Duffer's Question about Efficiency - - (Rocket construction: Launch to Orbit & Stages)

[JoeSchmuckatelli]

I keep getting this up to space (differing payloads).

I basically just build whatever I want, slap a Mammoth onto the bottom and a couple of big SRBs to the sides and then powerglide to an 80-100 KM orbit.  I've been having about this much DV left over after adjusting inclination to match the Mun's.  Second stage has a Poodle as my main interplanetary.  Whole craft still has 6k DV remaining, only 300 of which are left to feed the Mammoth.

From what I remember of my early days looking at the KSP tutorials, there's supposed to be some efficiency in dropping big heavy boosters.

On the one hand - why mess with success... but on the other? 

Shouldn't I break up that lower stage and drop the Mammoth about the time I lose the SRBs?  Maybe with a Mainsail or a Vector?  Aside from just doing it for gameplay purposes - anyone mind breaking this down Barney-style for me?

What are some of the good to better practices for building stages with different engines for the launch to orbit?

Edited March 18, 2023 by JoeSchmuckatelli

[FleshJeb]

http://www.projectrho.com/public_html/rocket/multistage.php

OK, it's a good link but a high bar.

On those rare occasions I used rockets, I usually used full throttle SRBs around an underpowered core stage. Light all on launch. Drop SRBs and use core to get just short of circularization. Finished the circularization with the upper (high vac ISP) stage. Upper stage would do the transfer burn to destination, and maybe the braking around the target.

Edited March 18, 2023 by FleshJeb

[Streetwind]

5 hours ago, JoeSchmuckatelli said:

Shouldn't I break up that lower stage and drop the Mammoth about the time I lose the SRBs?  Maybe with a Mainsail or a Vector?  Aside from just doing it for gameplay purposes - anyone mind breaking this down Barney-style for me?

What are some of the good to better practices for building stages with different engines for the launch to orbit?

The theory about rocket stage types goes like this:

• Stage 1 is called the booster. Its job is to throw the rocket out of the atmosphere and give it just enough speed that the second stage can do its job. For this purpose, it is equipped with engines that favor raw power over efficiency - in particular because atmospheric pressure is a thing. Rocket engines behave very differently at sea level pressure than in a vacuum, and you can optimize for each end of the spectrum or try to strike a compromise. Therefore a booster stage usually comes in two variants: either it has one or more powerful engines optimized for sea level pressure; or, it has some powerful solid motors to get it off the pad plus some lower-powered, so-called sustainer engines, which are the ones that try to strike a compromise between atmospheric pressure and vacuum operation.
• Stage 2 could be called the orbiter, although few people do. Its job is to accelerate the payload to orbital velocity. Because it begins operations after the booster has thrown it out of most of the atmosphere, it can rely on vacuum-optimized engines to get a higher specific impulse and therefore use fuel more efficiently. However, it still needs to field a certain amount of raw thrust: the booster has thrown it onto an arc, and if it doesn't succeed in transforming that arc into an orbit before it runs out of arc, then it will fall out of the sky. Thus, these orbital engines will be slightly less efficient than they could in order to deliver a bit more power. (That also makes them suitable as landing engines on airless bodies, incidentally.)
• Everything that decouples from stage 2 is called a spacecraft. It begins operations full in orbit, and thus does not need to worry about falling out of the sky anymore. The deep space engines that it uses can be tuned completely in favor of efficiency over performance. This is typically also where you would see high-impulse engines with alternative fuel types make their entrance, such as nuclear rockets and ion thrusters.

KSP2 helpfully sorts each of its engines into one of those four categories: booster, sustainer, orbital, deep space. The category is listed in a subheader directly below the engine's name in the part info window. So whenever you look at an engine, you can instantly see which role it is supposed to fill, and which stage it would traditionally be placed into. The Reliant is a booster engine, the Swivel is a sustainer, the Terrier an orbital engine, and the Ant a deep space engine.

So far the theory.

In practical application, however, launching from Kerbin is extremely easy. Kerbin is only one eleventh the size of Earth, so even though the gravity is the same, orbiting it requires a lot less energy. 2200 to 2300 m/s orbital velocity compared to Earth's 7500 to 7600 m/s. Think about it: an orbit is nothing other than moving sideways so fast that you keep missing the planet when you fall back to it. And if the planet is much bigger, then you need to move sideways much faster to keep missing it.

Because orbiting Kerbin requires so little energy, you can frequently just single stage to orbit (SSTO), wherein the booster stage carries you all the way into a stable orbit. In such a setup, you don't have an orbiter stage at all - you just have a spacecraft that detaches from the booster. This is what you've been doing. And it is a completely valid approach, if you like playing that way. It is no more and no less correct than the typical two-stage to orbit method used on Earth.

You may think that it is more mass-efficient to go two-stage, and you wouldn't be wrong; SSTO boosters tend to be very large and very heavy. On the other hand, though, using two stages to orbit will result in two stages which both have inefficiently low mass fractions, so you're not really getting your money's worth out of them... if, you know, you were actually spending money

So what KSP players in particular often do is organize the stages a little differently: the booster stage will go most, but not all the way to orbit (fielding in the neighborhood of 2800 to 3000 m/s dV). Then you have a transfer stage, which completes orbit insertion but is essentially a spacecraft and uses deep space engines. Its job is to move the payload where it needs to go - for example, it moves a Mun lander into low Mun orbit, or a Jool exploration spacecraft to Jool. Then the final thing you decouple achieves the actual goal of the launch, such as landing somewhere. On crewed missions, sometimes the transfer stage is reused for the trip back to Kerbin, waiting in orbit for the lander to come back from the surface and re-dock with it. This leverages the high dV potential of deep space engines and allows the lander itself to be smaller and lighter.

 

Edited March 18, 2023 by Streetwind

[JoeSchmuckatelli]

1 hour ago, Streetwind said:

theory about rocket stage types goes like this

Comprehensive!  Thanks! 

 

1 hour ago, Streetwind said:

helpfully sorts each of its engines into one of those four categories: booster, sustainer, orbital, deep space

I've not noticed this - will have to go look at it! 

 

1 hour ago, Streetwind said:

Kerbin is only one eleventh the size of Earth, so even though the gravity is the same, orbiting it requires a lot less energy. 2200 to 2300 m/s orbital velocity compared to Earth's 7500 to 7600 m/s

That is something I wasn't aware of.  Makes a lot of sense. 

 

1 hour ago, Streetwind said:

may think that it is more mass-efficient to go two-stage, and you wouldn't be wrong; SSTO boosters tend to be very large and very heavy. On the other hand, though, using two stages to orbit will result in two stages which both have inefficiently low mass fractions, so you're not really getting your money's worth out of them

That's the piece.  So I could break up my booster stage and put in a sustainer - but given the inefficiencies and realities of the Sim (much smaller planet, etc) - I'd effectively be doing it for role-play purposes rather than because there is a more efficient way to do it than what I'm already doing. 

[JoeSchmuckatelli]

10 hours ago, FleshJeb said:

OK, it's a good link but a high bar.

I'm enjoying the article!

This right here explains SS.

Quote

Note also that to get high mass ratios you need BIG rocket ships. That follows because there are minimum weights to many rocket parts. This is known as 'minimum gauge', meaning that you can't make it any thinner or lighter. Combustion chambers have to be rugged. Pipes must be thick enough to hold pressures that don't get smaller just because you're trying to make a small sized rocket.

 

Edited March 18, 2023 by JoeSchmuckatelli