Multiple Engines

[The Space Dino]

I was wondering how do multiple engines affect a rocket, and currently I think that adding more [identical] engines will only increase the rocket's TWR, which means more fuel tanks and more delta v.

Is my hypothesis correct?

Thanks

[Reactordrone]

More engine for the same amount of fuel will increase TWR and decrease delta-v. To maximize delta-v you want to minimize engine mass  since anything other than fuel is dead weight in the rocket equation, so you want just enough engine to give you the desired TWR and no more.

Edited June 15, 2016 by Reactordrone

[Mjarf]

From what I've seen increasing TWR doesn't necessarily decrease delta-v as much as reducing fuel mass or increasing payload mass. Doubling the TWR won't necessarily cut delta-v in half, decreasing fuel mass to half will. At least this is according to Kerbal Engineer, this mod seems to calculate delta-v precisely. It's true what Reactordrone says, overdoing the TWR will waste fuel, I always go for a balance between TWR and fuel efficiency. For a transfer stage you're wasting fuel with a TWR higher than 1, .5 is usually sufficient, unless you plan on doing very aggressive maneuvering and/or are bringing ISRU to re-fuel.

Doubling both engine thrust and fuel mass will increase payload capacity, so sometimes it's necessary to have multiple engine and tank assemblies if you're pushing a very heavy payload. I like to us mutliple engines on transfer stages because it looks cool and I like my Kerbals to be comfortable in a large command/habitation module.

[Streetwind]

5 hours ago, The Space Dino said:

I was wondering how do multiple engines affect a rocket, and currently I think that adding more [identical] engines will only increase the rocket's TWR, which means more fuel tanks and more delta v.

Is my hypothesis correct?

Thanks

In the sense that having more thrust lets you add extra fuel (while still being able to take off), and that adding this extra fuel without increasing the payload carried will increase dV - yes, you would be correct in your assumptions.

Rocket building is all about the mass fraction - the relationship between how much of the rocket is fuel versus how much of it is not. Adding another engine will add mass, but will also let you add multiple times that mass in fuel. Therefore, your overall mass fraction tends to improve, unless it already is hugging the theoretical maximum. This in turn means that you end up with more dV. I've written in much more detail about this here, if you're curious.

[Snark]

Also, bear in mind that dV is a complicated beastie.  Circumstances matter.  In particular, it matters a lot whether you're talking about launching from the surface of a planet (especially a heavy-gravity planet like Kerbin) to get to orbit, versus doing orbital maneuvering.

The reason why it matters (and the reason why the answer to "what's best?" is a complex "it depends" rather than a simple one) is because of gravity losses, which affect planetary launches but not orbital maneuvering.

Explanation in wall of text below, but what it boils down to is this, if you're trying to maximize your dV:

• For maneuvering in space, you generally want the lowest possible TWR you can get away with.
• For lifting off the launchpad on Kerbin, you generally want a TWR of around 1.5 off the pad, but less than that for your 2nd and later stages.

 

Okay, on with the details:

When you use your engines, what are you doing with them?  Specifically, are you using them to fight gravity (e.g. when you're accelerating straight up upon Kerbin launch)?

• If you're not fighting gravity (e.g. you're in circular LKO and doing a prograde burn)... then your TWR is completely irrelevant to your dV.  All that matters is your engine's Isp and the mass fraction (i.e. fuel percentage) of your rocket.  (You can see this in the rocket equation for calculating dV:  there's nothing there at all about thrust, it's irrelevant.)  Therefore, the best dV generally comes with very low TWR, for a couple of reasons:  first, the highest-Isp engines generally have very low TWR; second, you typically want as few engines as possible, since they're dead weight that counts against your dV.
• If you are fighting gravity (e.g. you're ascending straight up after KSC launch), then your TWR matters.  This is because every second you're ascending straight up, gravity is sucking 9.8 m/s out of you (on Kerbin, anyway), which is dV that's just flushed down the toilet.  This is called "gravity loss" and is a huge hit to your fuel economy when gravity is strong.  The only way to get around it is to minimize the time you spend fighting gravity (i.e. quickly get up above the thick part of the atmosphere, so you can tip farther over to the horizontal and spend your fuel in actually accelerating your rocket rather than fighting gravity), and the only way to accomplish that is with a high TWR.

So, for a launch from Kerbin, it's a balancing act.  On the one hand, you want to minimize gravity losses, in which case you want a high TWR-- the higher the better.  On the other hand, the rocket equation still applies to you (engines are still dead weight), so if you go too nuts with an overpowered ship, then you'll be wasting so much mass on engines that it more than offsets the benefit from reduced gravity loss.  Furthermore, there's another practical limit:  the faster you go while you're in the thick part of the atmosphere, the more energy you waste fighting aerodynamic drag.  If you go faster than terminal velocity on ascent, then the loss to drag outweighs the benefit due to decreased gravity loss.  This sets a practical upper limit on your speed, which in turn means it's not useful to have too much TWR, you'd just waste the capacity.  (Sort of like saying, it's not worthwhile driving a Lamborghini if you're going to be driving around city streets where traffic prevents you from going at high speed.)

In other words:  for liftoff, you need a happy medium in terms of TWR.  Too low, and you waste too much dV on gravity losses.  Too high, and you're lugging too much dead weight and wasting too much dV on aerodynamic drag.

So, I hear you ask, just what is that happy medium?

Answer:  It depends on your ship design (specifically, how big it is and how aerodynamic it is), but typically it's in the neighborhood of 1.3 to 1.5.  (Personally, I prefer 1.5.)  The more aerodynamic the ship is, the higher the TWR it can get away with.  Very large ships (i.e. multi-hundred-ton behemoths) can generally get away with a higher launchpad TWR, because they're less affected by aerodynamic drag and it's therefore worth their while to get up to speed faster.

Also, bear in mind that you don't want high TWR all the way up.  You need it off the launchpad to fight gravity, but pretty soon you're going to be tipping over into your gravity turn, and going less vertical and more horizontal.  Once that happens, a smaller fraction of your rocket power will be spent fighting gravity, which means you'll be wanting a lower TWR to get better dV efficiency.  So, typically, you want your 1st stage off the launch pad to have a high TWR (e.g. 1.5), then your 2nd stage usually kicks in when you're already tipped over 45 degrees or more and can be considerably lower TWR (e.g. in the 1 to 1.3 range), and your 3rd stage should generally be pretty low (is fine to be under 1).

[The Space Dino]

Thanks everyone for the very helpful tips!