Booster strategy, liquid fuel in addition to srb?

[Buster Charlie]

I play with stage recovery,  and srb usually land close enough to get maximum return.

So having said that I suppose taking a total loss on said boosters may change other people's ideas of what's a good idea.

Now I know people do staging with stop tanks for their main engine,  has anyone else experimented with a hybrid srb & fuel drop tank?

I've always heard you want the main engine firing in addition to the srb, because otherwise it's just dead weight,  that got me thinking; by the time you drop your  tanks the main tank is already depleted quite a bit. 

 

So I've been playing around with smaller SRB, with a small fuel tank on top,  capped with a chute. A fuel line feeds the main engine during ascent, hopefully I've calculated it so when the srb burns out,  there will just be a tiny bit of liquid fuel, the whole thing is jettisoned, and my main engine thank is still topped off.

 

But I'm wondering, am I gaining anything from this added complexity?

 

Am I just adding more air and gravity drag,  is it a bust? 

Edit: if this is a good idea,  would there be a market for hybrid solid rocket booster fuel tanks aso one piece? 

Edited February 29, 2016 by Buster Charlie

[Foxster]

Sounds like a cunning plan. 

You could be clever and have the SRB and drop tank coupled so that you wouldn't have to time it so close. Drop the SRBs when they are done, followed by the drop tank. 

Edited February 29, 2016 by Foxster

[Buster Charlie]

4 minutes ago, Foxster said:

Sounds like a cunning plan. 

You could be clever and have the SRB and drop tank coupled so that you wouldn't have to time it so close. Drop the SRBs when they are done, followed by the drop tank. 

Not a bad idea, I was worried about the added weight of extra decouplers, which is what made me think of combining the weight cost of a decoupled and parachute into two functions. 

But I guess if you gain back more delta v from the extra fuel, then it's worth it.

 

 

Edited February 29, 2016 by Buster Charlie

[Foxster]

Just now, Buster Charlie said:

Not a bad idea, I was worried about the added weight of extra decouplers, which is what made me think of combining the weight cost of a decoupled and parachute into two functions. 

True. Hadn't thought of needing two lots of 'chutes. Though you should only need some of the total number of 'chutes for each. Decouplers shouldn't add too much mass, unless you are really close to the edge with the craft. 

Needs a little experimenting. 

 

 

 

[Buster Charlie]

4 minutes ago, Foxster said:

True. Hadn't thought of needing two lots of 'chutes. Though you should only need some of the total number of 'chutes for each. Decouplers shouldn't add too much mass, unless you are really close to the edge with the craft. 

Needs a little experimenting. 

 

 

 

Now that I think of it,  a docking port Jr weighs even less than a decoupler...

[Norcalplanner]

This sort of design was used extensively in the later entries of my Cheap And Cheerful rocket payload challenge. Even without using Stage Recovery, a number of entries were less than 700 funds/ton to LKO. This technique was also proposed IRL on one of the Saturn V MLV varisnts which was never built.

[Brainlord Mesomorph]

YES I HAVE!

The LFO-SRB!!  

it works fine. yes the timing is an issue, you can either carry the spent SRBs, or jettison them with a little LFO left.

 

[Archgeek]

I suppose you could also reverse the timing issue and make it a non-issue -- just tweak things so the tank runs out just before the SRB cuts, and not worry about the empty tank since it's still being lifted just fine by what's left of its SRB and your core is still mostly topped-up when you do stage.

[Fearless Son]

In the old days, I used to attach liquid fuel tanks to the top of my boosters, draining them to the primary engines before dropping the boosters.  However, these days I find that inefficient.  I have not done the math, just feels a little too inflexible, a few too many parts, too much weight-shifting, etc.  

That said, these days I do find myself very fond of separate drop tanks for extra main-engine fuel.  These will usually be on decouplers, much in the manner of solid fuel boosters, and drain directly to the main engine's primary tanks.  They will then be dumped once they run dry, typically a little after the solid fuel boosters have spend themselves, but they are separately staged to give me finer control.  No reason they could not have some parachutes mounted on them and get recovered with the rest of the stuff too, especially because by the time I drop them I am well over the water and they should splash down not far from KSC.  

[Brainlord Mesomorph]

I did it for space (pardon the pun). My TechTier 5 Space Shuttle needed both droptanks and SRBs and there just wasn't room on the little thing to mount them both.

I'm in the SPH holding a fuel tank in the air and there was no place to put it ... except for these big green dots on the ends of the SRBs!

Edited February 29, 2016 by Brainlord Mesomorph

[boccelounge]

IIRC, a RT-10 Hammer set to around 55-56% thrust will burn out in the same time it takes a LT-T30 Reliant (at 100% thrust) to burn through 1 ton of LFO (e.g. an FL-T200 tank).

With a little static (on the pad) testing it's pretty easy to get a combination SRB/LFO droptank stack to empty at the same time.  I guess "math" would work too...  but I prefer my way.

Edited February 29, 2016 by boccelounge

[GoSlash27]

As @Norcalplanner said, we had good success with those designs in the cheap 'n' cheerful challenge, but a couple caveats:

1) The designs were optimized for completely disposable launchers, so YMMV when using stage recovery. You may find that other designs optimized for minimum cost of fuel work out cheaper.

2) This was only an effective strategy for fairly large rockets. When you get into smaller launchers, the cost savings of doing this is erased by the cost of additional separators, plumbing, struts, and sepratrons.

Finally, don't assume that you should "always" have the core engine firing just to get thrust out of it. Simple series 2- stagers can often be more cost-effective, and always are whenever using vacuum- optimized engines. There's a gray area in there where using another SRB to generate thrust is more cost- effective than using a LF engine with poor Isp.

Good luck!
-Slashy

 

[Snark]

I like this idea, but in practice I suspect it would be more inconvenience than it's worth for me. Too much variability in the way I build rockets: different SRB choices, different thrust limiter settings, different LFO engines. The math for working out how much LF tankage to use would be tedious. (Though perhaps okay for certain "standard"designs, where I repeatedly launch the exact same ship design.)

I'm also curious about how the mathematical justification works out for this kind of launch. On the pro side: let the main engine pull its weight the whole time, don't be dead weight. On the con side: SRBs typically have much crappier Isp than LFO engines, and in general it's better for dV if you burn your low-Isp fuel before your high-Isp fuel.

I suspect that the "which is better" answer will depend on various factors such as the size of the ISP disparity, the overall TWR, etc. Would be interesting to sort through the numbers.

[EVA_Reentry]

Arianespace's Ariane 44LP has both solid and liquid boosters. Check it out!

[GarrisonChisholm]

*selected this thread to read simply because Snark had posted*  

I have used this technique, it is very useful when you have an excess of thrust and you find your mains burning out a bit earlier than optimal.

[Skalou]

Yep, it works great, i use it in a trip to laythe ( for the research on the speed of sound)

Also, with a fuel line trick it initiates the gravity-turn without using the gimbal! (probably not the most efficient but really fun to make it works)

 

Edited February 29, 2016 by Skalou

[Geschosskopf]

7 hours ago, Buster Charlie said:

But I'm wondering, am I gaining anything from this added complexity?

Only the fun of meeting the challenge, the thrill of doing a cool trick, and bragging rights.  But in gameplay terms, not really  The way KSP economics work, you very soon reach the point where the payload is the largest part of a rocket's cost, and by the time you can afford to build such payloads, the price of the boosters is small change you don't worry about.  In addition, recoverable boosters are necessarily bigger, heavier, and more expensive than disposable boosters because they have to carry extra parts and/or fuel for their own recovery in addition to the actual payload, which cuts into whatever savings using them would give you.

 

[Brainlord Mesomorph]

14 minutes ago, Geschosskopf said:

  But in gameplay terms, not really  The way KSP economics work, you very soon reach the point where the payload is the largest part of a rocket's cost, and by the time you can afford to build such payloads, the price of the boosters is small change you don't worry about.  In addition, recoverable boosters are necessarily bigger, heavier, and more expensive than disposable boosters because they have to carry extra parts and/or fuel for their own recovery in addition to the actual payload, which cuts into whatever savings using them would give you.

 

I have to disagree.  Reducing your cost to orbit is key to building a large interplanetary fleet.

If you want to talk pure economics, your cheapest orbital lifter is an LFO-SSTO rocket. (given the 22k bubble).  Yes they are expensive to build but you can get 90% back if you parachute them in near the KSC.

I’ve reduced my cost to orbit from $2000/tonne to 600.

That pays for a lot more payload!

Edited March 1, 2016 by Brainlord Mesomorph

[Geschosskopf]

1 minute ago, Brainlord Mesomorph said:

I have to disagree.  Reducing your cost to orbit is key to building a large interplanetary fleet.

What do you call a "large interplanetary fleet" ?  I like to think I go fairly large myself.  My typical interplanetary payload is in the $300K-400K range and total launch vehicle price on the pad, including payload, is typically only $400K-600K.  Some ships approach a total cost of $1M, of which about $600K or more is payload.  IOW, the payload is 2/3 - 3/4 of the total ship price.  And I usually send out fleets of 6-8 such ships, so a whole fleet costs like $3-5 million and that's just 1 of several fleets I usually have going at once.

So, I throw millions at a time into space, the bulk of which is in the payloads.  And few of those payloads ever come home because they're for permanent colonies.  So if I can afford to throw away millions on what are effectively disposable payloads, I can certainly afford NOT to save a few tens of thousands on reusable boosters

 

[Brainlord Mesomorph]

By your own math the rocket is 40% of the cost of the flight.  If you only save half of that money by reusing the rockets, then for every five ships, you get a free ship!  Payload and all!

I think that’s well worth the cost of watching the thing re-enter.  And besides, I have fun seeing how close to the KSC I can throw them, it’s like darts!

And 3-5 million per mission?  How much money you have?  What contracts are you getting?  Or, what year have you time warped to?

I’m on day 300, I’ve got about two million, my first bots are on their way to Duna, and yes, I need to save money on my launches.

[Geschosskopf]

48 minutes ago, Brainlord Mesomorph said:

By your own math the rocket is 40% of the cost of the flight.  If you only save half of that money by reusing the rockets, then for every five ships, you get a free ship!  Payload and all!

No, because most of the cost of a booster is its fuel, which you can't recover.  The empty tanks and 2nd hand engines are chump change, not worth the time or trouble to bother with.

Seriously, you don't "win" KSP by having the biggest bank account, and your savings earn zero interest.  Thus, once you've bought all the building upgrades (and part entry costs if that's your thing), your money is useless unless you spend it.  So SPEND it!  If you can afford to send the rocket in the 1st place, then you can afford NOT to recover the boosters.

I see a lot of KSP players trying to squeeze blood from a stone, spending dollars to save pennies.  Recoverable boosters, refueling bases on Mun or Minmus, etc.  The economics just don't justify any of this.  Now, if folks find doing this fun and entertaining, hey, more power to them.  But there's no practical value to be gained in any of this.  

I think the reason for this is largely psychological.  At the very start of the game, you're really strapped for cash, so get in the habit of being frugal, and that lasts well beyond the real need for it.  Also, folks are intoxicated with SpaceX, or trying to implement some plan for lunar refueling from the real world.  In the real world, such things might be good ideas (although I'm quite dubious of the economic viability of SpaceX), but KSP isn't the real world.  It's way smaller so you don't need much dV to get anywhere, and the economy is completely different and totally artificial.  Thus, real-world economic justifications just don't carry over into KSP.

Quote

I think that’s well worth the cost of watching the thing re-enter.  And besides, I have fun seeing how close to the KSC I can throw them, it’s like darts!

Like I said, if the entertainment value appeals to you, knock yourself out.  Me, I'd rather spend my playtime actually using the payloads I put in space and so try to minimize the time I spend on the necessary evil of launching them.

Quote

And 3-5 million per mission?  How much money you have?  What contracts are you getting?  Or, what year have you time warped to?

I’m on day 300, I’ve got about two million, my first bots are on their way to Duna, and yes, I need to save money on my launches.

I'm somewhere near the middle of Year 16.  The tech tree and KSC have been finished for most of that time.  I have about $36 million in the bank.  I have so much money in the bank I can't spend it fast enough, mostly because I have so much stuff already in space I'm not doing many launches right now.  As a result, I'm also not doing many contracts.  I don't look at any that pay less than $500K, and of those I only accept ones that can be done by a ship already in space or involve a place I'm going to next anyway.  Mostly this means SENTINEL contracts at various planets, which get repeated business and pay quite handsomely for me doing nothing but accepting them.

So yeah, a $5-million fleet is a lot of money, but I can do that 7 times before I'm bankrupt.  This is why I don't care about reusable boosters.  But it's that way even early in the game, too.

Edited March 1, 2016 by Geschosskopf

[GoSlash27]

For real... your disagreement over KSP macroeconomics has nothing to do with the OP's question in the microeconomic sense. The OP just wants to know if combining SRBs and drop tanks makes for cheaper launches or not.

 The answer is that it does for bigger launches with core boosters that have good Isp at sea level and doesn't for small launches with vacuum- optimized core engines... at least where stage recovery is not used.

 The relative value of that cost savings WRT player time is a subjective thing and totally out of the purview of the question. You can both be completely correct or completely incorrect on that matter, but it's not what was asked.

 I recommend dropping the argument.

Edited March 1, 2016 by GoSlash27

[Brainlord Mesomorph]

jeez. touchy.

and sure, year 16 and 35 mill in the bank, none of it matters.

(heck you get to use the ion thrusters!)

EDIT: hi slash, the OP was asking if recoverable boosters were a waste of time, IMHO: no

EDIT 2: maybe I (and perhaps other KSP players) identify to personally with NASA's budget problems!

Edited March 1, 2016 by Brainlord Mesomorph

[FancyMouse]

I love hybrid launch stage because (1) stock SRB has no gimbal and (2) pure SRB often gives me too-big acceleration at the end of the stage.

And of course I put a dip of LFO on top of my SRB to feed central liquid engine. Oh someone's going to offer hybrid SRB with LFO? I'll buy it.

[Buster Charlie]

2 hours ago, Geschosskopf said:

Only the fun of meeting the challenge, the thrill of doing a cool trick, and bragging rights.  But in gameplay terms, not really  The way KSP economics work, you very soon reach the point where the payload is the largest part of a rocket's cost, and by the time you can afford to build such payloads, the price of the boosters is small change you don't worry about.  In addition, recoverable boosters are necessarily bigger, heavier, and more expensive than disposable boosters because they have to carry extra parts and/or fuel for their own recovery in addition to the actual payload, which cuts into whatever savings using them would give you.

 

I dunno I get like 98% of my booster cost back from stagerecovery, since they land so close to base.

 

Thanks everyone for the replies, I see this has really taken off and i'm glad I'm not the only one who's thought of this.