Did 1. 0 nerf SRB's too much?

[Hyomoto]

I can say they are no longer very useful. Dollar per tonne they just don't do it anymore and you can check their deltaV stats to back it up. Isn't the point of the SRB to be a cheap lifting option? Right now it's always cheaper to just build liquid boosters both in weight and cost. I think they still have a TWR advantage, but that's three strikes against and only one for.

[Redhaze]

I'm in the habit of using entirely SRB-powered first stages for lighter spacecraft up until three-seater pods. Even counting in the added cost of fins (due to lack of gimbaling) and structural parts like TVR400L quadcoupler my S1 SRB-KD25k solution is about √5000 cheaper and about as Dv-capable compared to the LFB KR1-2 version. (which is massively OP in it's own right compared to similar liquid engines, btw)

Other SRBs, however, not so much. They need a considerable un-nerfing to be useful again.

(Seriously. the LFB KR1-2, substracting the big orange tank it incorporates, has about 4 times the TWR of the S3 KS-25x4 and KR-2L+, it's ridiculous)

Edited May 12, 2015 by Redhaze

[Caelib]

Typically, if you're just strapping an SRB on to your rockets and not tweaking the thrust, then you're probably wasting your time using them. To get the biggest benefit from them, it is best to use them in tandem with a main engine so you can control your acceleration.

Here is how I use them:

1) install Kerbal Engineer mod (or whatever you like) so you can see the thrust-to-weight ratio for each stage.

2) build your rocket, place the SRBs in stage 1, radial decouplers in stage 2 and main engine with a GIMBAL in stage 3 (this placement is temporary)

3) look at your TWR in stage 1 (SRBs) - tweak the thrust until it reads ~1.0 (or a little higher if you like)

4) move your main engine from stage 3 to stage 1.

Now your TWR in stage 1 will be a lot higher than 1.0. When you launch, use maximum thrust from your main engine and slowly decrease it once you're above 100m/s so you're maintaining a steady-but-not-too-fast rate of acceleration. As you climb, your SRBs are going to be doing progressively more work than your main engine.

Why do all of this? You're trading thrust for additional burn time in your SRBs without over-accelerating. That being said, I find liquid boosters a lot more practical, but they are also a lot more expensive. Keep in mind that the whole point of SRBs is a low-cost, high-power first stage booster. I think very few people are actually taking advantage of their tweak-ability to increase the burn time.

Edited May 12, 2015 by Caelib

[karolus10]

Probably the most omitted fact about SRB's is that the fuel layer burns on the inner surface of the empty chamber drilled trough the middle. If the fuel and diameter is the same for each SRB then every each of them would work for (almost) same amount of time and thrust would be bigger proportionally to it's length.

[KerikBalm]

Tweaking SRBs for a longer burn time seems like a bad idea... You're nerfing their TWR... their main advantage, and carrying the mass longer. This is particularly bad if you have SRBs running in parallel with LFO rockets.

[Korizan]

Probably the most omitted fact about SRB's is that the fuel layer burns on the inner surface of the empty chamber drilled trough the middle. If the fuel and diameter is the same for each SRB then every each of them would work for (almost) same amount of time and thrust would be bigger proportionally to it's length.

Very true.

I am not even sure that KSP is using a thrust curve for there SRB's let alone which one they are using.

or for that matter is it even worth it to add those kinds of calculations into a game.

[Acet]

I'm doing the same thing but aiming at 1.4 TWR from the SBRs only. Used to do it already in 0.9 but then the target was around 1.6 TWR.

I try to avoid using that liquid engine earlier on a launch (even if I activate it on the lowest stage, it's usually throttled down), since liquid fuel engines have weaker ISPs lower in the atmosphere (some significantly so). For me those launch SRBs (with their ISP in the low 200s) are there to lift all that precious liquid fuel and its engines up to the point where the atmosphere is thin enough that they become more effective ISP-wise than the solid fuel boosters.

Using solid boosters like that makes even more sense on 1.0 since the atmospheric ISP of most liquid fuel engines has been heavily nerfed and the new drag model means that SBRs with properly tuned thrust will easily get you to 8000m high going at 300 m/s, high enough that liquid fuel engines are more efficient.

Using this technique a broader range of liquid fuel engines is viable for early stages since when you get up to 8000m pretty much all of them have an ISP which is maybe at worst 15% lower than vacuum ISP, so for example the LV-909 becomes a viable choice for the craft falling in the weight gap between the 48-7S and the LV-T engines, and similarly the "Poodle" engine provides an aerodynamically superior alternative to LV-Ts when your payload is of size "large" and even the KR-2L becomes a valid choice again for early stages.

Even the liquid fuel engines with the best atmospheric performance still benefit for using SRBs to raise them and their fuel above the thicker part of the atmosphere since their vacuum ISP is typically 20% better.

[KerikBalm]

"since liquid fuel engines have weaker ISPs lower in the atmosphere"

-Still better than the SRBs

"those launch SRBs (with their ISP in the low 200s) are there to lift all that precious liquid fuel and its engines up to the point where the atmosphere is thin enough that they become more effective ISP-wise"

A Liquid fuel booster can do the same thing.... and LFO mix is cheaper... and if you use fuel ducts so that your core stage can help lift, you can reduce (eliminate?) the TWR disadvantage (without going full asparagus stalks).

Yesterday I was using SRBs alot.... Today, I'm looking at the stats, and finding they aren't making as much sense as I thought.

[Caelib]

Tweaking SRBs for a longer burn time seems like a bad idea... You're nerfing their TWR... their main advantage, and carrying the mass longer. This is particularly bad if you have SRBs running in parallel with LFO rockets.

The TWR increases as it climbs because you're burning off mass and have less atmospheric drag. At liftoff, the TWR only needs to be slightly above 1. If they aren't tweaked, most people are probably starting with an excessive amount and this gets wasted by over-accelerating and/or causing the rocket to become unstable. Try it for yourself and just climb straight up with a tweakable test using ~1 vs ~2 and you will see a difference in your ascent.

NASA uses SRBs that provide about 75% of the lifting force at lift-off and burn for 2 minutes:

http://www.nasa.gov/returntoflight/system/system_SRB.html

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html

Edited May 12, 2015 by Caelib

[Ulzgoroth]

Since being introduced to the advantages of the LV-T45, I've considered my solid boosters justified for giving that critical first few km of height and allowing me to use a Swivel as my first liquid stage rather than a Reliant.

[NeMeSiS]

"since liquid fuel engines have weaker ISPs lower in the atmosphere"

-Still better than the SRBs

"those launch SRBs (with their ISP in the low 200s) are there to lift all that precious liquid fuel and its engines up to the point where the atmosphere is thin enough that they become more effective ISP-wise"

A Liquid fuel booster can do the same thing.... and LFO mix is cheaper... and if you use fuel ducts so that your core stage can help lift, you can reduce (eliminate?) the TWR disadvantage (without going full asparagus stalks).

Yesterday I was using SRBs alot.... Today, I'm looking at the stats, and finding they aren't making as much sense as I thought.

SRB's are cheap though, and you dont discard some expensive engines as well as fuel tanks compared to LFO's. They are an easy and relatively cheap way to get off the ground, and also an easy way to give other designs a slight boost for heavier-than-expected payloads. They barely have a use beyond that.

[Sky_walker]

NASA uses SRBs that provide about 75% of the lifting force at lift-off and burn for 2 minutes:

http://www.nasa.gov/returntoflight/system/system_SRB.html

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html

Arianespace uses SRBs that provide 90% of Ariane 5 lift for 130 seconds

http://www.arianespace.com/launch-services-ariane5/ariane-5-intro.asp

Also unlike KSP boosters these used by Ariane 5 have a gimbaled nozzle up to 7.3° (which is more than almost any engine in KSP).

Edited May 12, 2015 by Sky_walker

[panzer1b]

The raw isp was nerfed, you cannot say this isnt the case. That said, with teh new aero and a slight decrease in dV needed to get into orbit (1.0 it was perfectly balanced, 1.0.2 made the drag so high that its not THAT much better then 0.90), SRBs should still perform similarly to before. The only major nerf came to some of the earlier ones,m RT-10 for example lost some fuel, and it has a similar dry mass, not to mention less ISP and less thrust in atmo. Its not a bad booster, and still works great for some lighter payloads, but as before, SRBs only make any sense if you care about cost in career, they make no sense from a part count/payload fraction perspective. I myself only use them for very light payloads when a single tall srb or 2 will get me into a suborbital trajectory where payload can burn to circularize.

that said, i like the new RT-5, makes a great low-part count unguided torpedo. And you can cram twice as many torpedoes in the same space as with RT-10s (they were great for torpedoes but they were so bit you just couldnt have many of them on a smaller vessel), and they are actually better as most of the time you would use RT-10s at sub 50% fuel loads anyways. Then again, id have preferred a 0.6m srb, as now i still need to stick to sepatron clusters for my ibeam missiles, not that they dont work well, but they just take up way too many parts.

Edited May 12, 2015 by panzer1b

[MacroNova]

I'm getting lots of use out of SRBs on my 1.25 and 2.5m crafts, but I can easily see that ending when I graduate to 3.75m designs.

[Superfluous J]

SRBs are cheap, heavy, and inefficient. Perfect for the first stage. But you should drop them as quickly as possible. This means that you should not have 2 of them tweaked down to 50 percent, you should have 1 at 100 percent. Generally, you should use a single stage of the most SRBs you can use and not get too high TWR, then get rid of them.

Tweaking them down a little is fine, that's just dealing with the coarse granularity in your options. But if you need more dV than a single first stage gives you, you should add more liquid fuel and/or LOX boosters. They're lighter, more efficient, and not having to carry those heavy inefficient SRBs any longer than necessary will save you money in the long run.

IOW, SRBs are working just fine, for their intended purpose. We just got lazy because of all the unintended purposes they were good for in earlier versions.

[Renegrade]

NASA uses SRBs that provide about 75% of the lifting force at lift-off and burn for 2 minutes:

http://www.nasa.gov/returntoflight/system/system_SRB.html

http://science.ksc.nasa.gov/shuttle/technology/sts-newsref/srb.html

Yeah, SRBs produce enormous thrust in real life. The SS-SRBs produced upwards of 26MN of thrust, which is almost as much as the Saturn V's S-IC first stage. I've always felt that the KSP SRBs fell short in that regard.

(without the SRBs, the Space Shuttle's pad TWR would have been 0.62)

SRBs should get at least 207 vacuum Isp, to be "realistically scaled" relative to LF engines, assuming some LF engines are using LH2

If we assume Kerosene, no scaling is needed, and SRB Isp should reach 268...

KSP isn't about hardcore, detailed realism. It maintains the relationship that SRBs have lower specific impulse than most liquid fuel engines, which is good enough. The only change I'd make is to reduce the cost of the SRBs, and restore the lost fuel to the BACC (and buff the thrust somewhat).

[MacroNova]

Has anyone ever damaged a KSC building jettisoning their SRBs at low altitude? I'm astonished every single time it doesn't happen.

[zarakon]

Yes, I think they got hit too hard. In my 0.90 career I used solids all the time for first stages. Now in 1.0.x they just don't seem worth it.

I think they nerfed too many things about them:

- Much lower ISP. While the liquid engines got hit by around 15%, the solid boosters lost around 30%.

- Worse mass fraction (more dead weight per unit of fuel)

- Less fuel and less thrust on the RT-10 and BACC

- More expensive

The changes total out to about a 40% loss in the delta-V that you can get out of a solid stage, while the prices have increased.

In 0.90 I could make a simple orbital rocket with a solid first stage and liquid second stage. Now if I try to do that, my second stage has to be much larger to make up for the weaker solids, which then means my solid stage has to be much much larger, and it's just not worth using over an all-liquid rocket.

Pre-1.0, there were two main (practical) purposes for solid boosters:

1. Cost-efficienct first stage with a tradeoff of being heavier and harder to control. Weight is significant in career mode before the launch pad is fully upgraded.

2. As a band-aid when you've designed a rocket with enough delta-V but not enough launch thrust.

They still fill role #2, although obviously not quite as well. But #1 is pretty much gone. In addition to still being hard to control and even heavier than before, they are no longer significantly (or at all?) cost-effective compared to using all liquid.

Edited May 12, 2015 by zarakon

[Entropiated]

I've arrived late to the party, but here's my two cents: Unless you're launching something very small and light, SRB's shouldn't be a stage unto themselves.

As the name would imply, they are BOOSTERS. They should be used to help shoulder the load while still low down in the atmosphere. All of my launchers have SRBs (as many as 8) around a liquid fuel stack. Testing allows me to throttle back the SRBs so that my speed remains at or about 200 m/s while lower than 10,000 m. I've been playing with this setup since 0.90, and now that they've fixed the issues after the release of 1.0, I'm getting to orbit just as easy as I ever was.

[Dooz]

They also bumped up the separatron heat the main fuel tank explodes when the separatron fires, IRL Im pretty sure a small soild rocket Is not enough to make a massive fuel tank explode. It made me redesign my lifters

[Destroyer713]

Truthfully, I don't see what makes SRB's worse than a regular LFO Engine now.

The way I have them set up for my first Mun lander in Career mode, I have two Kickbacks paired with a Skipper in the first stage, giving me an initial TWR of 1.77. When they burn out, the Skipper is able to carry me the rest of the way out of the atmosphere. If I were to replace those Kickbacks with some LFO tanks and a pair of Reliants, however, my TWR plummets to just over 1.0, which isn't enough to get me out of the atmosphere anymore. And that's not even factoring in the fact it costs me an extra 1,900 funds to make the rocket with the extra LFO engines and tanks.

Now, granted, this rocket may be a bit overbuilt, since reaching a 85 km Apoapsis with it leaves me with just under half a Rockomax 32 tank of unused fuel. That, and I have yet to unlock Mainsails, so I don't really know how those perform compared to SRB's.

[Fleb]

They seem inferior to liquid boosters in everything but part count, even the slightly lower cost of using them isn't really worth it for me.

[Renegrade]

I've arrived late to the party, but here's my two cents: Unless you're launching something very small and light, SRB's shouldn't be a stage unto themselves.

So what you're saying is the Scout, Athena, Mu, Minotaur family of rockets are insane, as well as the Taurus, Pegasus, Start-1, PSLV, Shavit and Vega units? By units built, solid-fuel-only staged rockets vastly outnumber liquid-fuel units: Modern ICBMs are all solid-fuel as was stated earlier in the thread.

As the name would imply, they are BOOSTERS. They should be used to help shoulder the load while still low down in the atmosphere. All of my launchers have SRBs (as many as 8) around a liquid fuel stack. Testing allows me to throttle back the SRBs so that my speed remains at or about 200 m/s while lower than 10,000 m. I've been playing with this setup since 0.90, and now that they've fixed the issues after the release of 1.0, I'm getting to orbit just as easy as I ever was.

I bet your designs would work better and cost less with liquid fuel boosters with the current stats. Also 200 at 10km sounds slow - the souposphere thankfully died, time to move on. The 'solid rocket booster' terminology comes from the Space Shuttle, although in reality the "boosters" of the Shuttle are basically carrying it as a first stage. The hydrolox SSMEs produce about as much thrust as an angry squirrel.

[EtherDragon]

I'm also late to this conversation, and I have not noticed an issue with them being nerfed. I used SRBs in all of my launches for Career Mode. They offer a metric-butt-ton of thrust early on in the launch, right where the LFEs are really weak. Most of the time, I'm tuning down the SRBs thrust limiter, so they burn longer and don't get me to terminal velocity just as quick.

I also make a habit of using a fully automated (i.e. control free) gravity turn during my launches. After the initial push to 5degrees off of vertical, I turn SAS off and let the rocket fly itself. The only control I have is the throttle, to hit my milestones (e.g. 60degree pitch by 15km, 45 degree pitch by 25km, 30degree by 30km...) so I have to make sure I don't get too much thrust too early or I get too high too fast.

[Lord Aurelius]

I agree that SRBs are currently underpowered in KSP. As others have said, not enough thrust compared to LF engines, abysmal ISP, high cost and empty weight. This combination basically makes them useless in most cases except in early career.

They also aren't balanced against each other all that well. The Flea and Hammer have decent TWR, but really short burn times and terrible ISP. The big NASA SRB has a proper burn time, but a poor TWR. The BACC is just bad all around (still!).

There's also some gaps in the SRB coverage. We really need a long, thin 0.625m SRB, the Flea and Hammer just look silly. They look like they should be vacuum kickers, not surface boosters. On the other end, we need a large 2.5m booster (long and thin, please!) along with possibly a 2.5m low profile vacuum kicker.

5m rocket parts wouldn't be a bad idea either (along with the requisite 3.75m SRB).