Boosters... why?

smol

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#1
I've been exploring boosters the last few days. Near as I can tell, boosters are a pair of side rockets that are fired off in conjunction with the central core rocket, and then the boosters are dropped when they are empty (via side-separators or docking ports). Obviously the boosters should have a burn time that is less than the central core rocket. The core rocket often is a higher efficiency, lower thrust rocket that has TWR needs lower than the boosting portion of the flight. One clear disadvantage of the boosted stage vs simple staging is that the boosted stage is hauling around partially empty tanks, and empty tankage has weight.

My question is, under what conditions are booster rockets the preferred option?

I ask, because I've run some experiments with boosting / boosted vs. serial staged rockets, and I feel like in every case the staged rockets produce solutions that are lower initial weight. I know lowest total weight isn't everything (even if it is everything to me!), so I guess some possible answers are:
  • Beauty
  • Replicating an existing real-world rocket system
  • Don't have to light up a large number of core engines mid-flight (e.g. SFS user interface issue)
  • Running out of build space???
Are there any other technical reasons why a boosting / boosted flight plan is superior to simple staging, however you define "superior"?
 

Earl

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#2
Boosters can help increase the Delta V of a rocket without making it taller. That can make it more stable for Max Q.
Also, you can use a variety of engines more reliably. For instance, if you had a solid rocket motor on a stage, you wouldn't be able to adjust throttle or anything. With boosters, the core can still adjust the throttle. That's really all I can think of.

Also they look nice.
 

Horus Lupercal

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#3
One clear disadvantage of the boosted stage vs simple staging is that the boosted stage is hauling around partially empty tanks, and empty tankage has weight.
Your conclusion is back to front. Dropping boosters is getting rid of un-used engines and empty fuel tanks, lowering dry mass and increasing core stage efficiency. It's 'mid stage' staging.

Easy experiment for you.

Make a multi stage rocket. Any rocket you like, as long as the core has 3+ engines and work out the ΔV.

Then take stage one, divide the tank mass in half. Leave one half as the core, and then half the other half and split on each side.
Place your 3 engines down, one under core and one under each booster.
Work out the ΔV.

Thank me later.

Why though?

Easy.

Lets make a rocket. 50t payload, 50t second stage with a frontier engine.
Then a core. 100t tanks, 3 hawk engines.
Should give you this as a result
Screenshot_2020-07-10-21-40-56.png

216.5t wet mass, 2834m/s ΔV. That'll get 50t to LEO with some change to spare.

Now, lets make the same rocket, but boosted.
Added a pair of separators, so the total mass is higher, but the ΔV is higher for exactly the same amount of fuel at 2869m/s

Screenshot_2020-07-10-21-41-45.png


But, what about smol small?
Ok, how about this then...?

Screenshot_2020-07-10-21-41-16.png

11m/s more ΔV, 2.1t less weight.

And this isn't an optimum balance. The benefits get much better if you use a more efficient core engine, alter the booster cut off times etc.

The central point is this.
In an unboosted rocket, after 44.26 seconds you've still got all the empty tank weight and all the engine mass and you keep them all the way to MECO at 59.01 seconds. With boosted, at 44.26 seconds you get to drop 12.4 tons of useless wieght, whilst still having exactly the same fuel mass to burn and either the same ISP (if you're using all the same engines) or better because you now have a solo, more efficient engine to continue on until stage one separation.
 
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smol

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#4
Easy experiment for you.
I think I didn't ask my question correctly. Using your examples, there is an accepted topmost stage and payload, it weighs 106T, and it can give itself ~1570m/s of dV. The remaining rocket needs to lift itself and the topmost stage off of earth (TWR=1.6, but any upper stage needs a minimum TWR of 0.80), and it needs to provide an additional ~1270m/s of dV to complete the mission.

There are a few strategies that fit these parameters:
(1) a single lower half: 3 Hawks, 100T tanks, TWR=1.66ish, weight=216.5T
(2) a boosted lower half: (booster=2 Hawks, 60T tanks) + (boostee=1 Hawk, 40T tanks), weight=216.9T
(3) a staged lower half: (lowest=3 Hawks, 57.5T tanks) + (middle=1 Hawk, 37,5T tanks), weight=215.0T

Overall maybe this isn't the best example, as the combinations are all coming in very similar, between 215T-220T. There isn't much gain from breaking up this bottom at all. Note that we are being a little loose with the mission parameters - in real SFS there would need to be more separators in all of these scenarios, but again even with correct separators added the solutions are all between 215T-220T. Kind of a wash.

I certainly know of many scenarios where option one (a single bottom half) is clearly inferior (by weight, by fuel weight, by size, etc) to option two (a boosted bottom half) and option three (a staged bottom half).

My question is, assuming you have already made the decision to break up the bottom of a proposed rocket solution (that is, you will not take option 1), what are the scenarios where option 2 (boosterising the bottom) is clearly superior to option 3 (serial staging the bottom)? and what is the definition of superior (it doesn't have to be rocket weight)?

The reason I ask is because I have run many breakup->boost vs breakup->serial-staging scenarios, and so far I haven't found any real justification for going the booster route instead of staging route in SFS.

Earl has an interesting suggestion that using boosters instead of staging can increase dV without making the rocket taller. Given the limited build space in SFS, that kind of sounds legit. Any other reasons?
 

Horus Lupercal

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#5
There are a few strategies that fit these parameters:
(1) a single lower half: 3 Hawks, 100T tanks, TWR=1.66ish, weight=216.5T
(2) a boosted lower half: (booster=2 Hawks, 60T tanks) + (boostee=1 Hawk, 40T tanks), weight=216.9T
(3) a staged lower half: (lowest=3 Hawks, 57.5T tanks) + (middle=1 Hawk, 37,5T tanks), weight=215.0T
You need to display the overall ΔV for each as well dude if you're gonna make comparisons on which is best as the weights are pretty much the same, but there'll be a clear winner in output (bang for buck).
There is a 4th (but much more complex option) called asparagus staging, which is the most efficient method of rocket manufacture (mass to ΔV). But it's an absolute nightmare to implement in SFS cos of how the fuel transfer system works.
Essentially you have your engine pack (doesn't matter what those engines are) and your fuel mass. You divvy up the fuel mass into chunks, and put a separator between them. You then need to attach the engines somehow so they're attached to the topmost chunk. Then in flight, you transfer fuel from the bottom tank to the top tank so that it is always full, and then drop the bottom tank when empty. Rinse and repeat until all your stage is dry, and then stage for stage 2. Because you're constantly getting rid of dry mass as soon as it's becoming irrelevant, you're never hauling dead weight and your TWR / ΔV gets better and better as you go.

Obviously there is a zero sum with this as well, where too much staging becomes counter productive cos you've got a 300kg separator every 15 grams of propellant or something. And there is also a hard engine cap as there is a fuel transfer speed limit (1 ton per second in 1.4.06) and if your consumption exceeds that then you're gonna have issues.


Note that we are being a little loose with the mission parameters - in real SFS there would need to be more separators in all of these scenarios,
True, but it'll only change the numbers, not the advantages. If you add say another 2 separators, a capsule, and some payload fairings, yes it'll lower the ΔV and change the output numbers, but the naked advantage boosting gives you is still there.


but again even with correct separators added the solutions are all between 215T-220T. Kind of a wash.
That's hardly surprising. The rocket is tiny so you're not going to see enormous gains as there simply isn't the weight to lose. However, bigger rockets aren't going to see a 2.5t mass/30m/s ΔV difference. You go massive and then the gains go into 3 figures. An example is my Excaliber shuttles. Just doing in flight fuel transfer from the ET to the orbiter so it comes away sooner gives me an extra 300m/s ΔV. And the bigger the class of shuttle, the more ΔV I gain.


I certainly know of many scenarios where option one (a single bottom half) is clearly inferior (by weight, by fuel weight, by size, etc) to option two (a boosted bottom half) and option three (a staged bottom half).
If you're going exact (and not the peculiarities of SFS of whole fuel tanks), boosted / staged is always better than a single stage. That only changes when the rocket is so tiny that the additional toys required for staging / boosting are a prohibitively high proportion of the total rocket mass. That zero sum game where you have more separators the fuel tanks. The bigger your rocket is, the better staging / boosting works for you.


The reason I ask is because I have run many breakup->boost vs breakup->serial-staging scenarios, and so far I haven't found any real justification for going the booster route instead of staging route in SFS.
And so it should be. Because you have now created a 3 stage rocket. And 3 stages is always better than 2, boosted or not. And 4 stages are better than 3.
But if you then altered stage one of your x number stages to include boosters, you get more ΔV. If you made another stage inside stage one, it becomes a x+1 stage rocket and you get even more ΔV. Make it boosted for moar ΔV.
Rinse and repeat until again it becomes zero sum and you've got so many stages that the separators are doing all the lifting. Remember what I said earlier, boosters are mid stage staging and are better than having the stage as is but not as good as having another stage.


@Earl has an interesting suggestion that using boosters instead of staging can increase dV without making the rocket taller. Given the limited build space in SFS, that kind of sounds legit. Any other reasons?
Height, thrust and complexity. I know that for example with the planned Saturn V upgrades, they never intended on making it much taller aside from a bit stretching as it wouldn't fit into the VAB if they added another stage. That and lets be fair, firing 5 F-1 engines was bad enough on the ground. Having to do it at 10km up...?
All they were going to do to increase its lift capacity was go Kerbal and strap 260in SRBs to the side, make stage 1 and 2 a bit longer, uprate the F-1s and J2s and it'd do 200ton to LEO. Making an additional stage underneath the first one wouldn't have increased its lift capacity over that because of the low stage TWR anyway, but it would've thrown that 140tons much further.
Most of my shuttles are designed like that with a hard payload cap set with upper stage TWR but have a very high ΔV rating because 1.4.06 drag.

I've done some weird stuff with base game where I'm capped with how tall I can make things, but not wide. So I fitted pad assembly staged boosters to get the required 4500m/s ΔV I needed to get it to LEO.
This is also true if you're going really big, like kiloton and over but using more than one stage. For example, you could only fit 9 titans on one build screen width. This creates a mass cap for that particular stage and everything above it, because of TWR constraints if you're still in atmosphere. It's even worse if you're making a frontier upper stage, because a kiloton payload, plus the engine pack means that stage can only have about 50 grams of propellant mass or the TWR goes negative and you accelerate backwards instead.
The solution? MOAR BOOSTERS, added on the launch pad to the upper stages and dropped once the TWR was low enough for the 'core' to continue.

In Spaceflight Simulator, it simply comes down to your design ethic. If say you want to punt 300ton to the moon, then you can either say 'I want to make it x number of stages cos I like how it looks' or you can be ruthless and get your computer to continually add stages until there's about 30 of them, especially if you teach it how to do asparagus in deep space.

And then once you've gone firm on the number of stages, add boosters to the bottom stage regardless.

Cos MOAR BOOSTAZ is always better.​
 

Altaïr

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#6
That's a huge debate :p

In my opinion, between the 2 design options (first stage vs boosted stage), there's no clearly one that's better than the other on every point of view, even if I have a preference for the boosted design. That's an optimization problem in which you tend to gain on one side what you lose on the other side.

For the "first stage" design, the advantage is that when the first stage runs out of fuel, your second stage is full, which gives you the maximum delta-V available at that point. On the other hand, your first stage will do the job alone and will have to be more powerful.

For the "boosted" design, the advantage is that you have more thrust available in the beginning because the main core adds its power to those of the boosters (for a similar number of engines). However, after booster separation, the second stage is partially empty, meaning you have some dead weight to propell now.

As Horus said, there is an option that consists in transferring the fuel from the boosters to the main core to maintain it full until separation. That way, you cumulate the advantages of both designs. But as you can only transfer 1 ton of fuel per second, that method also has its own limits, especially for big launchers.

Overall I tend to prefer the boosted design, as it allows smoother transitions: as you know, TWR is important in the first part of the flight. But if we often look at the TWR at start, it must be remembered that because fuel is consumed, the TWR increases inflight. And this is even more true if you remember that gravity decreases with altitude (at Karman line, gravity is already lower by 20%). Problem, as you gain speed, you actually need less and less TWR, so it doesn't go naturally in the good direction :eek:

The booster design allows to adjust more smoothly the TWR ratio inflight: the boosters give you the kick you need in the beginning of the flight, your TWR increases, and then you drop the boosters: it means less dead weight from now on, and your TWR drops back to a more appropriate value. It even happened that I made a launcher with two levels of boosters to use this twice in a flight. Even if the main reason for this was because I hadn't enough thrust with one level of boosters. That's a cheap option to avoid redesigning a launcher :p

With a purely staged design, you can't tweak your TWR as accurately: when you perform your first stage separation, your TWR drops from "very high" to "very low", before slowly increasing again. Note that you can accomodate from that too: the last seconds of high TWR allow to put the launcher on a long ascending trajectory, it keeps climbing while the second stage starts burning, and by the time it reaches the top of its trajectory, its TWR became acceptable again. That's why I said there's no definitive answer to that debate.

Overall, I like the booster design though. Here is an example:
Screenshot_20200711-154208_Spaceflight Simulator.jpg
My favorite toy! :p

Here are its specifications:
Payload: 1350 tons
Main core: 1268.8 tons, 1200 tons of thrust, 290s Isp, 1053 tons of fuel, fuel consumption: 4.138 tons/s, burn time: 254.5s
Booster: 1180.3 tons, 2400 tons of thrust, 240s Isp, 996.75 tons of fuel, fuel consumption: 10.0 tons/s, burn time: 99.7s

Overall, the TWR at start is 1.2. With the same number of engine, the boosters alone could not lift that rocket if they were assembled in a single first stage.

The contribution of the main core is crucial here. However, on booster separation, I've already spent 40% of fuel from the main core, which represents 46.8 tons of dead weight. Sounds a lot in absolute, but that's a super launcher, it's not that much. That's still nearly 47 tons of dead weight I'll have to satellize, but that's the price to pay for the additional TWR at start, and that's quite a fair one. The only alternative would make me add more engines. In the end, if the number of engines or global thrust are the criteria for efficiency, the booster design is arguably the best approach here.

Also, one last word, one thing to be aware is that this debate is specific to SFS, it's quite different in real life.

In real life boosters (SRB) are relatively cheap to design (no turbopump, no complex plumbering, no cryogenic fuel...), so it's a good option to reduce costs. Apart from crewed module liquid fuel engines are the most expensive part of a launcher. That's also a good option to adjust the payload capacity of a launcher. Some rocket families rely on that: Ariane 6 will come in 2 versions: with 2 or 4 boosters. Ariane 4 could be used with 0, 2 or 4 boosters, and there was 2 kinds of boosters: solid and liquid (liquid was more performant but more expensive). The Atlas V can use 0 to 5 boosters too.
Some other rocket families use a pair of first stages to make a heavy version: the Delta 4 heavy or the Falcon Heavy. The Ariane 6 booster is also a Vega's first stage.
Overall, the logic behind the booster design in real life is rather in terms of versatility and cost reduction, so those are different considerations from SFS.
 

Altaïr

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#10
Oh yes, I remember now.

And totally not inspired by my least favourite launch system.
It inspired me only for the name though. I wanted something that doesn't sound too serious, so the SLS was a good choice for that. But apart from that it's better known as the launcher launcher now.