem of too little wing - usually people really mean it's hard to rotate, and it's usually a problem of main gear too far away from CoM (and CoL behind main gear will make things worse). Double check if it's what's happening.

And btw, to reach Jool I don't think you want to use LFO engines unless you really design for it. Even if you allow yourself to stop by Mun/Minmus to mine/refuel before leaving for Jool, it is still hell a lot of delta V. Think about it - if you have minimum 0.1TWR requirement, that's about 1m/s^2 accleration minimum. So with 2 nukes, they produce 120kN, so your maximum plane mass is 120t. 2 nukes only weigh 6t, you still have 114t to play around. You can even just substitute your current Poodle and a few LFO tanks with 2 nukes and all LF tanks and you can still have 0.1 TWR. It's a good start, better than achieve nothing.

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Posted (edited)

Yes, downsize the ISRU. Also add nukes, maximise your LF tankage and minimise drag. Add a bit of wing incidence to minimise fuselage drag, use the fuel-holding Big-S wings.

Something like this:

https://kerbalx.com/Wanderfound/Kerbodyne-Aqua

5UxKfup.jpg

4KKsy6A.jpg

agtMRiG.jpg

About 4,000m/s dV fully fuelled. Note the fuel proportions: just enough oxidiser to lift an apoapsis, LF and the nukes for the rest.

The seaplane ability and docking port are optional.

When building these, you need to take care that the drills are properly shielded by the cargo bay.

Right-click the drills and check their shielded state (activate aero data in the cheat menu) to be sure. There's a bug that sometimes causes the game to treat objects as unshielded even when they appear to be contained by the bay.

Edited by Wanderfound

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Posted (edited)

On 2017-5-15 at 0:28 PM, digger1213 said:

I have struggled for quite a long time to create a mining SSTO that could get itself to Laythe, and fly around... 

4inuAr7.jpg

I usually run into one of three or so problems.

1: Not enough TWR to reach orbit.

2: Not enough DV to reach orbit.

3. Flops around on the runway until it blows up. (Or fails to take off...)

Getting to orbit involves accelerating to 2200 m/s.    Engines, thrusting horizontally, provide the power to do this.      As you get closer to orbital velocity, orbital free fall effect supports more and more of your vessel's weight against gravity,  but until you get there the shortfall must be made up for with lift.

Drag is the enemy,  it's the main thing you want to minimise.    A low drag design can get through the sound barrier with one jet engine per 30 tons of takeoff weight,  and 60kn of closed cycle thrust per 15 tons (ie one nuke or terrier per 15 tons).  For oxidizer, you only really need 200 units or so per 40/50 tons if you've got one nuke per 40 tons.   If you've got one nuke per 15 tons or so, you can actually forget oxidizer altogether and go liquid fuel only.

Jet engines are very heavy and a draggy airplane will need a lot of them to get hypersonic. this adds loads of dry mass and cuts delta V.   I suspect your main problem is drag because I  can see potential problems from that one picture.   For us to fully diagnose it, show more angles or better yet share the craft file 

  • The main fuselage is a 2.5m stack, but it appears to end abruptly in a flat plate.  This creates huge drag.   There should be a 2.5m tail cone on the back or series of adapters tapering the back end down gradually to a smaller cone.    Looks like you got some side stacks of FT-800 tanks (the black and white stripey things).  Do they all end with cones or are there more flat faces poking the air stream?
  • Lots of stuff hanging off the outside.   I see drills (!) and radiators for a start.  I bet you got four way thruster blocks too?     These things should be inside a service bay or cargo compartment, and only be exposed after landing or while in space.
  • Pointy headed mk1 cockpit - because it is at the front it will tend to suffer overheating, you are better off with inline cockpit since the most efficient flight profiles involve flying at 1400m/s + on jet power.
  • Far too much oxidizer

As regards the mining setup,  you've gone for the big drill and converter.   They are more than 10x as efficient but also much heavier, which pushes the size of the craft up by a lot.    One thing to remember is that the rate at which it mines doesn't really affect you very much, since you can go into a high rate of time warp once you've started and be done in a minute or two either way.    What does matter is the craft having enough power to keep mining overnight.   If it runs out of juice the drills shut down and have to be manually restarted each morning, which gets seriously annoying.   Either bring a large amount of batteries or use RTG as your power source.

Also, those large radiators are useless.   They only pull heat from the part they are attached to , and won't stop the drills overheating.  Use the deployable thermal control system radiators instead.  Much easier to stow in a cargo bay too.

Building for low drag

Even if you build a craft that looks like a skinny glider, you'll find that in KSP > 80% of the drag is coming from the fuselage, not the wings.

  • Every stack must begin and end with something pointy.    Pointy = nose cone, air intake, or jet engine.
  • When joining parts via attachment nodes , both nodes must be of same diameter.   Eg.  Joining a 2.5m tank to a 1.25m tank = bad, high drag.  Joining a 2.5m tank to the 2.5m node at the fat end of a 2.5m to 1.25m adapter = good !
  • Keep stuff as  close to prograde as possible.   You should have enough wing that your spaceplane can get lift by flying with a nose angle only  5 degrees above prograde, at an altitude of 7km and a speed of 240m/s.     If it can't do that, you can either add more wing, or try angling the wings up a few degrees where they attach to the fuselage   The latter technique can be tricky to get right however.

Also, you need to be aware that some parts are better than others for drag.

Mk2 fuselage parts are terrible,  despite their sleek appearance, they have 2 or 3 times more drag than mk1.

Mk3 are similar to mk1 in terms of drag to fuel volume ratio.

2.5m are the best, having hardly any more drag than mk1 but much larger capacity.  However, they are fragile and there are no 2.5m liquid fuel tanks.

Perhaps make a craft with a 2.5m main fuselage?  Hitchhiker module, mk2 lander can (for control pod), 2.5m converter, and some service bays?    Keep your liquid fuel in the engine pods, big S wings and strakes?

Final drag saving tip - this is a minor one compared to the others, but remember what i said about every stack ending in something pointy?  The problem is that rocket engines, and RAPIERS, with rear attach nodes create much more drag than jet engines like panthers and whiplashes that do not.  The issue is that the game sees the unused rear attach node of the engine, and assumes your stack is ending with a flat face.    To get around this, you can put a cone on the back of the engine.  However, the collider that checks for blocked nozzles will see this and stop the engine working.  Solution?  Use the offset tool to move the cone so it's hidden inside the engine, that way the nozzle is not blocked.

https://kerbalx.com/AeroGav/SSTL-gavd

 

20170115153818_1_zpslntslp8n.jpg

 

The above ship is 60 tons, has 2 rapiers and 2 nukes.  Note that nearly all fuel is in the wings and mk1 tanks.  The main fuselage is all crew and cargo bay  (the rear cargo bay is upside down so the drills don't have to poke through the floor).   The only fuel in the mk2 sections are the LF/O in the bicoupler at the rear of the fuselage and the mk2 to mk1 adapter in the front. This is also the only oxidizer tankage on the whole ship.  Reaches kerbin orbit with 4000dV , so a  straight shot to Laythe is possible.

 

 

 

 

Edited by AeroGav
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Sorry for the extremely late reply. I have not had time to test yet, but I will try to minimize drag. I did try to fit the drills in a service bay originally, but they were way too big. Should I try an MK3 cargo bay facing downwards? I can't try these ideas right now, but I will try to get back when I can.

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3 hours ago, digger1213 said:

Sorry for the extremely late reply. I have not had time to test yet, but I will try to minimize drag. I did try to fit the drills in a service bay originally, but they were way too big. Should I try an MK3 cargo bay facing downwards? I can't try these ideas right now, but I will try to get back when I can.

You can get a big drill inside the 2.5m service bay if you're careful with the offset tool.

However, i'm not sure a Laythe miner will benefit from big drill and converter.   From what i remember, these things give you much more ore per unit of fuel, and the big drill has a much better mining rate.   The problem is you're limited by electric charge out on Laythe due to its distance from the sun.   So, out there you might be better off with just one small drill and one small converter, and a small thermal control system to stop it overheating.   Once into a high rate of time warp electric consumption decreases, and 4 x RTGs can keep up, but you'll need a few batteries to handle the transition.

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10 hours ago, AeroGav said:

You can get a big drill inside the 2.5m service bay if you're careful with the offset tool.

However, i'm not sure a Laythe miner will benefit from big drill and converter.   From what i remember, these things give you much more ore per unit of fuel, and the big drill has a much better mining rate.   The problem is you're limited by electric charge out on Laythe due to its distance from the sun.   So, out there you might be better off with just one small drill and one small converter, and a small thermal control system to stop it overheating.   Once into a high rate of time warp electric consumption decreases, and 4 x RTGs can keep up, but you'll need a few batteries to handle the transition.

Alternatively, use a fuel cell array. One of those can power just one big drill plus a refinery while still leaving you with a fuel surplus, provided you have a 5 star engineer. You also won't timewarp nearly as long or spend anywhere close to the amount of money 4 RTGs require. :wink: 

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3 hours ago, Wadusher0 said:

Alternatively, use a fuel cell array. One of those can power just one big drill plus a refinery while still leaving you with a fuel surplus, provided you have a 5 star engineer. You also won't timewarp nearly as long or spend anywhere close to the amount of money 4 RTGs require. :wink: 

Well,  the RTGs are coming back to Kerbin so money is not an issue.      With the fuel cell, you're going to want to more efficient big converter or you won't come out ahead, correct?

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1 hour ago, AeroGav said:

Well,  the RTGs are coming back to Kerbin so money is not an issue.      With the fuel cell, you're going to want to more efficient big converter or you won't come out ahead, correct?

2 small fuel cells easily hold up the 1.25m ISRU, given you have a 3star engineer and a single large drill.

Also instead of using a 2.5 service bay, maybe the 2.5m payload fairing could be useful (if OP is ok with clipping). I believe fairings have less drag and also less mass(relative to a similar size cargo-bay)?

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