Differences between using 8 solid boosters vs 4?

[Kerkulous]

Hi everyone, I just started a career game and I was given a mission to get into orbit. while building I attached 4 solid fuel boosters to get my rocket past the atmosphere. I know using 8 would make it faster and get a higher altitude, but why do I not see people who are part of guides on going into orbit (or any guide i have seen) do that? Is it unneeded or negitivley affecting the control?

[MR L A]

Firstly, welcome to KSP and the forums!

29 minutes ago, Kerkulous said:

Hi everyone, I just started a career game and I was given a mission to get into orbit. while building I attached 4 solid fuel boosters to get my rocket past the atmosphere. I know using 8 would make it faster and get a higher altitude, but why do I not see people who are part of guides on going into orbit (or any guide i have seen) do that? Is it unneeded or negitivley affecting the control?

Because it's a horribly inefficient design, but particularly "using 8 would make it faster and get a higher altitude" - this is wrong as it is not necessarily true and, technically speaking, the highest flying (far from kerbin) and fastest are almost never solid fuel designs.

1) You don't want to be going so fast you burn-up on the way out of that atmosphere.

2) SRBs have no throttle control which winds up ruining your gravity turn (part of the efficiency problem tbf).

3) You don't always need even one SRB to get into orbit, never mind 8 of them.

I'm sure somebody will give you a much more detailed response soon enough, but basically your fundamentals of the game (and basic rocket science) are a little off. I'd recommend Scott Manley, he has some extremely useful videos for beginners (even if some forum members do seem to have a problem with him for some reason). There have been a fair few updates since these were posted but the majority of it is still relevant.

 

There are also videos from the likes of Matt Lowne, Hazzard-ish, ShadowZone and many, many others that you will find useful, inspiring and down-right entertaining  

 

Happy exploding!   

[DeltaDizzy]

3 minutes ago, MR L A said:

I'm sure somebody will give you a much more detailed response soon enough

 

48 minutes ago, Kerkulous said:

Is it unneeded or negitivley affecting the control?

look in the spoiler for the simple answer, and below it for the complex one.

Spoiler

You need to get the most performance with the least mass. 4 solids do this just fine, but 8 add little performance and a lot of weight.

More SRB's and their fuel add more weight, which needs more thrust to lift it. Most of the time SRB's have such high thrust compared to their mass that this isnt an issue, but too many can negatively affect your rocket. This is due to the rocket equation.

It is easier to understand when plotted out like this:

The mass ratio is the mass of the rocket + fuel(wet mass) divided by the rocket mass without fuel(dry mass). As you can see(I cant make it any bigger sorry), your DeltaV goes up exponentially as the mass ratio decreases(as your fuel/rocket gets lighter). This is why you need to get the most performance with the least mass. 4 solids do this just fine, but 8 add little performance and a lot of weight.

[Zeiss Ikon]

You can fly a Mk. 1 Command Pod into orbit with no SRBs, just the right amount of tanks and a Swivel engine.  You can even keep enough fuel to deorbit and land in the ocean without a parachute (by propulsive braking just before impact).

What SRBs are good for is adding short term thrust, cheaply (compared to building a much bigger rocket).  The Space Shuttle used SRBs because the main engines couldn't even lift the full tank and orbiter, never mind push them into orbit on their own -- but the SRBs only burned for about two minutes, and by then enough propellant had been burned from the big external tank that the main engines could push the stack the rest of the way (another 6 minutes, approximately).  And  the SRBs were never replaced with better performing liquid fuel boosters (which might even have been made to fly back to the launch site -- more like a glider than a Falcon 9) because the SRBs were so much cheaper than a comparable super high thrust liquid fuel engine and tank stack.

[FinalFan]

2 hours ago, Kerkulous said:

Hi everyone, I just started a career game and I was given a mission to get into orbit. while building I attached 4 solid fuel boosters to get my rocket past the atmosphere. I know using 8 would make it faster and get a higher altitude, but why do I not see people who are part of guides on going into orbit (or any guide i have seen) do that? Is it unneeded or negitivley affecting the control?

I would disagree somewhat with MR L A and DeltaDizzy, but to be clear I also agree with some of what they say.  Let me elaborate. 
—Solid rocket boosters are usually the cheapest "kilonewtons of thrust per credit" you can get on your launchpad stage in a multistage rocket.  So it's cost effective to use them. 
—Adding "moar booster" as the saying goes does make your takeoff faster and faster (unless your rocket already has more TWR than the SRB, e.g. above 2.5 for the largest SRB and even higher for smaller ones). 
—However, SRBs are also quite heavy, so it's not without drawbacks:  if your launchpad stage also has a liquid fuel and oxidizer rocket, it's doing less and less of the work pushing the rocket.  SRBs can't steer like almost all LFO rockets can, so it's harder to control your rocket.  You can still use aerodynamic fins and control wheels (and even RCS), but it's not as easy as if you were using mostly LFO rockets.  MR L A is correct that if you are trying to make orbit you should be aiming for a launch profile close to the fabled "gravity turn" and this can be harder when packing SRBs. 
—Also, you can't throttle SRBs in flight.  You can build the rocket to fire them at less than 100% thrust, but once you hit the launchpad you can't change it from 50% back to 100% or whatever.  It's set.  This can occasionally be an issue since the thrust of an SRB that's almost dry will accelerate the rocket a lot more than one that is full on the launchpad.  You can have too much acceleration!  But you can throttle your LFO engine down to compensate. 
—LFO rockets that are designed for your launchpad stage are going to be lighter and more powerful than SRBs, even counting the fuel they have to carry (lighter compared to their power). 
—DeltaDizzy is right that the rocket equation would mean SRBs are just never good ... if you had to carry the empty ones all the way to the destination.  But if you ditch them as soon as they run dry, adding "moar booster" won't hurt the wet/dry ratio.  So be sure to plan your staging when using SRBs!  (I mean technically if your rocket had better than 4.3:1 ratio AND ran dry as fast as the SRBs did, but ... no.  I am not convinced such a rocket is even possible in KSP.) 

So to sum up, SRBs are less powerful, more heavy, less controllable, and all around just not as nice as LFO engines, and require more planning to boot ... but they're way cheaper.  And that makes sense!  I would guess that the reason you don't see them on guides is that adding SRBs is an extra complication to the rocket.  They are trying to teach someone the basics of getting a rocket to orbit or whatever, and messing around with extra stages might just be an unnecessary possible point of confusion.  Personally, I think that most people tend to use SRBs either in the very early part of the game as the sole launchpad engine (with LFO as the second stage on the rocket), or if they want to launch heavy stuff into orbit, but not so heavy that you'd need a stupidly huge amount of SRBs to lift off. 

If cost is no object, avoid SRBs.  If you care about a rocket's cost, go ahead and use them on the launchpad as much as you and your designs can tolerate.  (Unless it's an SSTO rocket, in which case the only part of the LFO rocket you lose is the fuel.) 

[AngrybobH]

To sum up the above answers (all good advice) in a basic usable way, use just enough SRBs to boost the initial TWR of the rocket to the desired amount. Then adjust the burn time (with the thrust limiter) to make the SRBs burn out when you have burned enough LFO in your tanks that the TWR of the remaining rocket is now good enough to do the job. What TWR you want depends on your rocket and probably other factors. I usually go for 1.4 to 1.6 for initial launch.

More speed in the atmosphere creates more drag, which requires more power to fight, which requires more fuel(or more engine). Only go as fast as is required to get the job done. Too fast = waste fuel on drag. Too slow = fight gravity for too long so also waste fuel.

Without mods you will not know your TWR unless you do the math yourself. But, you can just trial and error your way to orbit. If you don't already know, KER and MechJeb are the two most used mods for this information.

[FireKerb]

Basically, it's because adding more boosters will end up having less benefit.

Think of a motorboat. It can go moderately far. Now think of two motorboats tied together. It should go twice as far as the single motorboat, right?

Wrong.

Remember, you aren't just doubling the amount of fuel. You're also doubling the amount you have to push.

Now apply that same logic to the rocket. Doubling the amount of boosters will eventually only get you a little bit further.