FinalFan

How much do you use drop tanks?

How much do you use drop tanks?   

72 members have voted

  1. 1. When, if ever, do you use drop tanks?

    • All the time!
      7
    • Relatively often
      9
    • Sometimes
      21
    • Rarely
      26
    • Never
      9
  2. 2. What best describes the drop tanks you use?

    • Any excuse, any size
      21
    • Go big or go home
      12
    • Relatively small ones
      25
    • Landers, not launchers
      16
    • Launchers, not landers
      11


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There was a challenge thread a while back based around lowest cost per ton of cargo for vertically launched rockets using LFO engines and SRBs. I recall that many of the successful entries had fuel tanks on top of clusters of Kickbacks; the tanks would feed the center core, and were sized so they ran dry at the same moment the SRB clusters would be staged. Sounds like the OP is on the right path, imho.

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4 hours ago, 5thHorseman said:

Hm. I think of it more as a hybrid. The SRBs aren't using that fuel as well as the center engine(s), which to me is a key part to Asparagus staging.

But anyway, I tend to use it as well and have even worked out a trick with KER to figure out how much fuel to bring. It involves tweaking the fuel in the tanks and srbs up and down to see when the time to stage stops changing. Just fiddle and you'll see it.

The only (other) times I use drop tanks are:

  • Long interplanetary burns where I'm only using the engine(s) I want at the end of the trip for the ejection burn, and am okay with the lowered TWR during ejection. This is fairly frequent, especially in career before I've unlocked the whole tree.
  • Landing on big worlds, even Mun in early career but mostly Tylo, Moho, Eeloo and Vall. And Dres if I'm landing an asteroid there but I've only done that the once :D The reasoning here is that you don't need a big TWR at the start of your burn but do want a decent one at the end, so shedding mass while keeping engines is a good thing.

Although I disagree with Vanamonde elsewhere, I don't really disagree with his characterization of what I described as "a variation of asparagus staging".  I actually said myself in the OP that "you might call [it] 'SRB hybrid asparagus'."  While it's true that the SRB doesn't use any of the LFO it's pushing, it's not that much of a stretch from normal asparagus: 
—Normal:  LFO fuel with LFO engine underneath.  Some LFO goes to the center stage and some LFO goes to the booster. 
SRB hybrid:  LFO fuel and SRB fuel with SRB engine underneath.  All LFO goes to center stage and all SRB goes to the booster.  Bonus points for timing the LFO and SRB fuel to run out at the same time. 
The only difference is that the dropped section has two types of fuel, which might require more careful rationing. 

P.S.  I love the typo in your signature:  "And even if you DO do everything correctly, an unplaned-for mountain can totally ruin your day." 
I'm assuming that you meant "unplanned".  If I'm wrong, let me know! 
But taking "plane" to mean "to make smooth or even; to level", an unplaned-for mountain is one that you failed to remove from your flight path :o

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24 minutes ago, FinalFan said:

P.S.  I love the typo in your signature:  "And even if you DO do everything correctly, an unplaned-for mountain can totally ruin your day." 
I'm assuming that you meant "unplanned".  If I'm wrong, let me know! 
But taking "plane" to mean "to make smooth or even; to level", an unplaned-for mountain is one that you failed to remove from your flight path :o

Haha no it's a flat-out typo. Which I'm fixing now :)

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Something like that?

Ijo6OKm.png
I am also using the final Terriers to help the Poodle.

...it is probably overpowered, but I am going to save some poor fellow from the Mun and take him/her together with four tourists to visit my station there, before we return.

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I've only ever used drop tanks on shuttle-like designs.
 

Though I do have a really nice TSTO space plane (runway launch) that drops its jet engines - not drop tanks I know. Not overly efficient cost wise either but it is cool as all heck.

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Ok I come down on both side of this.

I haven’t ever put tanks on top of my SRBs.  Personally I like to do the first stage on only SRBs (I fire the main stack engines only for a couple of seconds to clear the launch pad).

SRBs are much cheaper thrust so instead of drop tanks why not just use more SRBs?

 

I do disagree with Vanamode though.  Drop tanks can be very useful even within the rocket equation in a myriad of situations.

1. Using 1 Nerva.  I can’t exactly cut it in half so a drop tank on it is ideal and increases DV (youvarent carry dead tanks).

2. Small rockets in the 10-20 time range.  Often there are no equivalent smaller rockets to divide the work.

3. Etc

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17 hours ago, FinalFan said:

Although I disagree with Vanamonde elsewhere, I don't really disagree with his characterization of what I described as "a variation of asparagus staging".  I actually said myself in the OP that "you might call [it] 'SRB hybrid asparagus'."  While it's true that the SRB doesn't use any of the LFO it's pushing, it's not that much of a stretch from normal asparagus: 
—Normal:  LFO fuel with LFO engine underneath.  Some LFO goes to the center stage and some LFO goes to the booster. 
SRB hybrid:  LFO fuel and SRB fuel with SRB engine underneath.  All LFO goes to center stage and all SRB goes to the booster.  Bonus points for timing the LFO and SRB fuel to run out at the same time. 
The only difference is that the dropped section has two types of fuel, which might require more careful rationing.

I've come to use the hybrid more and more.

While it is harder to tweak for optimum I often use standard launchers so I only have to tweak each class of launchers once.

One additional advantage is that when the boosters/droptanks are dropped, the central core is still fully fueled (-ish) which makes it easier to get the planned TWR for that stage.

IM(nv)HO

// Curveball

Edited by Curveball Anders

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I mostly use droptanks on planes, but the stock KSP didn't have an aerodynamic model which would also be fairing friendly, so I made one for myself:

 

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5 hours ago, Rocket Farmer said:

SRBs are much cheaper thrust so instead of drop tanks why not just use more SRBs?

Are SRBs cheaper than their equivalent in LFO thrust out of your launch engine? I don't actually know if they are or not, but that is a good reason to not use them if it is, and I'd never really thought about it. I just always assumed (without thinking about it) that a can of gas is cheaper than a can of solid gas with a nozzle and whatever else makes solid boosters work.

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honestly, I don't find drop tanks that useful.  the empty tank itself is usually fairly light compared to most engines that would be useful for launch.  Meaning you really aren't saving much mass over adding extra tankage to your core stage or a booster, plus you're making the rocket more draggy then it needs to be without adding more thrust to compensate.  @Vanamonde is right, if you add an SRB to the bottom of the tank to make up for the extra drag and mass, it's not really a drop tank so much as an inefficient asparagus booster, and if your main engine doesn't drain the tank before the SRB burns out, you're dragging the extra weight of that spent booster.  You're better off either going with full up LFO asparagus boosters or sticking with SRBs.

On 6/5/2018 at 10:30 PM, FinalFan said:


—A Kickback's full thrust was almost enough to cancel the weight of itself plus a Jumbo tank and two cone tanks. 
 

This runs afoul of the rocket equation, you're adding all that weight just to cancel it out, and you'll only get a slight boost in Delta-V for it.

Edited by Capt. Hunt

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1 hour ago, 5thHorseman said:

Are SRBs cheaper than their equivalent in LFO thrust out of your launch engine? I don't actually know if they are or not, but that is a good reason to not use them if it is, and I'd never really thought about it. I just always assumed (without thinking about it) that a can of gas is cheaper than a can of solid gas with a nozzle and whatever else makes solid boosters work.

A single Kickback costs 2,700 funds, and puts out 650 kN of thrust for a little more than a minute.  To get equivalent performance from an LFO engine and tank you'd need a Skipper (5,300 funds) plus (if memory serves correctly) a Rockomax 16 and a Rockomax 8 (2,350 funds) for a total cost of 7,650 funds.  The Skipper and fuel tanks will weigh less, and be more versatile due to gimbaling and throttling, but they can't compete with the Kickback on cost.  

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One more thing in case some folks aren't aware - the idea of putting fuel tanks on top of SRBs has a real life basis.

Although it was never built, the proposed Saturn V/4-260 would have had four massive SRBs with fuel tanks on top of each SRB feeding the center core.  The solution was intended to solve the structural and manufacturing problems associated with stretching the S-IC first stage (a modification found in several other Saturn V study variants), while still massively increasing the total payload weight to orbit. 

More info can be found here: http://www.astronautix.com/s/saturnv4-260.html

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On 6/9/2018 at 11:08 AM, Rocket Farmer said:

Ok I come down on both side of this.

I haven’t ever put tanks on top of my SRBs.  Personally I like to do the first stage on only SRBs (I fire the main stack engines only for a couple of seconds to clear the launch pad).

SRBs are much cheaper thrust so instead of drop tanks why not just use more SRBs?

 

I do disagree with Vanamode though.  Drop tanks can be very useful even within the rocket equation in a myriad of situations.

1. Using 1 Nerva.  I can’t exactly cut it in half so a drop tank on it is ideal and increases DV (youvarent carry dead tanks).

2. Small rockets in the 10-20 time range.  Often there are no equivalent smaller rockets to divide the work.

3. Etc

For me, "moar SRB" has a limit when the number of them just gets too ridiculous, either aesthetically or practically.  Usually there's a sweet spot for me of how many I can make share a smaller number of decouplers.  In addition, while SRBs are cheaper than the full LFO package, they are only cheaper than JUST adding tanks if the engine is a dedicated vacuum engine.  Even a Rhino pretty much breaks even on pure cost (for just the fuel to run it while the SRBs are burning, assuming the engine is "free" since it's already there anyway), and it adds a lot of thrust that in turn reduces gravity losses.  See below. 

On 6/9/2018 at 1:24 PM, Curveball Anders said:

I've come to use the hybrid more and more.

While it is harder to tweak for optimum I often use standard launchers so I only have to tweak each class of launchers once.

One additional advantage is that when the boosters/droptanks are dropped, the central core is still fully fueled (-ish) which makes it easier to get the planned TWR for that stage.

IM(nv)HO

// Curveball

The "additional advantage" is shared by traditional asparagus, so I'm not really sure what you meant to say it's advantageous in relation to. 

21 hours ago, 5thHorseman said:

Are SRBs cheaper than their equivalent in LFO thrust out of your launch engine? I don't actually know if they are or not, but that is a good reason to not use them if it is, and I'd never really thought about it. I just always assumed (without thinking about it) that a can of gas is cheaper than a can of solid gas with a nozzle and whatever else makes solid boosters work.

See below. 

20 hours ago, Norcalplanner said:

A single Kickback costs 2,700 funds, and puts out 650 kN of thrust for a little more than a minute.  To get equivalent performance from an LFO engine and tank you'd need a Skipper (5,300 funds) plus (if memory serves correctly) a Rockomax 16 and a Rockomax 8 (2,350 funds) for a total cost of 7,650 funds.  The Skipper and fuel tanks will weigh less, and be more versatile due to gimbaling and throttling, but they can't compete with the Kickback on cost.  

While that's true, the question was if the SRB was cheaper than the equivalent in LFO thrust out of the launch engine.  After doing some simplified math comparing a Kickback to a Rhino, the answer appears to me to be "Adding LFO tanks (tanks alone) is cheaper unless it's a vacuum engine".  The Rhino was pretty close to the break-even point:  if you assume that the average ISP is halfway between sea level and vacuum then the Rhino's tanks are either slightly more or slightly less expensive on a "kilonewtons per credit" basis depending on which tanks you buy, but that ISP average is probably too generous for a mere 63 seconds of runtime out of the sort of thing you'd slap a Rhino on.  The Rhino's ISP is 205 at sea level; only the Poodle, Terrier, and Ant are worse among the LFO engines.  The next best is a tie between the Swivel and the Twitch at 250, so I think it's safe to say that these and any other LFO engine would get more kilonewtons per credit than a Kickback. 

Of course, you still need good launchpad TWR, but unless I've made a mistake somewhere I see no need to refrain from firing an LFO engine that isn't a dedicated vacuum engine (and adding fuel for that purpose).  I imagine that even the Rhino comes out ahead after considering the reduction in gravity losses that extra thrust would give you—assuming you didn't put moar SRBs under the bell! 

I do have to question your claim that the Kickback "puts out 650 kN of thrust for a little more than a minute".  The wiki reports that the KB's kN output ranges from almost 600 at sea level to 670 in vacuum.  Isn't a 650 average much too optimistic? 

Having said all that, your post does demonstrate pretty well that SRBs will get many more kilonewtons per credit than traditional asparagus. 

Edited by FinalFan
KB kN question

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49 minutes ago, FinalFan said:

The "additional advantage" is shared by traditional asparagus, so I'm not really sure what you meant to say it's advantageous in relation to.

In relation to just SRBs.

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On 6/9/2018 at 5:13 PM, Capt. Hunt said:

honestly, I don't find drop tanks that useful.  the empty tank itself is usually fairly light compared to most engines that would be useful for launch.  Meaning you really aren't saving much mass over adding extra tankage to your core stage or a booster, plus you're making the rocket more draggy then it needs to be without adding more thrust to compensate.  @Vanamonde is right, if you add an SRB to the bottom of the tank to make up for the extra drag and mass, it's not really a drop tank so much as an inefficient asparagus booster, and if your main engine doesn't drain the tank before the SRB burns out, you're dragging the extra weight of that spent booster.  You're better off either going with full up LFO asparagus boosters or sticking with SRBs.

This runs afoul of the rocket equation, you're adding all that weight just to cancel it out, and you'll only get a slight boost in Delta-V for it.

To your first point, it depends on how long you would otherwise be hauling the tankage proposed to be dropped, and also how soon you plan to drop it (i.e. how much drag is even a factor considering you launch at 0m/s). 

[edit:  And really, think of it this way.  If you're going to have a given LFO tank that is empty when the SRBs separate, what is worse:  the delta-v lost to the drag penalty of having it on top of the SRBs up to that point, or the delta-v lost to both gravity and acceleration for the empty tank's mass from separation to as far as the rest of the stage is going?]

For the rest, I am aware of the drawbacks you mention, but I am not convinced you are giving appropriate weight to the advantages.  Why is 59 seconds of additional Twin Boar runtime only "a slight boost in Delta-V"? 

When you say full LFO asparagus is better than doing it with SRBs, are you factoring in cost or going by pure physical effectiveness? 

Edited by FinalFan
One more thing

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22 hours ago, Capt. Hunt said:

This runs afoul of the rocket equation, you're adding all that weight just to cancel it out, and you'll only get a slight boost in Delta-V for it.

Dv and Rocket equation is a very bad indicator for cost efficiency of first stage. For non reusable small to medium-big launchers, using SRBs in the first stage almost always reduces the cost of the launch, while augmenting its mass.

If you use both SRBs and LFO engines, which is justified for mainsail and above classes of launcher, you are going to stage them at some point, and you might as well ditch some empty tankage. Macollo uses that technique in the video below to beat this challenge :

which consists in building the cheapest non reusable launcher (in fund per ton of playload in orbit).

and it does use fuel tanks on top of SRB, and even does in a way which decreases drag, not increase it. Also, as said above, in addition to the space shuttle being basically just that, Lfuel tank carried by SRBs, it has been cosidered for an heavy (Saturn) Rocket launcher.

23 hours ago, 5thHorseman said:

I just always assumed (without thinking about it) that a can of gas is cheaper than a can of solid gas with a nozzle and whatever else makes solid boosters work.

SRB are actually very cheap IRL too. Their solid propellant is not solid gas (an oxymore btw, at least in standard conditions), nor an unstable solidified gas, but a solid propellant possibly completely unrelated to any liquid fuel. For example, gunpowder is the earlier rocket solid fuel (and the earliest rocket fuel), used by medieval China and Corea.

Now both solid and liquid fuel are relatively very cheap, and usually amount to 1% of the price of a launcher (several percents in KSP, still not the main part).

A solid rocket booster is just solid fuel wrap in metal or whatever. You ignite the bottom and it burns, and because of the shape of the wraping, it will produce thrust (it can even produce a varying, though predetermined, thrust). That make them extremely simple, reliable, and cheap.

A liquid fuel engine on the other hand... They vary a lot in complexity, some can gimbal, some can throttle, some can be ignited several times or indefinitely (most can't). But what makes almost all of them more complex than a SRB is that after ignition, you have to pump fuel into them, and sometimes even pump filler (eg uncompressed helium) into the liquid fuel tank to maintain pressure and fuel flow (the few exceptions are engine who use the acceleration of the ship to push fuel into them ; they may make for simple design themselves but require other engines to be fired). That makes them much more complicated and thus expensive than their solid counter part.

Edited by Kesa

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1 hour ago, Kesa said:

Their solid propellant is not solid gas (an oxymore btw, at least in standard conditions)

Sorry I was using "gas" as a synonym for "fuel," not as one of the 3 (or 4, or 17 or whatever there are now) states of matter.

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7 minutes ago, 5thHorseman said:

Sorry I was using "gas" as a synonym for "fuel," not as one of the 3 (or 4, or 17 or whatever there are now) states of matter.

No problem, I was just being nickpicky (although I also genuinely did not get that you were using gas in that meaning, which is perfectly fine then, I even see it as a nice pun).

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Also, as @FinalFan pointed out, you can't count the LFO engine into "are more SRBs better than putting drop tanks on the SRBs that are already there" because you're bringing the engine anyway. And while pumps and extra pressure requirements and whatever are important in real life, I'm more curious about in the game. As in, which should you do to be more efficient, cost-wise?

And now that I think of it, the answer is obvious: When detaching the SRBs, it is always better to have full tanks than partially empty ones, unless somehow those full tanks are heavier than a smaller, partially filled one. So, it is always better to put dropsparagus tanks on top of your SRBs because it means you're just putting less tanks on your center stage.

Edited by 5thHorseman
I had my "lighter" and "heavier" mixed up. So I fixed up.

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The reason why I like the hybrid version is mainly economical (and a slight hint of OCD).

A standard aspargus 'booster' is far more expensive than an SRB with a LFO tank on top, and can, with a little fiddling, provide the same boost.

Edited by Curveball Anders

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I actually have used Drop tanks once, in my moho refuelling mission to boost DV from the 4 Nervs. I dropped them before the insertion burn.

over the whole SRB hybrid thing - I really don’t think its anything close to an asparagus or onion design; I see as extra boosters lifting a drop tank for a while and then being dropped.

And using Drop tanks usually means you have a single centred engine, with drop tanks arrange radially, so I don’t see how did the ‘extra engines’ enter the debate at all.

Infact, I generally don’t use SRBs at all, unless I’m doing something like a very easy Kerbin system contract, when I pair SRBs with Skippers. heavy payloads end up being onion staged and even heavier ones like the launcher used for said Moho refuelling mission using asparagus. Pain to fly though, as the tanks were the size of your average mammoth rocket and kept crashing into engines. Just to send the payload (350ish tons) to LKO (86x96) took a total launch weight of 2.7 MT.LFO do offer more versatility, unless you really are short of funds. There generally comes a point in a career save that it begins to function like sandbox and even the largest rockets barely cost anything.

Edited by Xurkitree

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If a single 'launch stage' does not give me the dv(or twr) I need to put my space stage where I want it, then my first go-to is kickbacks with tanks on them.(but extra kickbacks may be added if launch twr is too low).  Generally with 62* main engine lf/sec worth of lf in the lfo tanks.

This generally gives me a 2.5 stage rocket that can get even pretty sizeable self-fueling rockets wherever they need to go for their first top-off.

(once I have a fuel station in orbit this is only needed for much larger rockets as not needing that extra ~2km/s for a mun refueling landing on top of the launch dv helps a lot) 

Edited by Terwin

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On 6/10/2018 at 5:58 PM, Curveball Anders said:

The reason why I like the hybrid version is mainly economical (and a slight hint of OCD).

A standard aspargus 'booster' is far more expensive than an SRB with a LFO tank on top, and can, with a little fiddling, provide the same boost.

Note that this doesn't make economic sense with hammers: you're better off burning hammers instead of the liquid rocket at all.

for LV-30: 2 hammers + 2 side decouplers + 2 fuel pipes + 2 FLT-100 tanks = 595 thrust, 2450 funds
  3 hammers +  1 central decoupler = 593.7 thrust, 1000 funds

for skipper: 2 hammers + 2 side decouplers + 2 fuel pipes + 2FLT-400 tanks = 963.8 thrust, 2900 funds
  5 hammers +  1 central decoupler  = 989 thrust, 1400 funds

(I don't think anyone would replace mainsails with hammers.  But I think the math still works).

And of course if you had more hammers off on the sides this doesn't change anything (unless you used more than 2 couplers, then using a grouped cluster only gets cheaper).  Just add the additional hammers to your new cluster.

Note that for anything but hammers, you run into both stability and control issues (expect to add 2460 in funds just in 4 AV-R8 winglets [you should be able to get away with 2 aligned N-S]).  Stability of rockets balanced on kickers is tricky, unless they are small swivel and/or terrier rockets.  You *can* of course mount kickers or thumpers to the sides of these rockets, and just build the hammer cluster to fall away first.  I doubt the hammers will take you so far vertical that you have a problem with your pitch over (gravity turn) [if not, give it a little angle towards the East before launch].

I still like pure kicker first stages, but the new aerodynamics model makes them extremely difficult.  Expect to mount most of your thumpers and kickbacks on the side.

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On 6/10/2018 at 3:58 PM, 5thHorseman said:

 

And now that I think of it, the answer is obvious: When detaching the SRBs, it is always better to have full tanks than partially empty ones, unless somehow those full tanks are heavier than a smaller, partially filled one. So, it is always better to put dropsparagus tanks on top of your SRBs because it means you're just putting less tanks on your center stage.

Sorry, you missed the other option.  Load up on SRBs and don’t fire the center stack until the SRBs are done.  

Ill try some modeling on this tonight but last time I looked more SRBs instead of drop tanks had better starting thrust (high early thrust is more efficient to get up to speed early) plus cheaper in credits.

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On 6/15/2018 at 8:33 PM, Rocket Farmer said:

Sorry, you missed the other option.  Load up on SRBs and don’t fire the center stack until the SRBs are done.  

Ill try some modeling on this tonight but last time I looked more SRBs instead of drop tanks had better starting thrust (high early thrust is more efficient to get up to speed early) plus cheaper in credits.

How did your calculations turn out?  I did some very basic calculations on a Mammoth versus a roughly equivalent number of Kickbacks and it seemed like the drop tanks would weigh less and be cheaper.  (6 KBs versus 63 sec. worth of drop tank; the Mammoth has 6.3 KBs of thrust on the launchpad.)  But maybe I oversimplified my math and thereby came to an erroneous conclusion.  

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