Asparagus or just srb ? the cheaper option

[looper_1234]

i know asparagus staging is super efficient in terms of weight, but is it cost effective ?

would a pure srb stage get the same job done cheaper ?

[Superfluous J]

SRBs are always cheaper for launching from Kerbin. Asparagus is always more expensive.

The only way asparagus is "cheaper" is weight. SRBs are heavy so hauling them to your destination and landing them is a bad idea. That's why Tylo and Eve landers are frequently asparagus staged instead of SRB'd. The Tylo lander is more expensive, but it's lighter so getting it there was - all things considered - cheaper.

[Alshain]

I agree with what 5thHorseman said. As far as getting to orbit goes, radial LFO engines just add a huge cost compared to an SRB.  A pure SRB is also going to be cheaper but that is a little further extreme as it will be a bit harder to pilot and harder to balance just right, especially if you want to do a hands off turn.

Your simplest balance of piloting vs cost is to have a 2 stage LFO center stack with SRB's radially.

Why 2 stage if I said LFO engines are expensive?  Because eventually fuel costs do add up.  LFO engines in KSP can be simplified into 2 categories.  Low Atmosphere and High Atmosphere/Space.  As you ascend you will transition from one category to the other.  Low Atmosphere engines will chew through fuel the higher you go and so you will need a lot more of it, which just makes the whole thing heavier and you need more engine.  It's a problem that feeds on itself and just gets more expensive.  Having a small kicker stage with a vacuum rated engine for final orbiting maneuvers is a better balance and easier to fly.

So in short, 1 kicker stage for final orbit, 1 lower stage, radial SRB's as needed to augment fuel and/or thrust.

 

Asparagus is not totally useless though, it's very useful for dropping mass on interplanetary burns when SRB's aren't practical.

Edited October 8, 2016 by Alshain

[problemecium]

Heh.

My concerns about cost have always led me to just go SSTO. Not spaceplane, necessarily, but basically all of my launch rockets since very shortly after I started are a single stage that parachutes back down into the ocean for recovery. It admittedly makes for a lousy payload fraction, but I at least like to tell myself it makes for a much cheaper cost per trip.

[looper_1234]

That makes sense, I've being using asparagus staging exclusively ever since i heard of them. but scaling up is very costly.

 

[Vectura]

4 hours ago, Alshain said:

Low Atmosphere engines will chew through fuel the higher you go  

I was under the impression that the thrust was what changed, not the fuel flow?

[Scotius]

How much mass do you intend to lift? Launching big station cores or heavy landers (for Tylo, Eve and mining) is not really practical with extremely massive stack of SRB's. My standard heavy launcher can heave - up 70 tons to LKO using 7 Jumbo tanks and Mainsails (with fuel left for spare after achieving orbit). I shudder to think how many SRB's i would have to pile up to do the same. And back in the day - before game engine optimisations? Good Kraken - that would be a nightmare

[GoSlash27]

Back in 1.0, we found that the cheapest option for large disposable lifters was a pseudo- asparagus arrangement. SRBs are used to lift LF&O drop tanks, and are arranged asparagus style to feed a cheap general purpose LF&O core.

http://s52.photobucket.com/user/GoSlash27/slideshow/KSP/CnCRocketFactory/Cheep135

For smaller payloads, SRBs serial staged behind vacuum engines was cheapest.

@Nefrums created a pseudo- asparagus lifter that cost less than $600/ tonne.

Best,
-Slashy

[KerikBalm]

A key consideration for going beyond LKO is cost per ton.

SRBs are bad for ascent vehicles on other worlds because they are more massive, so you need a more massive transfer stage, and a more massive lifter... which means more expensive lifters and transfer stages.

When you don't have to lift the SRBs anywhere before using them, use SRBs.

[magnemoe]

2 hours ago, Scotius said:

How much mass do you intend to lift? Launching big station cores or heavy landers (for Tylo, Eve and mining) is not really practical with extremely massive stack of SRB's. My standard heavy launcher can heave - up 70 tons to LKO using 7 Jumbo tanks and Mainsails (with fuel left for spare after achieving orbit). I shudder to think how many SRB's i would have to pile up to do the same. And back in the day - before game engine optimisations? Good Kraken - that would be a nightmare

True, crossfeed rules the very heavy launches. As this tend to be huge ships or bases designed for interplanetary trips its usualy an benefit to finish the orbital burn and circulation using its own engines, using an drop tank or refuel in orbit.  
 

[Alshain]

5 hours ago, Vectura said:

I was under the impression that the thrust was what changed, not the fuel flow?

Fuel flow doesn't change, fuel efficiency changes.  That is why each engine's stats has a listing for Vacuum ISP and Atmospheric Sea Level ISP

[Alshain]

5 hours ago, Scotius said:

How much mass do you intend to lift? Launching big station cores or heavy landers (for Tylo, Eve and mining) is not really practical with extremely massive stack of SRB's. My standard heavy launcher can heave - up 70 tons to LKO using 7 Jumbo tanks and Mainsails (with fuel left for spare after achieving orbit). I shudder to think how many SRB's i would have to pile up to do the same. And back in the day - before game engine optimisations? Good Kraken - that would be a nightmare

Three.  This is my 70 ton lifter at 114,000.   A 7 Orange Tank/Mainsail combo costs around 137,000.  So you are throwing away about 23,000per launch using asparagus (and my design is most likely not the most cost efficient either).

Edited October 8, 2016 by Alshain

[Scotius]

At the point i need heavy lifters in game, contract payments usually come in a six digit range (or more). I can afford to throw away a bit of income for the sake of my nerves

[magnemoe]

Miner 3 renamed Minmus main, used an 1+2+2 aspargus+ SRB too go to minmus, took some time to get right, found that nosecones on top did not have any noticeable impact, probably because of all the radial stuff. Too large to put in an faring, KER is lying it had far more dV as it reached Minmus, even with the huge drag, 
Yes had I had something in orbit to refuel I could save a lot however the munbase one miner was too tiny to fuel it in LEO, it was in Minmus orbit in case it needed help circulating and landing. 
 

[ZentroCatson]

I find SRBs most useful for kickstarting a rocket with a Kerbin sea level TWR of >1.00, that's what they're there for IRL. Past Kerbins atmosphere, they're useless. If I have a launcher with high TWR that needs more delta-V, I just attach some drop tanks. Asparagus is really useful for ascending from Eve or Tylo, some people also use it for their LV-N powered interplanetary transfer stages, though I haven't used that method. 

IRL small SRBs are used for getting smaller payloads into orbit. They're called payload assist motors. They are used because they're more reliable than engines with liquid propellant.

Edited October 10, 2016 by ZentroCatson

[Terwin]

For small satellites, I'll generally use a single kick-back as the launch stage, but any larger than that and I go with a liquid core.  Generally my launch stage has a single LFO engine and as many SRBs as needed to get a decent starting acceleration, when I need more fuel, I tend to add it on the SRB stacks before adding it to the core.(As the initial core stage is generally close to 1.0 TWR when fully fueled, adding any more fuel to it generally means I need to resort to Thuds if I don't want to start slowing down when I stage my SRBs, but adding ~60 seconds worth of LFO on top of the kick-backs just means that my lifting core starts as if launched after 60 seconds of SRB assisted acceleration) 

[eddiew]

Cheaper, but less controlled. As @ZentroCatson says above, I'll use them to get something moving off the launch pad if its TWR is a bit low, but it's rare that I still have them attached by 10km because they make flying an efficient gravity turn so darn hard. When you're verting on 4-5 TWR at the end of the SRB burn, you just end up in a steeper ascent than you wanted and have to pack more delta-v into the launcher overall than it really needs.

[Nich]

As everyone else has said they should only be used on the first stage off kerbin.  In addition they should only be used at full throttle.  I have found the optimal point to be around 1110-1300 m/s dv from the SRB.  Any more and staging to an LFO second stage gets you more dv for less.

The major problem of the Flee and the Thud are the radial decoupler costs more then the SRB.  Thumper is the First SRB that makes sense kredit wise.  (Thud and flee can be used inline cost effectively)

[KerikBalm]

In my 1.2 pre release career, I'm having quite a bit of success using a single kickback as my first stage for most of my small rockets.

I forsee keeping it this way until I have 3.75m engines. I think the dV to orbit is just too low to make asparagus attractive.

Its completely different on Eve, of course. I have yet to see an eve lander using SRBs, even if ti was just hyper edited there... I wouldn't be surprised if an SRB first stage worked ok for eve though. I should start a cheapest eve sea level lander challenge, instead of lightest (which was won by asparagus lifters, of course)

However, lets figure it costs 500 funds/ton to send something to Eve. Add that cost on to parts according to their mass, and SRBs won't look appealing any more.

[magnemoe]

2 hours ago, Nich said:

As everyone else has said they should only be used on the first stage off kerbin.  In addition they should only be used at full throttle.  I have found the optimal point to be around 1110-1300 m/s dv from the SRB.  Any more and staging to an LFO second stage gets you more dv for less.

The major problem of the Flee and the Thud are the radial decoupler costs more then the SRB.  Thumper is the First SRB that makes sense kredit wise.  (Thud and flee can be used inline cost effectively)

Use the 50 fund small hardpoint for the small SRB's. perhaps even the medium, the Thumper tend to get an seperatron, its also more of an sustainer than a TWR boost, they are also nice to use with variable trust, you have one set with 100% trust and the other with say 60%  
For advanced use put Thuds outside the ThumperS
 

[Nich]

@magnemoe Unfortunately Advanced Aero is a bit to late in the tech tree for me to use it with most of my rockets in addition it is in a tech line that I never unlock until the very end of the science tree.  What are the kerdits/ton of whiplash based SSTOs?  I know Rapeirs can get down into the 50's but I have never looked at the whiplash in much detail.

 

[Draco T stand-up guy]

After reading this thread I built this:

It's about 5000 funds cheaper than what it would cost with full LOH tanks and engines. Drops from ~17k to ~12k.

Edited October 11, 2016 by Draco T stand-up guy

[Plusck]

On 08/10/2016 at 5:59 PM, Alshain said:

Three.  This is my 70 ton lifter at 114,000.   A 7 Orange Tank/Mainsail combo costs around 137,000.  So you are throwing away about 23,000per launch using asparagus (and my design is most likely not the most cost efficient either).

I'm pretty sure a Mainsail-cored asparagus design could do the same for a cost of about 75-85k funds.

[Alshain]

4 hours ago, Plusck said:

I'm pretty sure a Mainsail-cored asparagus design could do the same for a cost of about 75-85k funds.

Sorry but no.  That's not possible.  The cost of 7 tanks, 7 mainsails, and 8 decouplers (including the payload/upper stage decoupler) is 137,000.  I priced that out before I even posted.  That's not even including fuel lines or nosecones for those huge radial stages.

Here it is right here.  I don't know how I could meet his description and strip the cost any more than that.

Edited October 11, 2016 by Alshain

[Scotius]

Agreed, but Mainsails and Jumbo tanks come much earlier in Tech Tree than Mammoth and big tanks. So, either you wait for them or use what you have