Why asparagus liquid fuel and not use solid boosters instead?

[THX1138]

Why asparagus liquid fuel engines and not use cheaper solid boosters instead?

[WhiteKnuckle]

A. Because you can't throttle them.

B. Because they have lousy ISP.

[Alshain]

What ever gets you there at the lowest cost, go for it.

Liquid fuel has in-flight throttle control where SRB's can only be throttled in the VAB. They are useful for an initial kickoff, but when you get higher you don't need all the thrust you did when you were on the ground. Their ISP isn't great, but they are cheap.

Largely, it depends on what I am launching, often I use both, but when I get in the high atmosphere I definitely want throttle control.

[MarvinKitFox]

Why asparagus liquid fuel engines and not use cheaper solid boosters instead?

Pretty much the same reason you use an internal combustion gasoline engine in your car, rather than burning straw in a steam engine.

The straw is way cheaper than the gasoline, but your performance is... a bit less.

Also, when you try to do something a bit tougher, you get to appreciate the performance increase.

Flying a SRB rocket to moho and back is about as easy as building a straw-powered steam Concorde.

[WuphonsReach]

Why asparagus liquid fuel engines and not use cheaper solid boosters instead?

SRBs are still a bit overpriced for the performance they give. Actually, a lot of things in KSP are out of balance with regards to cost...

[Red Iron Crown]

Why asparagus liquid fuel engines and not use cheaper solid boosters instead?

Mostly from old habit. Before career mode was added and for an update or two afterwards there were no monetary concerns, so players generally tried to optimize for mass. Asparagus is very, very efficient mass-wise, so it was a good design strategy for that.

Now that costs are a thing the equation has changed, it's cost-to-orbit rather than mass-to-orbit for career players. In the sandbox it's still the same, though.

[sal_vager]

Another reason may be that you can't pump solid fuel from one SRB to another

To be asparagus, the SRB's in each pair would need to have lower thrust or more fuel than the pair that precedes them and this can be difficult to get right, as has been mentioned they cannot be throttled so if your pair of SRB's has too much or too little thrust or fuel for their portion of the flight envelope you can find yourself not reaching space today.

Edited February 3, 2015 by sal_vager

[GoSlash27]

Mostly from old habit. Before career mode was added and for an update or two afterwards there were no monetary concerns, so players generally tried to optimize for mass. Asparagus is very, very efficient mass-wise, so it was a good design strategy for that.

Now that costs are a thing the equation has changed, it's cost-to-orbit rather than mass-to-orbit for career players. In the sandbox it's still the same, though.

^ This unless you happen to be doing something like leaving Eve. In that case, you want lots of DV and excellent payload fraction so asparagus is still the way to go.

Best,

-Slashy

[Torquemadus]

SRBs need to be thrust limited to increase their burn time. I usually set mine to 70%. This allows the rocket more time to climb out of the soupy lower atmosphere to altitudes where the atmosphere no longer holds back the acceleration of the rocket. This means that at the point when the SRBs are low on fuel and giving their best TWR, their thrust is being used to rapidly accelerate the rocket, not push against the atmosphere. This in turn increases the altitude and speed the rocket is able to reach before staging, which leaves less work for the second stage to do.

While great as first stages, I've never liked the performance of solids in upper stages, so I prefer to use re-useable liquid fuelled upper stages instead.

[KerikBalm]

Asparagus got established before there were any funds, so there was not cost difference.

Then .24 came out, and costs mattered.

Now 0.90 is out, and size matters-> well, until lyou have a lvl 3 launchpad.

If you want to launch a more ambitious mission before upgrading the launchpad, then you may want to do an asparagus design.

Also, for something like eve: if you can cut your ascent vehicle mass by 1/2, then everything below that can be cut by half, and you can save quite a bit of money.

But as a first stage that can be as massive as you want (since you don't have to move it anywhere) - SRBs work nice if you're not mass limited.

[pincushionman]

Now, do you mean "why do you asparagus" or do you mean "why do you use liquid boosters?" Because asparagus (or onion) staging is not simply using liquid fuel boosters. It's using liquid fuel boosters where the boosters' fuel tanks feed both the boosters and the core stack. That way you can use your core's engines at the same time as the boosters, but when the boosters run dry and get ejected, your core is still full of fuel. And that gives you options you don't have with a traditional booster setup.

But if you are asking about using LFO engines as traditional boosters, that's a performance vs. cost issue.

[sal_vager]

Actually the original asparagus design makes no mention of transferring fuel, though it did use liquid kerosene rockets

Source, an image of the textbook page showing where the first known use of the term "asparagus" for rocket boosters comes from.

[maltesh]

Actually the original asparagus design makes no mention of transferring fuel, though it did use liquid kerosene rockets

Source, an image of the textbook page showing where the first known use of the term "asparagus" for rocket boosters comes from.

I can't seem to find a scan of the full page, unfortnately.

However, in the post where Klopchuk brings the concept to the attention of the KSP community he states:

Anyway, Ed Keith developed a concept that might be familiar to many of your designs:

1) seven cylindrical rockets mated side-by-side

2) all seven engines burning in parallel at lift-off

Please see attached photo of page 144 of the textbook.

However, he also proposed automatically pumping propellant from some of the rockets first, so two of the rockets could be discarded as soon as possible, followed by the next two, and so on, until you are left with the center rocket.

Edit: And then, of course, there's the question: If you're using seven identical boosters, and aren't pumping fuel the way Klopchuk says Tom Lodgson says Ed Kieth's proposal says, why would you be only dropping two at a time?

Edited February 4, 2015 by maltesh

[kujuman]

Ever since we got roll control on gimbals, the lack of gimbal on SRBs has made them even less attractive, particularly for less than balanced loads or loads that tend to spin. Now, I love SRBs (otherwise I wouldn't have fixed them), but the stock SRBs (not the LET or separators) aren't really that fun for building launchers because they have so many shortcomings, particularly the inability to design a burn profile for them.

Even the largest SRB won't contribute that much dV to a launcher of any size, but what it will contribute is thrust. So I do liquid boosters if I need dV, and SRBs for additional thrust.

[pincushionman]

Actually the original asparagus design makes no mention of transferring fuel, though it did use liquid kerosene rockets

Good to know. But I don't think I've ever seen a thread in this forum about asparagus that didn't inclide a vampire core.

…Now watch you find one for me.

[Sidereus]

If I just need a bit extra dV to get into orbit, I'll slap on some SRBs. Otherwise, they never seem worth the extra problems that come with using them.

[Laie]

As a side note, solid fuel is the most expensive means of propulsion in the game. If you check what the stuff costs and how much thrust you get out of it, even a Poodle at sea level will be more cost-efficient. What makes them worthwhile (sometimes at least) is the cheap case -- discarding a spent booster is much cheaper than the equivalent amount of Poodle and liquid fuel tanks. But with deouplers being as expensive as they are, that still often doesn't work out.

[panzer1b]

I use SRBS alot with SSTOs. Or should i say when im too impatient to actually SSTO in a SSTO, ill just boost it to like 30km on say 3 of those massive SRBs, and then just finish off with the jets/rockets of the actual craft.

Anyways, for serious matters, they dont have much use. because i dont play with money enabled too much (i mostly do sandbox as ive gotten too used to the no limits of sandbox, and career seems to just limit me after being used to pre career days. In career its good early game, and it is very cheap, but SRBs are both heavy, inefficient, and arent exactly that much cheaper per what you get out of them to bother, + they tend to add alot to part counts.

only other use ive found SRBs to excell at, is anti-ship torpedoes, gl surviving a RT-10 in the face, almost always cuts a ship in 2 provided i can actually hit something with a unguided RT-10.

[THX1138]

A. Because you can't throttle them.

B. Because they have lousy ISP.

Who cares about ISP when the fuel is that cheap? All that matters is the 650 thrust you're getting for ~79 seconds. Carbon emissions aren't factored in.

Pretty much the same reason you use an internal combustion gasoline engine in your car, rather than burning straw in a steam engine.

The straw is way cheaper than the gasoline, but your performance is... a bit less.

Also, when you try to do something a bit tougher, you get to appreciate the performance increase.

Flying a SRB rocket to moho and back is about as easy as building a straw-powered steam Concorde.

I wasn't suggesting the final stages should be SRBs, just the lifting stages.

As a side note, solid fuel is the most expensive means of propulsion in the game. If you check what the stuff costs and how much thrust you get out of it, even a Poodle at sea level will be more cost-efficient. What makes them worthwhile (sometimes at least) is the cheap case -- discarding a spent booster is much cheaper than the equivalent amount of Poodle and liquid fuel tanks. But with deouplers being as expensive as they are, that still often doesn't work out.

That's the thing - you do have to discard the engines in early stages. Also, the decouplers aren't such a big deal if you attach multiple boosters directly to each other instead of to their own decouplers.

Edited February 4, 2015 by THX1138

[Superfluous J]

I don't asparagus much anymore. Most of my ships start with solids and end up with LFO rockets to get into orbit and beyond. However, when I DO choose to not go with solids, it's usually more to do with the fact that I'd need like 30 of them than anything else.

[Slugy]

If you are not recovering stages, which you generally won't for rockets in stock, using solids for as many early stages as possible is the almost always the cheapest way to go, with ~$1500/tonne to orbit fairly easy, and ~$1000/tonne possible.

Good asparagus will get you the lowest weights and best payload percentage, but beware asparagus designs that gain little over more normal rockets except to add complexity!

This all assumes you don't want to abuse jets - which will always win cost and weight for moderate payloads.

[Empiro]

As a side note, solid fuel is the most expensive means of propulsion in the game. If you check what the stuff costs and how much thrust you get out of it, even a Poodle at sea level will be more cost-efficient. What makes them worthwhile (sometimes at least) is the cheap case -- discarding a spent booster is much cheaper than the equivalent amount of Poodle and liquid fuel tanks. But with deouplers being as expensive as they are, that still often doesn't work out.

Unless you're playing with a part recovery mod, then you'll be discarding early stages anyways. Liquid engines not too expensive, and liquid fuel is pretty cheap, but liquid fuel tanks are extremely expensive in KSP. SRBs are a cheap way to increase your TWR for your launch stage, and let you get away with a smaller, more efficient engine in your core.

[SRV Ron]

A better solution is using a pair of the three types of SRBs with the RT-10 being the first to stage followed by the BACC , then the KD25k. If you do tweak the thrust, do so evenly among the three types of boosters. Plan on having to do an orbital turn with the KD25k still burning.