When to move from 1.25m to 2.5m parts.

[Chequers]

Hi Kerbanauts.

New player here, playing through a career mode without quicksaves or reverts, so it's fairly slow going through the tech-tree. I've really enjoyed the vast variety of parts so far, and have tried to create craft that utilise all of the different engines, rockets, and utility parts as I unlock them.

Sometimes, this looks like upgrading or improving current designs for ships, to help them do their jobs better. For example, I have a tourist vessel that was enhanced by replacing the Swivel engine with a Reliant engine, and using a stablility enhancer for attitude control on ascent.

But I just unlocked a couple of nodes that give me access to bigger radius parts, such as the Skipper and Poodle engine, which will involve completely redesigning some ships. It's a tantalising prospect, but I'm wondering whether my planned missions warrant these new parts, of I should wait for more ambitious pursuits to interplanetary space.

At the moment, I've made two succesfull landings and returns from the Mun to Kerbin; the first by delivering a science package to the surface, and then landing a Kerbal to retrieve the data and return. The second (to avoid having to launch two missions) was done 'apollo style', with a seperate lander and command module.

I've also sent a science probe which is currently making a high pass of Minmus, and won't return for another week of in game time (I tend to do more local missions to pass the time, rather than time-warp entire weeks).

I want to continue exploring the Mun, and make a polar landing. I think this can be achieved with my current 1.25m ship designs with some tweaking to account for a less efficient polar orbit... but would utilising some of these 2.5m new parts save me some valuable cash? I've had a quick play around with building a ship that seemed to be more cost for the same delta-v result. But I've not tested enough to make that conclusion.

tl;dr - when is a good time to transition from 1.25m parts to 2.5m parts? What sort of missions warrant these larger parts?

 

 

[HvP]

Hello @Chequers

You seem to have a pretty good grip on the game, and congratulations on your progress so far. As far as when it's best to start using 2.5 meter parts, I think that the increasing diameter and weight of your payload are the primary factors which demand such larger sizes. And there are usually choices in every diameter which scale efficiently with the size of those diameter stacks.

You shouldn't confuse size with efficiency. The most efficient engines in the game are 1.25 meters in size if you consider the ISP or "fuel efficiency" of an engine. What the smaller engines usually lack is maximum thrust, but even this isn't always true. The "Vector" is 1.25 meters in diameter but has one of the highest thrust outputs of any single nozzle engine in the game. You pay for this compact powerhouse, literally, as it's very expensive. As you unlock more of the tech tree I think you'll find that there is usually an engine to fit your current stack size which will efficiently move your payload.

There is never going to be a hard and fast rule as to which specific engine to use for your payload, but the goal is always to achieve the minimum required thrust to operate your vehicle while maximizing delta-v. Sometimes this means using a larger engine which provides a higher absolute thrust. Or sometimes it's better to cluster a few smaller, and potentially more efficient, engines for similar total output at higher ISP efficiency. You won't always get a performance increase just by attaching a larger engine. In some cases, a larger engine could reduce efficiency due to its extra mass.

Also, the environment in which the engines will operate is crucially important. A lower stage booster demands high thrust, but efficiency is secondary since it will usually be discarded before you leave the atmosphere. But an interplanetary transfer stage doesn't necessarily require high thrust because you have plenty of time to complete your maneuvers in orbit - maximizing delta-v is key. A surface lander however will depend on a high enough thrust-to-weight ratio to safely touch down, while also having an engine bell that isn't so long that the landing legs don't have clearance to touch the ground. But the game gives you several options to experiment with - there is no one right way to do it.

So in short, my advice would be this. A larger engine isn't necessarily an "upgrade" if you already have an engine which fits the size of your rocket that is getting the job done.

Factors such as the diameter and weight of your payload, the ISP or fuel efficiency of the engine, the environment the engine will be used in, the weight of the engine itself, if you need engine gimbal for thrust vectoring, and whether or not it's possible to achieve similar results with multiple smaller engines or one large engine... all add into the decision of what parts to choose.

It might seem a bit complicated - it is rocket science after all. But luckily this is a game which encourages experimentation and rewards novel approaches with the satisfaction of designing and flying your very own space craft in whatever way you personally like. Fine tuning your craft is certainly part of the fun, but there's not one right way to do it.

[Chequers]

Thanks for your detailed response @HvP!

I’m glad that newer parts don’t make older ones obsolete, but essentially enable more complexity and options for completing missions.

I have learned that bigger doesn’t mean better - and very much enjoy the challenge of creating the most elegant solution to a problem - for as optimised a budget as I can.

From your explanation, I see now the fallacy of trying to compare Skippers to Swivels. It’s not about whether one is more efficient, it’s more about the right tool for the job. 

Like using a ride on lawn mower vs a hand mower to cut a small square of grass.

I’ll evaluate whether the greater power of a larger engine as a lower stage offsets the cost and ISP profile of current 1.25 engines for my Mun CM and lander payload, and go from there.

Edited March 19, 2020 by Chequers

[The Aziz]

I didn't stop using 1.25m rockets even when the time came for using 3.75m. Different purposes, you still can send tiny interplanetary probe on a top of single core 1.25 rocket powered by Swivel and then Terrier as second stage. You need a bit more deltavee? Just add side boosters and there's still no need for 2.5m vehicles. And with careful planning, you can launch the whole thing for less than 20k 

And in fact, my small rockets are the easiest to fly, compared to bigger vessels.

It's all about the payload and mission requirements. 

Edited March 19, 2020 by The Aziz

[Chequers]

11 minutes ago, The Aziz said:

I didn't stop using 1.25m rockets even when the time came for using 3.75m

...Parts get even bigger!?

I too seriously enjoy making tiny probes. This little science probe on its merry away around Minmus uses the 'Spark' engine and a single Oscar-B tank, and it's adorable. I think the 'Propulsion Systems' and 'Miniaturisation' tech nodes are my favourite unlocks so far.

[Superfluous J]

A good rule of thumb is to try to get about 2000-3000m/s out of a stage before dropping it. This means generally that your rockets will be 2 stages to orbit, and that 2nd stage may take you to Mun but you'll need a couple in space to do anything else.

The vast majority of my payloads are 1.25m. I only go up to 2.5m when I start sending 3 Kerbals places. However, once you start having trouble launching something off the pad with 2000m/s and a decent (I think about 1.3 or so personally) TWR, it's time to make the launch stage larger. Then you'll want to launch more, and you'll need to make the orbit stage larger. then you'll want more, and that launch stage won't cut it and you'll have to up it to 3.75m.

If you ever do a Jool 5 (I recommend it! Though go to Duna and Gilly and Dres first, they'll give you tons of personal experience that will make Jool a lot less daunting) you may even get to the point where you're launching 3.75m things into orbit. then you'll be wishing you had 5m parts.

(psst. There are mods with 5m parts)

[AHHans]

2 hours ago, Chequers said:

I’m glad that newer parts don’t make older ones obsolete, but essentially enable more complexity and options for completing missions.

Indeed!

If there is one recommendation that I give for rocket design, then its: start from the top and go from there to the bottom of the rocket. I.e. design the payload first, and then start thinking about the booster needed to get it where you want it to go. If the job is to land a science payload on the Mun or Minmus, then 1.25 m parts are perfectly fine. If you want to get a station that can hold a few thousand units of fuel into orbit, then you probably want 2.5 m parts. If you want to actually haul several thousand units of fuel into orbit then you might want something even bigger.

You may want to challenge yourself to try out every part that there is in the game. But it's also fine to decide to see how far you can get with tiny probes and stick to 1.25 m parts for most of the game. One thing I like about KSP is that there are so many ways to play it.

[Chequers]

I've had a crack at designing a 2.5m lifting stage. It essentially costs the same (it's actually a few thousand cheaper), but radically simplifies the design of the lifting stage, and enables me to tuck the lander and command module within a nice streamlined fairing. I'll be interested to see how it performs on my next Mun mission.

Thanks for all your tips - what a great community!

Exodus I

Exodus II

[CBase]

For me it is all about drag: Bigger sizes have less surface per volume. Note that even sides do produce drag.

So center core + 2x side ? Fine. But before going for center+3x side I usually switch to 2.5m.

Actually at the next steps ( I am using SpaceY up to 10m ) a factor of 2 is enough to switch for next diameter.

 

Edited March 21, 2020 by CBase

[magnemoe]

On 3/19/2020 at 8:36 AM, The Aziz said:

I didn't stop using 1.25m rockets even when the time came for using 3.75m. Different purposes, you still can send tiny interplanetary probe on a top of single core 1.25 rocket powered by Swivel and then Terrier as second stage. You need a bit more deltavee? Just add side boosters and there's still no need for 2.5m vehicles. And with careful planning, you can launch the whole thing for less than 20k 

And in fact, my small rockets are the easiest to fly, compared to bigger vessels.

It's all about the payload and mission requirements. 

This, for probes, small parts and single kerbal rescue  out to Minmus,  1.25 m i perfect. 
2.5 meter is nice for medium payloads, as KSP is a bit easy mode this is more like 30 ton if you use boosters. 
Then its 3.5 for the large stuff like planetary bases 

This is an probably the Bop or Val base getting base topping up in Minmus orbit by an probe tanker. 

5 meter is for the crazy stuff. 

Spacedock Moonshadow, Left part is Minmus express, ending in the drag plate, the three radial rotating living spaces was added later. This thing can hold 90 something Kerbals and need to do it.
Unfortunately its an thing who make i7 and treadrippers run away and hide in fear and shame. 
Adding the top ring would increase lag and was not needed as Kerbals was rotating out to other destinations. 

[Chequers]

Sheesh! Very impressive! I can only imagine getting to that stage in my KSP career.

The 2.5m parts certainly make some of the more difficult calculations easier, but at an increased cost. I still enjoy the challenge of finding a more cost-effective solution to my Mun missions with the 1.25m parts. There's just something about each stage getting to exactly where it needs to, without wasting any fuel. It's just satisfying!

But it might just be me - I really don't enjoy the whole 'asparagus staging' concept. It doesn't feel very realistic, nor very elegant. I'm confident I'm just handicapping myself though!

Edited March 21, 2020 by Chequers

[jimmymcgoochie]

Minmus is actually easier to get to and from than the Mun despite its inclined orbit, as the delta-V required to reach it is not much higher than for the Mun but its orbital and escape velocities are much lower and the gravity is substantially less. If you're planning to send a crew of more than one at a time, you're going to need to start using larger parts and realistically you need to build a ship in orbit in order to make crewed trips to other planets which will require 2.5m and most likely 3.75m stages on your launch rockets. Probes/landers/rovers might get away with not using anything larger than 1.25m though.

If you're a fan of smaller parts and probes, you could try using the RLA Reborn mod which has a multitude of half-sized (0.625m) parts including fuel tanks of varying sizes, engines and reaction wheels; it also gives you some souped-up versions of the stock probe cores OKTO, HECS and QBE with better stats. There are also some nice small parts in ReStock+ and there's also OctoSat which unlocks later on the tech tree but contains a lot of part options to create your own modular satellites, and fits on top of a 1.25m stack. I think KNES has a lot of different sized parts from 0.3125m to 1.25m in various increments, although the sheer number might be a bit overwhelming and since they're modelled on real rockets the stats might not be particularly good for some/many of them (I don't use it myself but have tried it out). And if you like doing probe missions, look up ProbesPlus which has replicas of many real missions including Voyager, Cassini-Huygens and Mariner probes.

Building small rockets means you can only carry small payloads but there's something very satisfying about turning a QBE core into a science lander and parking it on Duna, or putting a relay satellite into orbit of the Mun using only 0.625m parts and for under 9000 funds (and that's something, considering I did it using JNSQ which makes the planets nearly 3 times bigger and roughly doubles the delta-V needed to get anywhere). 

Back to the original question...

Building bigger inevitably runs into the problem of diminishing returns- to get more range you need more fuel, which adds weight, which means you need more thrust, so your engine needs more power, but that needs more fuel, which adds more weight, etc. etc. It's a bit like how long-haul flights use a third of their fuel just to carry the rest of the fuel, but considerably worse because rockets are inherently less efficient than air-breathing jets and need to carry their oxygen with them too.

One way around this is to use strap-on boosters, whether that's solid fuel boosters ore just more liquid-fuelled stacks stuck on the side; in the latter case use fuel ducts to funnel fuel from the boosters to the central rocket to drain the boosters first, leaving you with a full tank in the middle once the boosters empty. For SRBs, sticking some extra fuel tanks on the top and setting the decouplers to enable crossfeed also gives you additional range , and I always do that when using SRBs.

A 1.25m stack with three or four solid boosters on the first stage should be able to get into orbit of Duna or possibly Eve in the stock solar system, if you use the NERV engine on your top stage and are willing to try aerobraking to eke out your fuel when slowing down into orbit at the other end, but going further afield will probably require larger parts. 2.5m parts are recommended for interplanetary missions if you're not planning to come back, and 3.75m parts are required if you're launching 2.5m stuff into orbit e.g. to build a space station or Mun base. Crewed missions always require larger parts to be used due to their larger size and weight and the necessity to carry enough fuel to return to Kerbin at the end of it (hopefully you aren't abandoning your poor Kerbonauts on other planets!). Launching something like a research lab into orbit will probably require a 3.75m first stage, possibly with extra boosters, followed by a 2.5m second stage and if necessary a third stage for docking the lab to a space station/ship under construction.

One last piece of advice- put parachutes on your lower stages and boosters and use the Stage Recovery mod, it will return most of the dry cost of each launch which is especially effective when you're launching large rockets. Why would you not want to get money for the parts that would otherwise just fall into the ocean and be destroyed?

[Chequers]

Thanks @jimmymcgoochie for that detailed advice - really appreciate you taking the time to come up with a few suggestions.

This playthrough, as it's my first, I want to at least complete the tech tree before exploring mods; but I'm making a list of ones that look interesting, and quite a few of those you've listed look great.

It'll take a while though; without quicksaves/reverts, and having cash-cost to unlock tech-nodes, I'm still very much in the Kerbin system; haven't landed on Minmus yet, but currently have my first manned mission en-route to orbit it. It's within a day of arriving, which is exciting.

I absolutely agree with the satisfaction of completing a mission for low cost; there's just something so rewarding about maxisming every Kerbal-buck. I think that's why I like missions with the smaller parts, and micro-sattelites and probes. Just feels right.

My main challenge currently is refining my Mun landing missions. I've completed 5 landings and returns so far, but have never gotten it quite right. With 1.25m parts, if taking a full science payload down to the surface with the Kerbal, I don't quite have enough fuel to get them home, and have had to make a couple of subsequent rescues from Mun orbit. Proabably perfect for Minmus though, from what you've said.

I've gotten it pretty spot on by delivering a science payload to the surface of the Mun before hand, and then sending a very simple direct return lander with a Kerbal to retrieve the science data. But that's two launches.

But with 2.5m parts, it feels very inefficient. It costs almost double (~35-40k) with my 'Apollo-style' setup, and each stage feels like overkill - meaning I have too much power and fuel. I think I can refine things down, I've just unlocked the Mainsail, which can comfortably lift the entire stack on its own. It's just very expensive to keep testing each stage to get the Delta-V calcs spot on.

[RizzoTheRat]

I find the Skipper is a really useful first stage engine in the earlyish game, wither with an adaptor to connect to 1.25m upper stage, or with a faring as you've done.

 

As for Mun landings, I find drop tanks work really well in the early game.  A capsule with a smallish fuel tank (FL-T200) and a Terrier has enough dV to get from the surface of the Mun back to Kerbin, and enough thrust that it can carry 3 extra fuel tanks on radial decouplers to get to the Mun and land.  you can also leave the science payload on the Mun to save fuel on the return journey,  Sticking the landing gear on the radial tanks means it copes well with slopes too, but is a wide payload to launch so the wider first stage and a faring make sense.