Multi stage help

[Lankspace]

How does one decide weather to use a Multi stage Rocket or  just a single setup with lots of Boosters?

I have looked at the Web and lots of help on building but nothing on the theory?

Sorry or am I missing something?

 

[Starhawk]

When you build a rocket using a single stage, you will face severely diminishing returns as fuel/engines are added to the craft.

In order to deal with this, one builds in stages, creating a craft with sections that are used up and thrown away.

Generally, each stage that is discarded (staged away) is quite a bit larger than the one above it.

This can be easily seen in the case of a rocket like the real-life Saturn V.

Kerbin is much smaller than Earth and this results in a much smaller velocity needed to achieve low orbit.  This means that the ships can be much smaller than RL rockets, but staging is generally still required in order to create craft that can make orbit, let alone go anywhere significant after that.

There is much much more to say on this topic, but that will give you a general idea to get started with.

Good luck and ...

Happy landings!

[HebaruSan]

Different engines are optimized for different parts of your trip to orbit. For example, the Terrier is excellent in vacuum but useless at sea level. So if you want to use a Terrier, you need to use some other engines to get it into space. And once you're there, you might as well discard those other engines.

But the main reason is the role played in the rocket equation by dry mass, the part of your rocket that isn't fuel that you're pushing around. If you landed on the Mun and returned with a single stage rocket, by the time you got back you'd have a tiny amount of fuel sloshing around in gigantic heavy fuel tanks, and you'd be burning it in huge overpowered heavy engines (since they had to be enough to lift your vessel out of the atmosphere when it was filled with fuel). That same amount of fuel would go much further if your vessel was smaller, i.e. the upper part of a staged design.

[Zhetaan]

@Lankspace:

Others have covered theory of construction well enough that I don't need to repeat them.

In terms of theory of design, however, I can say that the overall goal is to get something into space such that it doesn't:

• cost too much,
• blow up on the way there,
• fall back down later, and
• fail the mission.

Usually for real-life space programs, cost is the deciding factor.  There are a lot of missions that don't fly because they cost too much.

For KSP, the practical issue of getting something to space--and making it stay in space--intact is more important.  The main difference between the two designs you gave as examples involves drag versus thrust:  a rocket with boosters has more of both.  Sometimes, that's needed, and the reason for that is because getting off the ground requires you to fly in what would otherwise be the most inefficient way possible.

I will assume that you know about how to manoeuvre in space.  If not, then please accept that you generally want to change your orbit by thrusting in the prograde/retrograde direction, with normal added if you need to change inclination.  Radial burns (at right angles to both prograde and normal but meaning roughly toward or away from the planet) are the least efficient because they don't add much useful velocity to your rocket and don't add any inclination at all.  There are a couple of very specific use cases for them but normally only as corrective manoeuvres.  When you launch, however, your rocket begins by going straight up--which is to say, radial out.

Because of both the atmosphere and the gravity, it is necessary to include a radial-out component in your launch burn.  Obviously, radial-out keeps you from falling down to the surface, but it also gets you out of the atmosphere, which otherwise parasitically drains prograde velocity.  Spaceplanes get this radial-out component from lift, which is what makes spaceplanes potentially so efficient, but wingless rockets cannot take advantage of that.  However, with the right rocket design, you can immediately begin adding prograde velocity at launch in such a way that gravity and drag cancel the radial part at the correct rate to match the declining need for radial velocity--that is to say, you want gravity and drag to work against you so that when you don't have any radial velocity left, you don't need it anymore, either.  That's the idea behind a gravity turn.

Ultimately, your choice for a rocket design is tied to how well it will fly to orbit.  'Well' can be defined as a matter of cost, weight, thrust, or a combination of these and other factors.  One thing to remember is that because of the way gravity works, you will only need to provide a specific value of thrust to counter it, so anything above that value is thrust that you can use for other things, such as going to orbit.  Side boosters provide that extra thrust (which means, practically, that you can tip the rocket a bit more on your way to orbit), but that also has you going faster in the atmosphere which increases drag and heat and wastes fuel--there's also such a thing as too much thrust.

On the other hand, a long inline rocket will fly very well through the atmosphere--provided that it is pointed in the same direction that it flies.  Also, with less thrust generally available, a larger proportion of it must be used to counteract gravity.  This requires a longer gravity turn and wastes fuel--there's such a thing as too little thrust, as well.

You are always going to 'waste' fuel to fight gravity in any surface launch; the object is to find a design that wastes less of it.

[MrSystems]

I agree with @Zhetaan; I usually make the determination based on the following two factors:

• Which setup costs less?
• Which setup is less likely to topple over in flight?

Suppose your payload + upper stage has a mass of 50 tons, and you need 2,200 m/s dV from your main stage + booster.  If you try to get it all from a liquid-fueled engine with min total TWR of 1.8, you're looking at a Mammoth + 97 tons of fuel, which costs $58,400.  If instead you split it to 1,400 dV from the main stage + 800 dV from the boosters, you're looking at a Mastodon* with 43 tons of fuel and cost of $16,600, plus three Kickbacks at a cost not including separators and struts of $8,100, which is about $30,000 less expensive than the other option. (*If you don't have Making History, it would be a Mainsail with 45 tons of fuel, for $5,500 more than the Mastodon but still $25,000 less than doing it without SRBs.)

In real life big rockets use boosters for a reason--they're a less-expensive way to add thrust and delta-V at launch.

Two more tips: The Small Hardpoint and Structural Pylon, which are in the 'Structural' category in the VAB and unlocked with Advanced Aerodynamics and High Altitude Flight, can be used as decouplers for SRBs and cost much less than ordinary radial decouplers. And the mod SpaceY-Lifters has a great assortment of SRBs that provides a lot more flexibility than the handful of stock options.

I made a calculator for use in Excel that will help with planning.

[Lankspace]

46 minutes ago, MrSystems said:

I agree with @Zhetaan; I usually make the determination based on the following two factors:

• Which setup costs less?
• Which setup is less likely to topple over in flight?

Suppose your payload + upper stage has a mass of 50 tons, and you need 2,200 m/s dV from your main stage + booster.  If you try to get it all from a liquid-fueled engine with min total TWR of 1.8, you're looking at a Mammoth + 97 tons of fuel, which costs $58,400.  If instead you split it to 1,400 dV from the main stage + 800 dV from the boosters, you're looking at a Mastodon* with 43 tons of fuel and cost of $16,600, plus three Kickbacks at a cost not including separators and struts of $8,100, which is about $30,000 less expensive than the other option. (*If you don't have Making History, it would be a Mainsail with 45 tons of fuel, for $5,500 more than the Mastodon but still $25,000 less than doing it without SRBs.)

In real life big rockets use boosters for a reason--they're a less-expensive way to add thrust and delta-V at launch.

Two more tips: The Small Hardpoint and Structural Pylon, which are in the 'Structural' category in the VAB and unlocked with Advanced Aerodynamics and High Altitude Flight, can be used as decouplers for SRBs and cost much less than ordinary radial decouplers. And the mod SpaceY-Lifters has a great assortment of SRBs that provides a lot more flexibility than the handful of stock options.

I made a calculator for use in Excel that will help with planning.

There are lots of useful posts too many to respond to, this one quoted has a nice spreadsheet to aid in calculations thank you ALL for the help. Most of this I have learned over the few weeks of playing by trial and error and using You-tube. I use the multistage system I also use the asparagus staging system. I wish I had found this Forum sooner.

[bewing]

One reason to use a setup with a lot of SRBs (rather than multi-stage) is if you need the bottom-most part of the main stack to be accessible from the ground when you launch it. It's an unusual thing to need, but it happens to me when I launch my first Mobile Lab in the game.

Another is if the thing you are trying to launch is basically an SSTO spaceplane, and you are launching it vertically. It can be difficult to mount SRBs behind the spaceplane's engines -- but strapping a couple SRBs to the sides or wings sometimes works.

One other thing that I like doing is having a standardized first stage liquid fuel booster that just barely makes it to LKO. (If it needs a little extra help making it to LKO, then I strap SRBs to the sides of it.) Once I have several of these boosters in LKO (each with a little fuel remaining) I dock them together. This quickly makes a huge orbital fuel dump for me to visit -- when I need fuel, or when I have some extra and I'm just about to deorbit, so I can give it away.

[Starman4308]

When optimizing for mass, frequent staging should be your preference, especially asparagus/onion staging. The quicker you shed dry mass, the better. Dry mass is the enemy of the Tsiolkovsky equation.

When optimizing for cost, it gets a lot trickier, since engines and decouplers are fairly expensive relative to propellant tanks. Because of that, cost-optimized launch vehicles tend to be a bit propellant-overloaded, becoming heavier and likely incurring extra gravity losses relative to a mass-optimized vehicle.

Stock is relatively forgiving in terms of how often you stage, since fuel tanks are relatively heavy and expensive. Staging "too early" helps your delta-V, ameliorating the cost of excess decouplers/engines. Staging "too late" likely means a slight propellant overload, and while fuel tanks are relatively expensive... they're still cheaper than engines.

[StrandedonEarth]

On 2/6/2019 at 12:19 PM, MrSystems said:

The Small Hardpoint and Structural Pylon, which are in the 'Structural' category in the VAB ... can be used as decouplers for SRBs and cost much less than ordinary radial decouplers.

Thousands of hours and probably thousands of launches later, I keep forgetting about these. I really need to give that a try. 

[Tyko]

I'll launch with the fewest stages possible to get achieve my mission. Why add more stages/engines/complexity than you have to?

If I can get all the way to orbit with one stage (SSTO) I'll do it. If not I'll first try adding boosters so my center core engine can still fire at launch and isn't just dead weight. If that's not enough I'll add another stage stacked on top of the core. 

A two stage stack with side boosters should be sufficient to launch most payloads. When you get into truly monstrous payloads you may need more.

[Curveball Anders]

When aiming for cheap launches (as in standard multi launch crafts like UpDownTaxi (aka Soyus) or Progress/Refuel) I go for SRB with LF tanks on top (hybrid aspargus).

I aim to drop the core launcher just shy of 70k orbit.