How much do you use drop tanks?

[FinalFan]

Note:  Drop tanks as distinct from asparagus boosters that have an engine using the type of fuel that is in the tank. 

I was inspired to create this poll because I was curious after I found myself beginning to use drop tanks in certain circumstances.  I hadn't been using them much previously—I'd used a little asparagus a few times but no tanks without LFO engines attached, except when I stuck a couple T800s on top of Kickbacks once or twice.  But then I realized an interesting coincidence: 
—One Jumbo tank with a cone tank on top and bottom was enough to fuel a Twin Boar for 59s, or two sets could fuel a Mammoth for 62s. 
—A Kickback ran for 63s at full thrust
—A Kickback's full thrust was almost enough to cancel the weight of itself plus a Jumbo tank and two cone tanks. 
Therefore, it seemed to me, you could get a large drop tank at relatively little launchpad TWR penalty that would conveniently run dry at close to the same time that its accompanying SRB ran dry. 

Tanks and Kickbacks are a lot cheaper than tanks and Mammoths.  So the last couple of times I have built a heavy-lift launchpad stage, I've tried the tactic of adding an appropriate number of Kickback-assisted drop tanks, then adding a few more KBs if I need more launchpad TWR (to a seemingly reasonable limit). 

Now I'm sort of curious where this lies on the spectrum of "Nifty trick"—"Welcome to the crowd"—"Rookie mistake".  On paper, it seems to me that this thing you might call "SRB hybrid asparagus" is a cheap alternative to traditional LFO-engine asparagus, but is that somehow an illusion? 

[Vanamonde]

Aside from landings and takeoffs, you normally are not in a hurry to change your velocity and low thrust is generally as good as high thrust. So with drop tanks if you dump all that mass but keep the engines, your ship probably has more thrust than it needs. Excess thrust means you're carrying more engines than you need, which is mass which is not contributing to the mission you want to perform and making your craft less fuel-efficient. 

[KerikBalm]

Does a shuttle style external tank count? I would think that it does.

1 hour ago, Vanamonde said:

Excess thrust means you're carrying more engines than you need, which is mass which is not contributing to the mission you want to perform and making your craft less fuel-efficient. 

I've done single LV-N designs with LF drop tanks. Since you can't drop half a LV-N, it makes sense.

[A Soviet Tank]

I've only ever used them on landers, ditching tanks on the surface as sort of a memorial that I was there.

[Streetwind]

With your username, it would seem like the "tanks" you are leaving behind would be more of an invasion force than a memorial...  

[Zeiss Ikon]

In my current stock career, I've used drop tanks on launchers, but quickly reached a point where they needed engines (and then larger engines), which made them liquid fuel strap-on boosters.  For landers on the Mun and Minmus, however. they make a nice way to widen the base (cuts down on tip-over incidents) without carrying dead weight.  All but my first generation of landers have used drop tanks (which were often empty before deorbiting for the landing, due to the lander doing its own transfer from Kerbin and capture at the Mun or Minmus).

With the landing legs on the drop tanks, I don't dump them on the surface, but rather after I've done the initial boost, at the same time I start to tip over to build orbital velocity.

[Gargamel]

45 minutes ago, Streetwind said:

With your username, it would seem like the "tanks" you are leaving behind would be more of an invasion force than a memorial...  

Groan... but you get a like for that one...

 

I mainly just use them if a lander needs a little more fuel left over for ascent, but @Vanamonde is correct, that does reduce the efficiency somewhat depending on the design.   But if you design it so that you are under powered for the descent, and lose the tanks right before you need all the twr you can get, then you should be ok.  

I also like to use them on working plane designs, but need a bit more range for whatever reason. 

[blakemw]

I don't use them much.

Partly from playing hard career and more staging = more problems.

Also the benefits are kind of marginal. Dirty secret is: most the payload to orbit benefit just comes from making the rocket bigger rather than cleverer. A rocket which is twice as big straightforwardly delivers twice as much payload to orbit. A rocket which is twice as clever (in terms of fancy staging) probably only launches 5% more compared with a simple two-stage rocket.

[winged]

I use drop batteries for electric aircrafts.

Edited June 6, 2018 by winged

[FinalFan]

6 hours ago, Vanamonde said:

Aside from landings and takeoffs, you normally are not in a hurry to change your velocity and low thrust is generally as good as high thrust. So with drop tanks if you dump all that mass but keep the engines, your ship probably has more thrust than it needs. Excess thrust means you're carrying more engines than you need, which is mass which is not contributing to the mission you want to perform and making your craft less fuel-efficient. 

I agree with your underlying objection, but not necessarily its conclusion.  In the OP I described drop tanks that are supported by SRBs instead of liquid engines.  Do you have an opinion on that? 

P.S.  Also, I recall one time I used drop tanks just because I was uncomfortable with how tall my rocket was becoming (I'd forgotten all about it until now), but that's an edge case unrelated to the OP. 

[wumpus]

7 hours ago, Vanamonde said:

Aside from landings and takeoffs, you normally are not in a hurry to change your velocity and low thrust is generally as good as high thrust. So with drop tanks if you dump all that mass but keep the engines, your ship probably has more thrust than it needs. Excess thrust means you're carrying more engines than you need, which is mass which is not contributing to the mission you want to perform and making your craft less fuel-efficient. 

The logic doesn't make sense: dropping tanks reduces mass, which means the engines are going to be more efficient.  Granted, the stated engines were mammoths, which are fine for launch but not always ideal after all the drop tanks are gone.

One other favorite trick of mine is to place a small/medium tank below the capsule/probe and a larger tank above it (you might need a lander for stability).  Use the top engine first, jettison it, and keep going with the bottom one.  Granted, this normally only makes sense with terriers and poodles which are cheap and light, but you still get the efficiency of "bamboo staging" in something that works.

Without the souposphere, drop tanks generally replace asparagus staging (although twinboar asparagus likely has advantages).  Often aerodynamic concerns outweigh the advantages of drop tanks, although combining them with SRBs make more sense.  Note that for small rockets, it often makes more sense to stage a cluster of hammers with a single decoupler.  The thrust from a hammer is high enough that an extra hammer is cheaper than an extra decoupler, and two hammers likely out thrust the main rocket (and don't ask about the cost of at least two fuel tanks and lines).

[riki2048ksp]

I've used drop  tanks  twice.  The  first  craft to use them was a  tourist  orbiter  capable  of  intercept the Mun,  orbit it  and  return to  the  surface  of Kerbin without a  heat shield, and I used drop tanks  to  have  some  more  fuel.  The  second time I had to deliver  two  docking  arms to Minmus station, and they were drop  tanks with  docking  ports

[Kesa]

2 hours ago, wumpus said:

The logic doesn't make sense: dropping tanks reduces mass, which means the engines are going to be more efficient.

He says it's inefficient compared to dropping both the tank and the engine, not compared to not dropping anything.

9 hours ago, Vanamonde said:

Aside from landings and takeoffs, you normally are not in a hurry to change your velocity and low thrust is generally as good as high thrust. So with drop tanks if you dump all that mass but keep the engines, your ship probably has more thrust than it needs. Excess thrust means you're carrying more engines than you need, which is mass which is not contributing to the mission you want to perform and making your craft less fuel-efficient. 

I expect drop tanks to be mostly used for launchers and landers. Anyway, they can help make the mission more fund efficient (albeit more massive or costy upfront), due to recovery of the most expensive parts.

 

Airbus has (had) an interesting take on reusability with Adeline and it inlvolves a drop tank.

Edited June 6, 2018 by Kesa

[Snark]

11 hours ago, FinalFan said:

Now I'm sort of curious where this lies on the spectrum of "Nifty trick"—"Welcome to the crowd"—"Rookie mistake".  On paper, it seems to me that this thing you might call "SRB hybrid asparagus" is a cheap alternative to traditional LFO-engine asparagus, but is that somehow an illusion?

Drop tanks are a perfectly reasonable engineering solution to a reasonable variety of rocket problems.  They're like anything else in KSP-- not a universal solution, but in the right situation they can be quite handy.

I don't use them all the time, but I make liberal use of them in the particular situations that I find them to be applicable.

There are three main situations where I use them.  In descending order of usage frequency:

• LFO tanks atop radial SRBs.
  • Usage:  Very frequent-- the large majority of my launches off the pad.
  • Rationale:  This is simply a win-win, and I always do this whenever I launch with SRBs (which is most of the time, I like them a lot).  I mean, heck, why wouldn't I?  The SRBs are already firing and are going to get jettisoned pretty soon anyway.  My center LFO engine is firing and burning fuel-- so I might as well let it burn fuel from the radial boosters while the SRBs are firing, so that when I jettison them, I've got a nearly full load of fuel.
  • Size:  These fuel tanks tend to be relatively small, because I usually use the smaller SRBs (Hammers, Thumpers), and they burn out pretty quick, so I only want enough LFO on them so that the tanks are empty by the time the SRBs burn out.
• High-dV nuclear-powered interplanetary ships.
  • Usage:  Fairly frequently.  I use LV-N for 30-50% of my interplanetary missions, and I tend to use droptanks whenever I have such a mission.
  • Rationale:  The LV-N is the go-to engine for ships that need very high dV.  So, if I'm using an LV-N... dV is what I'm trying to optimize for.  This means I want to minimize dead weight as much as possible.  That means they tend to be very low-powered ships, because TWR isn't a priority, and engines are dead-weight, and an LV-N is heavy.  Usually I only have one, unless the ship is truly ginormous.  Optimizing for high dV also means ditching dead weight at the earliest possible opportunity, so I don't want to be hauling along empty fuel tank space.  Droptanks make a lot of sense, there.  A common mid-sized LV-N design I'll use is a 1.25m, 4-tons-LF center stack, with six radially attached droptanks that are either 2 or 4 tons of fuel each.  Works great, gives scads of dV.
  • Size:  Anywhere from small to large, depending on the scale of the ship.
• Biome-hopping low-to-medium-gravity vacuum landers.
  • Usage:  Occasional; typically two or three times per career, useful for exploring Mun and/or Minmus.
  • Rationale:  I play career mode, and in the early-to-mid game there's a section of career where I want to make a mission or three to go to the Mun and/or Minmus, hop around to as many biomes as I can get away with, and strip-mine them for science.  Biome hopping takes a whole lot of dV on the Mun, and drop tanks are an easy way to get it.  I generally tend to do this only a little and only in early career, though.  In mid-to-late career, I'm more likely to have an orbiting mothership where the lander can refuel (obviating the need for droptanks), and in late career I've maxed out the tech tree anyway so I don't have any reason to go biome-hopping like that.  The technique is mainly useful for worlds with Mun-like gravity.  Much lighter worlds (like Minmus) don't need much dV and therefore don't need the droptanks, and heavy-gravity worlds (like Tylo) need high TWR so I'm more likely to go asparagus than droptanks.
  • Size:  Small.  Typical use case will be a small Mun lander with a couple of side-mounted 2-ton tanks.

 

 

[Cavscout74]

16 hours ago, KerikBalm said:

I've done single LV-N designs with LF drop tanks. Since you can't drop half a LV-N, it makes sense.

This is my primary use for them right here.  

I've also used ferry tanks on my Pumera SSTO to get enough dV to transfer to Laythe, that are dumped once the transfer burn is done since I keep it permanently around Laythe for interface flights.

Edited June 7, 2018 by Cavscout74

[StrandedonEarth]

Along with LV-N missions, I’ve found drop tanks to be quite useful for Tylo landings, although a crasher stage can be used instead. 

[roboslacker]

I use conventional staging for most of my ships.

[FinalFan]

2 hours ago, Snark said:

(snip snip)

• LFO tanks atop radial SRBs.
  • Usage:  Very frequent-- the large majority of my launches off the pad.
  • Rationale:  This is simply a win-win, and I always do this whenever I launch with SRBs (which is most of the time, I like them a lot).  I mean, heck, why wouldn't I?  The SRBs are already firing and are going to get jettisoned pretty soon anyway.  My center LFO engine is firing and burning fuel-- so I might as well let it burn fuel from the radial boosters while the SRBs are firing, so that when I jettison them, I've got a nearly full load of fuel.
  • Size:  These fuel tanks tend to be relatively small, because I usually use the smaller SRBs (Hammers, Thumpers), and they burn out pretty quick, so I only want enough LFO on them so that the tanks are empty by the time the SRBs burn out.

Great post, and it helped me get back in the right mindset.  The SRBs aren't fitted for the tanks--it's the other way around.  We're just eliminating the deadweight of having dry tanks on the core vessel when the boosters drop off.  I knew that, but I was starting to slide into tweaking the SRBs' runtime to match the tanks' instead of the other way around.  

[Vanamonde]

A tank on an SRB isn't a drop tank. It's a variation on asparagus staging. 

As for the logic of using drop tanks, look at the rocket equation again, guys. You'll see that thrust is not a term in the math. Apart from landings and takeoffs, which I specifically excluded, redundant engines contribute mass to the equation but nothing else, reducing fuel efficiency. 

I'm not saying there are categorically no situations in which I would use them, but as a general rule they are counter-productive. 

[FinalFan]

49 minutes ago, Vanamonde said:

A tank on an SRB isn't a drop tank. It's a variation on asparagus staging. 

As for the logic of using drop tanks, look at the rocket equation again, guys. You'll see that thrust is not a term in the math. Apart from landings and takeoffs, which I specifically excluded, redundant engines contribute mass to the equation but nothing else, reducing fuel efficiency. 

I'm not saying there are categorically no situations in which I would use them, but as a general rule they are counter-productive. 

Hang on, now!  I think you're wording this a bit too strongly.  First off, thrust actually does matter to a certain extent.  Aside from convenience, longer burns use more fuel for the same maneuver due to, for example, reduced Oberth effect. 

Secondly, the fact that dropping dead weight means that you have now become "over-engined" doesn't retroactively make the overall design overengined if the vessel needed it at some other point in its life.  Every rocket that's 3/4 of the way to running dry has "redundant engines" but it's silly IMO to say they're all poorly designed.  And, as others said, some designs are not amenable to asparagus staging.  Reasons can include mission considerations or cost considerations (recovery of engines). 

I don't know why you say drop tanks are counterproductive.  The only drawbacks I see are design hassle, aerodynamic losses, and connection overhead—design hassle probably being the biggest issue most of the time. 

[Kesa]

2 hours ago, Vanamonde said:

A tank on an SRB isn't a drop tank. It's a variation on asparagus staging.

I can see why you say that and I guess I even agree

Quote

 As for the logic of using drop tanks, look at the rocket equation again, guys. You'll see that thrust is not a term in the math. Apart from landings and takeoffs, which I specifically excluded, redundant engines contribute mass to the equation but nothing else, reducing fuel efficiency.

Rocket equation tells you should get rid of dead weight ASAP, stage as often a you can. Ideally, you should be throwing fraction of fuel tanks as they get dry, and fractions of engines as you no longer need them. In practice, in KSP, the wet to dry ratio of tanks is the same from oscarB to the Kerbodyne, meaning if you want, you can have many tanks for one engine, meaning if you follow the rocket equation, you'll be dropping drop tanks in between regular stages, where you drop engines.

I challenge you to make the rocket of 18t with a detachable empty mk1 pod and a chute as a playload, a single reliant as an engine, and the biggest possible Dv

Spoiler

It uses drop tanks.

Drop tanks is not about carrying useless engines, it is about dropping useless tanks (save for things like the Space Shuttle or Adeline, which are about both, for understandable reusability reasons - and fit the launcher category).

 

IRL, one drop tank is often too many drop tank, if anything because big fuel tanks have a better wet/dry ratio, or because simplicity cost and reliability supercede mass optimisation, but in no way because of the rocket equation. And even then, the point of drop tanks for aircraft is more flexibility, modularity, than efficiency. I can understand using them for a similar purpose for a generic partially reusable nuclear interplanetary stage, which receives various amounts of fuel depending on the destination, in both RL and KSP.

Edited June 7, 2018 by Kesa

[eddiew]

Sometimes I use them to get a spaceplane to orbit that doesn't quite have enough fuel. In this example, I wanted to explore a smaller world than Kerbin and would never need that much fuel again, so I kicked off the excess dry mass early to minimise the weight I'd be sending interplanetary  

Outside of spaceplanes, very situational. I prefer interplanetary vessels not to leave debris if possible and usually spec them to do the full round trip and be refuelled again in LKO.

[FinalFan]

51 minutes ago, eddiew said:

 

Is that a Stargate reference?  Oh, that takes me back...

Edited June 8, 2018 by FinalFan
Spelling

[eddiew]

3 hours ago, FinalFan said:

Is that a Stargate reference?  Oh, that takes me back...

More of a Star Wars hyperdrive ring  

[Superfluous J]

On 6/6/2018 at 6:57 PM, Vanamonde said:

A tank on an SRB isn't a drop tank. It's a variation on asparagus staging.

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 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.