Rocket Design Help?

[Scarecrow71]

So, I am not a rocket scientist.  Shocker!  Because of this, I find myself in the same old habit of creating ships up and down, with no idea how to effectively craft a ship that needs to get wide.  Or use different types of engines (such as the new Hydrogen engines).  So I get what I always have gotten:  a long, slender ship with a crap-ton of solid boosters at the bottom that struggles to get off the launchpad and needs serious help to not spin uncontrollably.  Sure, I can get decent-enough craft to fly to Mun and Minmus.  But my interplanetary rocket design skills leave a lot to be desired.  In KSP1 I can force my way interstellar.  And I could very well turn on infinite fuel...but I feel like that defeats the purpose of the game.  At least for me.

I therefore am coming to the community to ask for tips/pointers/help in designing craft for interplanetary missions.  How do you guys get them to be sleek and working?  How about widening them instead of always going slender and long?  In KSP2, are you taking advantage of being able to build multiple assemblies at once, and if so, how are you able to reconcile that and then put the ship together?

I've viewed the existing KSP1 tutorials, but for some reason none of them really dive into how to build a rocket, rather focusing on "We are going to Eve, so I built this thing here (with sped up video to not bore you), and off we launch".  Which really doesn't help me to understand the HOW of doing it, you know?

Any help here would be awesome and keen.  Until then, I'll keep brute-forcing slender ships!

[Streetwind]

Hmmm, that's a very general question. I can link you to this writeup I once did, which is admittedly meant more for newcomers, and also this one, which talks about optimal rocket stage design from a mathematical standpoint. I don't know if either is what you're looking for, but maybe you'll find something helpful.

They were written for KSP1, but 99% of them probably apply just as much to KSP2.

[Sorcie]

Here's the list of points I got from your post:

• Help creating rockets that aren't getting too long. Help with scaling up rocket designs. Advise on rocket width vs length aspects

Try Scott's Video below or hit me up and I would be more than willing to demo a build. There's too much to express in a forum post.

• Advice on other engine types

Nuclear and Ion are awesome just harder to use. Haven't tried new KSP 2 engines.

• Help with reducing rotation during launch

Turn off gimbal on all external engines off the core. Put more reaction wheels on the core. Avoid 4x symmetry until necessary.

• Help designing sleek purposeful interplanetary missions

Practice and build with purpose according to a good plan.

• KSP1/KSP2 Sub-Assembly usage

I use it for Rovers and avoid redocking for missions when possible. Simulate any scenario in the VAB for your mission plans. The sub-assemblies help with this in KSP 1.

 

I love building in KSP 1. I really really want to help but there's literally so much to say here it would require actual demonstration. I've been playing KSP 2 a little but it's just so buggy I can't build anything serious. I typed on this for a while and just gave up. Message me I'll give you a personal discord tutorial on my building style methods if you want. Try digging through Scott Manley's video library I'm sure he has something back in the day with some build strategy.

Here's Scott showing some basic and advanced discussion about ascent design. I have no idea your experience or knowledge level so  yeah! Try this: https://www.youtube.com/watch?v=XZMkQvbk0zw&list=PLYu7z3I8tdEm5nyZU3a-O2ak6mBYXWPAL&index=6

 

[Aldaris5]

Hi ScareCrow,

here are some handy advise: 

1) Try to minimise payload (the actual spacecraft you want to send to other planets) this should be compact and small as can be. 
2) Think of the building backwards: you build the spacecraft, then the tank with engine which it uses in deep space (small like the Pug or Terrier depends on size) then a larger tank and engine you will use to leave Kerbin SOI. Reliant,  Swivel will do it. Don't build it too long though two medium size max. If you have External fuel pipes, you can put on 2-4 fuel boosters (rockets with engines) and feed the EFP to the centre. Finally you build a large lifter from the surface of Kerbin. Orange tank max twice. Maybe fuel boosters. Skipper or Mammuth engines. Then outside attach SRBs if you want. You need stabilisers for keep the whole thing in line. You can use Monopropellants too but consider their weight. Use fins and winglets to control them in the air.

3) Stages triple checked. 
4) use struts to solidify. That helps especially in lower stages. Decouplers usually cause a lot of noodle effect. For example: when put on the SRB decoupler (side) is usually in the middle. Put at least one strut on the to and the bottom near the engine. Also connect the SRBs together for solidity.

4) Use cones on top of the boosters and fairings on the actual spacecraft. Winglets/fins on the bottom near the engine. Lifter stage should have a solid large one. No need to control while in the thickest atmo.

5) Learn how to asparagus fuelling. You can google it.

6) TWR! Figure out the stages TWR. Kerbal Engineer Redux (mod) is the best way to do it. If it is under 1 rocket won't lift, you waste fuel. Over 1.18 is good, higher the better. But! In atmo over 2 is actually create too much drag. 

7) If the rocket start to spin you most likely did built something not aligned properly. Check the build. Use the tools in VAB to align everything.

8) You need around 4500 to leave Kerbin SOI, you have to have it without the Deep Space stage.

Hope these help.

God speed! Brave Kerbonaut! 

Edited March 18, 2023 by Aldaris5

[JoeSchmuckatelli]

If long and skinny is the problem - start using some of the fat parts. 

Build your rocket in stages and save them under different names. 

I usually have a lander /rover / comm probe as the end stage.  So then I need to build a stage to get that from orbit to the destination - basically fuel cans and Poodle or Labradoodle. 

The next part is pick a or b.  

A.  Build a booster with a a TWR above 1, using something like the Mammoth and a pair of the big SRBs and hit the green button or

B. Build a short sustainer stage to complete the orbit / give you a boost to your destination and then slap on a shorter booster (ditch a fuel can) than you built in A - still ending up with a TWR above 1.

Profit 

Do try to stay away from multiple levels of SRBs, and work on gravity turns - like try to be close to 45 degrees by 4km... It works! 

Edited March 21, 2023 by JoeSchmuckatelli

[Streetwind]

21 minutes ago, JoeSchmuckatelli said:

45 degrees by 4km... It works!

Was that a typo? Because unless you have something like a >2.0 TWR on the pad, that's going to fall out of the sky in a hurry. And once atmospheric heating is enabled, that'll get as toasty as a reentry from the Mun.

The general advice for rockets with sane TWRs of around 1.3 to 1.4 is to aim for 45 degrees pitch at 10km altitude, erring slightly on the side of going higher. (Because you can correct too high by throttling down, but you cannot correct too low by throttling up, since you're already at max throttle.)

 

Edited March 21, 2023 by Streetwind

[JoeSchmuckatelli]

5 hours ago, Streetwind said:

Was that a typo? Because unless you have something like a >2.0 TWR on the pad, that's going to fall out of the sky in a hurry. And once atmospheric heating is enabled, that'll get as toasty as a reentry from the Mun.

The general advice for rockets with sane TWRs of around 1.3 to 1.4 is to aim for 45 degrees pitch at 10km altitude, erring slightly on the side of going higher. (Because you can correct too high by throttling down, but you cannot correct too low by throttling up, since you're already at max throttle.)

 

I'm a complete Neanderthal at this!

I start nosing at 50m/s and then keep feathering the D key to keep the dot in the white carrot on the edge of the prograde circle, which if I do it right keeps me on the edge of falling out of the sky.  I rarely get to 45 by 4k but I am usually there between 6 and 8 by trying.   

Once past the 45 line I usually hit prograde hold and watch for 80 or 100 km depending on the altitude I want. 

I generally only reduce throttle if I know I'm over 1.2 TWR and that is for the SRB burn. 

(Love to hear a better way!) 

Edited March 22, 2023 by JoeSchmuckatelli

[JoeSchmuckatelli]

@Scarecrow71

@Streetwind

Here's what I do with my OP rockets.

Critiques welcome!

 

[Streetwind]

That's not a bad launch. Especially the second stage flight, that was pretty much perfect. Yes, even though you didn't throttle up - or perhaps because you forgot You still had enough thrust anyway. That you got your periapsis above ground before your apoapsis reached your target altitude is a sign your ascent went really well, especially when time to apoapsis stays below 2 minutes at the same time. You also stayed hands-off, so you get an A+ for the second stage flight.

Now, some notes: you said you had about 1.8 TWR on the pad, so you throttled down to two thirds thrust, to get it to 1.2. That that did not work as you perhaps expected, because the flight throttle only controls liquid engines. So the Mammoth throttled down, but the two SRBs didn't. As a result, your rocket probably had around 1.5 to 1.55 liftoff TWR. That's how you survived passing 45° by 8 km altitude. Indeed, you even had to force the rocket to turn, because it would have gone straighter on its own due to its high thrust. If you want to throttle an engine down to avoid excess TWR, I recommend adjusting the engine's thrust output in the VAB instead of using the flight throttle; that way, you can also see the effect of your changes reflected in the engineer's report. (Bonus tip: don't aim for 1.2. Aim for 1.3 instead.)

Second note: you didn't, strictly speaking, perform a gravity turn. The term is called that because gravity is what pulls the rocket onto its course, and this happens hands-off - in other words, no steering input required beyond a single initial pitchover. In this video, you actively steered the rocket the entire time, all the way to stage separation. This is not bad, per se; if that works for you, and perhaps you even enjoy the task of manual piloting, then by all means, have at it. I just wanted to point out the distinction.

Third note: the wings attached to your SRBs are mounted off-center, and thus unsymmetric  I recommend leaving them off entirely; adding control authority to objects mounted on radial decouplers can invite flexing, and when something flexes while producing thrust, you get unstable flight. Your rocket already seems to have plenty of control authority from thrust vectoring alone.

Fourth note: stage separation was ugly. The burnout sequence of stages was wrong, and as a result, you nearly had the SRBs impact the engines of the second stage. That definitely needs a redesign! Since you have 1.8 TWR at full throttle, you can easily add another tank to the core stage. This will cause the Mammoth to burn longer, so you can ditch the SRBs while the vessel is still under thrust. (Obviously, ensure that the SRBs detach first in that scenario.) Additionally, slide the SRBs down just a little bit further on the decouplers (and slide the decouplers up as necessary). This changes the point where the decoupling force is applied. If it is applied further up on the SRBs, their noses will tend to push outwards, instead of inwards, making them less dangerous to your rocket. Another option for cases where SRBs and core stage burn out at the same time is to not use radial decouplers at all, and simply attach the SRBs fixed. You leave them behind together with the core stage,

If you want, you could pass me the craft file, and tell me the mission requirements. I'll give you a redesigned variant and explain each change and why I made it. If you'd rather tinker yourself based on the recommendations above, that's fine too - it's just an offer.

 

Edited March 22, 2023 by Streetwind

[JoeSchmuckatelli]

3 hours ago, Streetwind said:

you could pass me the craft file, and tell me the mission requirements. I'll give you a redesigned variant and explain each change and why I made it

That is a kind offer!  

I'm going to take what you wrote above and go play with it.  I'm one of those guys who really don't start learning until I get my hands on stuff - so it's actually better for me to make mistakes and learn from them than to watch someone who really knows what they are doing. 

It's like when I was learning carpentry - I had this guy who was an amazing woodworker who could spend an hour telling me what right looks like - but until I got my hands on the piece and tools I couldn't feel what right was.  While this involves some waste - I learn faster through failure than by striving for perfection! 

3 hours ago, Streetwind said:

he burnout sequence of stages was wrong, and as a result, you nearly had the SRBs impact the engines of the second stage

Yeah - I need to do the math rather than guesstimate!

3 hours ago, Streetwind said:

Third note

This is a Yaaarg moment.  Can't tell you how often I've been frustrated by the SRBs and not known what was going on. 

 

3 hours ago, Streetwind said:

This changes the point where the decoupling force is applied

Never realized it was this sensitive. I just plop them into what looks like alignment.  Will also play around with this to figure best practices! 

3 hours ago, Streetwind said:

perhaps you even enjoy the task of manual piloting

So I've flown a variation of this profile ever since I learned about gravity turns.  Never knew that what I was doing wasn't a gravity turn!  So yeah I kind of do enjoy it b/c I'm generally on the edge of flipping the rocket (success often requires failure and revert) 

 

3 hours ago, Streetwind said:

no steering input required beyond a single initial pitchover

 

Okay - so talk me through this? 

16 hours ago, Streetwind said:

general advice for rockets with sane TWRs of around 1.3 to 1.4 is to aim for 45 degrees pitch at 10km altitude, erring slightly on the side of going higher

... 

3 hours ago, Streetwind said:

Aim for 1.3 instead

.

Let's say I built a sane rocket... What does that single pitch over look like? 

 

[Streetwind]

The initial pitchover is slightly different from rocket to rocket, depending on its starting TWR and engine selection. Usually, whenever I build something completely new, I do a few test launches and reverts to figure out how it best likes to fly, although experience lets me make educated guesses on where to start.

Let me give you a sample rocket for which I know precisely when to pitch how much, so it's easier to replicate and we both talk about the same thing when questions arise.

Place the downloaded file at: C:\Users\Your_User_Profile_Here\AppData\LocalLow\Intercept Games\Kerbal Space Program 2\Saves\SinglePlayer\Your_Savegame_Name_Here\Workspaces
Then, next time you load this savegame, you should see a new workspace available for loading in the VAB. It might not have a thumbnail, because that might only generate once you have it loaded at least once, I don't know for sure.

The rocket was something I designed on a whim to figure out how I could do a crewed Minmus landing and return with the lowest possible mass. Not sure this is the actual lowest possible, but I was fairly pleased with how this one performed.

It has a launchpad TWR of 1.265, and additionally, an engine with a fairly small margin between sea level Isp and vacuum Isp. So it only barely has suitable thrust, and doesn't gain much additional thrust as it raises through the atmosphere. Both factors mean that this rocket prefers a somewhat more conservative turn, something like 12km at 45° pitch.

Upon launching, wait until your speed is around 60 m/s. Then, using the D key, pitch over five degrees. That's half of the smallest circle on the navball, very gentle. Toggle SAS Hold Prograde, take your hands off the controls, and watch. The next time you should need to touch anything is during stage separation and second stage ignition (spacebar twice). Then, wait and watch some more.

As your apoapsis altitude goes past 55km-ish, you can start throttling down. This delays the point at which your apoapsis climbs to its target altitude, and stops the time-to-apoapsis from running away from you; so you burn for longer and closer to apoapsis, which lifts your periapsis more and reduces the final orbit insertion dV. Feel free to go down to something like 20%, so long as your apoapsis altitude still keeps rising reasonably. You do want it to exit the atmosphere before too long, after all.

Eventually, your apoapsis will be where you want it (I usually target 80km), so cut the engine and coast. Once time to apoapsis is down to like 15 to 12 seconds, slowly throttle up again. Dragging the throttle with the mouse gives great precision control. You want to find the point at which time to apoapsis stops decreasing, then throttle back again. Slowly let it creep closer to 0 as you watch your periapsis rise up, using throttle control to keep the apoapsis just in front of you until you have a nice circular orbit.

And then - congratulations, you flew to orbit with only spacebar and throttle control, plus a single touch of the WASD keys! If you want, you can try and see if you can do the Minmus landing and return. Since you launched into an equatorial orbit with the exercise, you'll want to intercept Minmus at AN/DN and may need to timewarp for Minmus to be in the proper position. There's not enough dV margin for a 6° plane change in low Kerbin orbit.

 

Edited March 22, 2023 by Streetwind
Typos, typos everywhere!

[JoeSchmuckatelli]

20 hours ago, Streetwind said:

once atmospheric heating is enabled, that'll get as toasty as a reentry from the Mun.

I just went back and read this.

...I am so building a list of bad habits during EA that I will have to unlearn once the game is closer to functional!

[JoeSchmuckatelli]

3 hours ago, Streetwind said:

congratulations, you flew to orbit with only spacebar and throttle control, plus a single touch of the WASD keys!

So I really appreciate your sharing of the craft and flight profile.  Had some fun trying to figure this out.  First attempt: too agro on the initial turn... but the second went well.  I would really have to play with your suggestion about throttling (and have much more confidence in what I'm seeing on the Navball) to get efficiency just right.  

Thanks!

 

The failure.

 

The Success!

[Streetwind]

Congratulations! Yes, the second flight really nailed it. Exactly 12km by 45°, which is how this rocket likes it.

Now you can try and build some rockets of your own with different TWRs, and see if you can figure out how they like to fly.

Note that you can vary the speed at which you pitch over and the amount you pitch over. The longer you delay (and the faster you are), the harder you need to turn. For example, this rocket, which wants a very small pitchover maneuver, could have maybe gone to 100 m/s and then made a bigger pitchover which may have been easier to place on the navball. Turning earlier is more efficient, though, so I tend to go for early turns.

If you want, you can even have a rocket with a high TWR that is tilted over sideways just slightly in its launch clamps. With the right angle and TWR combination, that'll fly to orbit on its own without any WASD input at all