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MPDerksen

Double Mun orbit rescue ship help

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1254cuf.jpg

so here is the ship I used to rescue 2 derelicts from LKO.  I now have a similar mission(s) with a pair in nearly identical Mun orbits.  Ap, 13.600Km, Pe, 10,500Km.
I'm still far from the best at maximizing my fuel to get into orbit, certainly.  As it sits, I can get both rescued, but then run short on getting home.  Perhaps simply add 2 more primary boosters to make it 6?  I also read something about "asparagus staging"?  so I ditch 2 at a time?  

Second "issue" is that once I ditch the outer boosters, the main rocket is a bit difficult to rotate.  Am I a bit shy on reaction wheels and need another?

TIA,
Michael

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On 2/27/2019 at 6:13 AM, Zhetaan said:

You can do that.  I doubt you need that much thrust, though.  Consider adding a fuel tank to the core stack before you add more engines.  A general rule is to have enough fuel in your stages that they burn for about two minutes each, though that rule has to be relaxed a bit for asparagus staging, which brings me to the next point.

Here's a free sketch of what asparagus staging looks like, from the bottom of the rocket:


Fuel lines (flow direction shown with arrows):
       B2 <---- B3
      /
     /
    B1 ---> C <--- B1
                   /
                  /
       B3 ----> B2


Good luck!

Totally started from scratch using your (and others') advice.  I also re-watched a S. Manley video on efficiently getting to orbit.  The below design got Jeb down onto Minmus, and home with over 1,000dV to spare.  I could use a smaller lander fuel tank for sure in the future.  I ended up getting over 600 science by doing EVA over various biomes, surface sample, flag, etc.  I even landed back home in the desert for a new Kerbin biome science.  Jeb got his second star, and I got to unlock some fun stuff.

Next big mission is to rescue a stranded Kerbal from the backside of the Mun.  I'm thinking of replacing the storage box on this design with a simple lander can so she has a seat.  But just a quick question about that.  If I mount a thermometer on the outside of the fuel tank, and jettison it before I land, I lose the science, right?  Unless I transmit, I have to actually return the instrument to get the points?

Last, since I'm likely to use far more fuel than I should to land on the Mun, should I keep the design as is?  or select a smaller tank?

rkn79v.jpg

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1 hour ago, Aegolius13 said:

I wholeheartedly reject that scandalous allegation.  As the record clearly indicates, I stated:

 I remain a member in good standing of the Order of MOAR BOOSTERS.

Elon Musk would agree with you. MOAR BOOSTERS

SpaceX-Falcon-Heavy-first-photos-677x102

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The key to asparagus staging is plumbing together your boosters with fuel lines (or crossfeed), so that all your engines are draining from the outermost engines, then you drop them, then everything drains from the next set. So in this rocket you'd have lines going from one pair of outer boosters to the other pair, and then from the second pair to the core.

Going to six boosters is not a bad idea, particularly if you asparagus-stage them. 

Other delta-v enhancing ideas:

-Your upper stage looks a little low on fuel; maybe add a little more here?  If you're close to orbit at this point, you don't need much TWR.

-You could also consider adding an extra stage at the top, with a Spark and a small amount of fuel.  

-You're probably generating quite a bit of drag with those lander cans, and (to a lesser extent) the batteries and solar panels.  You could consider putting a fairing around your upper stage.  You could also look at alternatives to the landing cans, depending on what parts they have available.  They're intentionally bad at atmospheric stuff (draggy and not very heat-resistant).  

-Are those Swivels or Reliants on the bottom?   I find it's pretty effective to use Reliants on the outer boosters and a Swivel on the core stage.  You get lots of thrust from the Reliants (meaning you can put more fuel above them than with a Swivel), better upper-atmospheric ISP from the Swivel once you lose the boosters, and enough control authority to make everything work.

-You can also consider upgrading to 1.875 (if you have Making History) or 2.5m parts. 

 

Regarding rotation, what are the conditions at time you're having the problem?  E.g., what is your approximate height, speed, and are you continuing to burn the core engine?  If Swivels, you should have more than enough control authority.  Even if the center stack is a Reliant, your fins would likely be enough. If anything, I'd expect this rocket to be unstable since those lander cans are causing a lot of drag per above. I would think the 3 pods have plenty of reaction wheel power in vacuum, but they're not really good in atmosphere 

 

-----

Finally, shameless plug for a guide I recently posted, which might have some helpful thoughts on these kind of issues.

 

 

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As @Aegolius13 says, an alternative to the two lander cans would be a good idea.
By far the best solution - from a weight and aerodynamic perspective - is to use a crew cabin (1.25m, 0.5 tonnes per occupant) instead. The disadvantage is that you then have only one way in to the vessel (via the top pod) and will need to shuffle crew around inside and on EVA to get the rescuees in, and you no longer have the lander cans' reaction wheels to help manoeuvering (which also means you absolutely must keep perfect retrograde hold on "surface mode" on re-entry).

Agreed also for the asparagus staging: place and stage the boosters in pairs, with fuel lines from the first pair to drop to the second, then from the second to the core.
An alternative that might get you further is to replace one pair or both pairs with SRBs. You can still add small fuel tanks to the top and keep the fuel lines, so that when the SRBs are ready to drop, their associated tanks have emptied. It's a bit tricky to time without using mods, but as an example one Reliant will empty one FL-T400 in almost exactly the time it takes for one Hammer to burn out at 95% thrust...

Since you have steering fins, you could also get away with replacing the Swivel with a Reliant and an additional small tank on the core.
And you could replace those fins with elevons (elevon 1) to reduce the weight a touch.
Also there is no need for two struts per booster on that size of craft. I've found that one strut is fine until you get really massive and complicated boosters (or specifically, for inner boosters where they themselves have outer boosters strutted to them).

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Switching the staging and fuel connections might be enough, if it's not I'm pretty sure adding extra stacks WILL be.  I don't like the pair of landing cans myself, the cabin's a better option ... up until you possibly flip.  If you go with the cabin, you need a heat shield (the weight actually helps your alignment), and if you end up flipping anyway, align PROGRADE and pitch up to use body lift to slow descent, because you're not getting back retrograde.

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8 hours ago, MPDerksen said:

Perhaps simply add 2 more primary boosters to make it 6?

You can do that.  I doubt you need that much thrust, though.  Consider adding a fuel tank to the core stack before you add more engines.  A general rule is to have enough fuel in your stages that they burn for about two minutes each, though that rule has to be relaxed a bit for asparagus staging, which brings me to the next point.

8 hours ago, MPDerksen said:

I also read something about "asparagus staging"?  so I ditch 2 at a time?

Here's a free sketch of what asparagus staging looks like, from the bottom of the rocket:

Fuel lines (flow direction shown with arrows):
       B2 <---- B3
      /
     /
    B1 ---> C <--- B1
                   /
                  /
       B3 ----> B2

Usually, all of the stacks are attached to the core with decouplers, but the fuel lines run in the order shown from the first boosters to be staged (in this rocket, the B3 boosters) to the next boosters (B2) to the next boosters (B1) to the core stack (C).  You're running a rocket that only has two sets of outer boosters, but the principle is the same.  What makes asparagus staging attractive (and what gives it improved delta-V) is that it sets up the fuel flow so that all of the engines (in the example, all seven; in your rocket, all five) use the fuel from the outermost boosters first.  This means that those boosters drain at lightning speed, but once they are empty and staged away, you're left with a fully-fuelled rocket.  Now the remaining engines drain the next set of outermost boosters, but since there are fewer engines, it takes longer, and when those boosters are empty, they are staged away, leaving a fully-fuelled rocket ... and so on.  In the end, when you get down to the core stack, it's full of fuel, has an appreciable downrange velocity, and is likely fairly high in the atmosphere.

To make it, you have to attach the boosters in pairs, for balance.  In theory, you could attach quads and have a valid asparagus arrangement that way, but that gets a bit crowded and it's too much for rockets that have only as many as six boosters.  Personally, I simply make a booster, attach it with 2x symmetry, and then copy it.  I only add the fuel lines after I've fixed the staging (you have to be very careful to get the staging correct, or else you'll stage away half of your fuel).  Also remember to put all of the engines in the bottom stage:  they need to ignite all at once.

8 hours ago, MPDerksen said:

Second "issue" is that once I ditch the outer boosters, the main rocket is a bit difficult to rotate.  Am I a bit shy on reaction wheels and need another?

You can always add a reaction wheel, but I think your problem can be solved with a different approach to control authority.  Others have said to switch your fins--and that is good advice; the Delta-Deluxe fins you're using now only have 20% control surface--so pick something such as the AV-R8 Winglets, which are 95%, or the suggested Elevon 1, which is 100% and cheaper than either the AV-R8 or the Delta-Deluxe.  Others have also said to change your central Swivel engine for a Reliant, and that is possibly good advice, but not necessarily.  If you stage away your core stack and make orbit on your upper stage, then yes, do that, because you will only need the extra rotational control when you are in the atmosphere and the fins are working for you.  If you use the core stack to make orbit and possibly also as a transfer stage to get to the Mun, then keep the Swivel because in space, fins are decoration.  Also, the Swivel is slightly more efficient in space than the Reliant (only ten seconds, but why not?).

Using SRBs instead of liquid boosters is a valid approach but you can't get asparagus efficiency out of boosters that can't share fuel.  On the other hand, SRBs provide cheap thrust for what they cost.  It's worth exploring.  There's also nothing stopping you from using strap-on boosters with enough kick to get your rocket in the air with the main engines off, and relying only on fins for attitude control until you drop the spent boosters and ignite the main engines--this gives you a sort of 'poor man's asparagus', so feel free to try it.  The booster-with-an-LFO-tank-on-top idea is a good one, as well.

Another trick to use is to set the fuel flow priority on your stacks so that the fuel drains from the bottom up, rather than the top down.  This has the benefit of pushing the centre of mass forward as the tanks drain, which gives your fins (and Swivel if you keep it) more lever arm, which gives more control.  To do it, set the fuel flow priority (I believe you need Advanced Tweakables, which is in the difficulty settings, turned on for this) on the lower tanks to a higher number than the upper tanks.  Fuel drains from higher priority numbers first, so you want the numbers to increase as you go farther out from the core stack.  When you set the staging, the game usually advances the fuel flow priority by ten for each stage (to give you some room to tweak), but sometimes it gets confused with asparagus staging, so be certain to check.  In my example above, the core would start at priority ten (the upper stage is normally priority zero), and the boosters would be twenty, thirty, and forty, respectively.  Leave the upper tanks as they come but set the bottom fuel tank in each stack to priority eleven, twenty-one, thirty-one, and so forth.  If you have three tanks in a booster, then the middle tank is twenty-one and the bottom is twenty-two, and so on.

That being said, I prefer to have a reaction wheel anyway, because I like what I will call a 'snappy' rocket.  You should have plenty of wheel, though, if you're using lander cans.  Be certain that you remove the monopropellant from the pods if you don't intend to use it.  It's not a lot of mass but it is unnecessary.

Also, as @Aegolius13 said, lander cans are not really meant for atmospheric use and are very draggy.  Drag at the nose of the rocket causes control problems, too.  Consider a fairing or the Crew Capsule, as others have suggested.

Struts are also draggy, and as @Plusck said, you don't need two per booster.  The fuel line doubles as a strut (I doubt it's got the same strength, but it still does something) and to be honest, the only thing you need to do for this design is keep the boosters from shaking too much.  One strut is perfectly sufficient for that.

Good luck!

Edited by Zhetaan

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5 minutes ago, MPDerksen said:

Next big mission is to rescue a stranded Kerbal from the backside of the Mun.  I'm thinking of replacing the storage box on this design with a simple lander can so she has a seat.  But just a quick question about that.  If I mount a thermometer on the outside of the fuel tank, and jettison it before I land, I lose the science, right?  Unless I transmit, I have to actually return the instrument to get the points?

Actually, no.  You can EVA and take the data from the instrument.  The data can be stored on a Kerbal, in any crewable pod, or in a special science box (it shows up under the Science tab in the editor).  I once tried a trick where I sent a probe with a full science suite to--I don't remember where, but the probe landed, took a lot of experimental data, and then had the science box plus necessary hardware only return to Kerbin as a sort of rocket courier for a fraction of the mass and cost.  It also left the experiments in the biome so that when I did eventually send a crewed mission, I could reset and reuse the same parts; I just loaded the probe onto a rover and had a fine time.

Of course, if you do jettison the thermometer, then you won't be able to get the temperature data from any new Kerbin biomes when you return.  I generally prefer to keep the small experiments; they're very expensive but also very light, so I try to recoup the cost when I can.  Consider mounting it on the pod next to the hatch (don't obstruct the hatch!); it can't cause drag if it's in a fairing on the way up, and it won't harm anything on the way down.

14 minutes ago, MPDerksen said:

Last, since I'm likely to use far more fuel than I should to land on the Mun, should I keep the design as is?  or select a smaller tank?

If you could land on Minmus and return with 1,000 m/s to spare, then you are close, but should probably add another fuel tank for a Mun landing.  The difference in required delta-V is about 400 - 500 m/s one way, so that rocket doesn't have much safety margin.

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7 minutes ago, Zhetaan said:

If you could land on Minmus and return with 1,000 m/s to spare, then you are close, but should probably add another fuel tank for a Mun landing.  The difference in required delta-V is about 400 - 500 m/s one way, so that rocket doesn't have much safety margin.

Hmm.  Based on the design above, how would I add more fuel?  a few baguettes?  I don't want to make it taller by putting another tank on the bottom.  I've looked at dozens of picture online to get examples, and they seem much smaller, and efficient than mine already.

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While that is all well and good, you're also all forgetting the basic rule of KSP. "When in doubt, add more boosters!" @MPDerksen, I also have a terrible time maximizing my ascent to orbit, but if you add enough dV, unless something falls critically *cough DANG IT mod cough* you should be fine. Oh, and @Aegolius13? Nice plug!

Edited by Kerballing (Got Dunked On)

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1 hour ago, Kerballing (Got Dunked On) said:

While that is all well and good, you're also all forgetting the basic rule of KSP. "When in doubt, add more boosters!" 

I wholeheartedly reject that scandalous allegation.  As the record clearly indicates, I stated:

On 2/26/2019 at 10:29 PM, Aegolius13 said:

Going to six boosters is not a bad idea, particularly if you asparagus-stage them. 

 I remain a member in good standing of the Order of MOAR BOOSTERS.

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1 hour ago, GoatRider said:

Elon Musk would agree with you. MOAR BOOSTERS

I for one am still waiting for the real "Falcon 9, " with three Falcon Heavies bolted together.  Nothing less will suffice. 

 

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So all very interesting,  but I remain clueless how to add more fuel full the above lander....

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Hmm... @MPDerksen do you have any larger fuel tanks? Because if you do, (wait for it!) add the larger fuel tank and then ADD MOAR BOOSTERS to cope for the added weight of the payload! Other than that, I'll be watching to see what the experts come up with. Maybe a detachable top tank of fuel?

Edited by Kerballing (Got Dunked On)

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12 minutes ago, MPDerksen said:

So all very interesting,  but I remain clueless how to add more fuel full the above lander....

Do you have the small grey tanks unlocked? If so, the easiest thing is to add a pair of drop tanks. Add a pair of the small decouplers to the top stage, just under the batteries would be fine. Add a pair of those grey tanks, then pile a couple more on top and under those. If you have the small nosecones unlocked, add them top and bottom. Make sure the decouplers are set to stage after your Terrier fires. Enable crossfeed on the decouplers, then check that the fuel priority on the drop tanks is a higher number than the core tank (or use fuel lines if you aren't using advanced tweakables).

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3 minutes ago, Plusck said:

Do you have the small grey tanks unlocked? If so, the easiest thing is to add a pair of drop tanks. Add a pair of the small decouplers to the top stage, just under the batteries would be fine. Add a pair of those grey tanks, then pile a couple more on top and under those. If you have the small nosecones unlocked, add them top and bottom. Make sure the decouplers are set to stage after your Terrier fires. Enable crossfeed on the decouplers, then check that the fuel priority on the drop tanks is a higher number than the core tank (or use fuel lines if you aren't using advanced tweakables).

OH DANG CALLED IT (to a decent amount of closeness... I said on the top while he said on the side... close enough. It's within the 10% margin recommended for dV... wait wrong thing)

Edited by Kerballing (Got Dunked On)

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mission complete, and both rescued with plenty of fuel to spare.  I used a bigger main and just two SRBs.  Was simple enough once I was willing to part with the original design.  Appreciate the help.

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