(PS4) Rotating Control Parts

[Bakkerbaard]

Let's face it: Jeb is an idiot. He saw Apollo 18 on his weekend off and promptly took a rocket that was clearly underfueled and headed to the Mün to find Münspiders. But he's my idiot and I want him back, so now I have to send up a two-kerbal rescue lander because he took Bob with him on a dare. 

This poses two problems: It has become painfully apparent how little I know of space travel and I need to send up a two-kerbal rescue lander.

I know PC users have all kinds of mods and things available, like that thing that shows the TWR. The PS4 ("Enhanced" my cheeks) does not seem to have any extra readout available, so I just kinda eyeball it, as in: "This one's about Münar sized." More often than not this results in not being able to return and with the amount of launches I've made by now, the trainings I have struggled through ("press R1+triangle to talk to me", press it, nothing, "You dun did something wrong, but I'm not saying what!") I have established that I need to go lighter.

I am now experimenting with all kinds of ridiculous configurations (because it's fun!), but I'm kinda limited by the fact that when I want to flip a control part (usually one of those brownish discs I forget the name of that look like reaction wheels) on it's side I get a notification saying that the *control part* is not facing in the right direction and that might result in a screwy navball. So here's my conundrum: My ship goes upward, but I want the control disc facing left, like a wheel, not a pancake. I need to use a control disc because the two MK.1 capsules need to remain empty for the pickup (I'm thinking two MK.1s are lighter than one two-kerbal spaced part).

As an extra little hurdle, I plan to have an engine (all of this is going to be experimented with tonight, we're in the theoretic stage now) facing right so that once in the vacuum of space this crooked little spaceship will rotated 90 degrees and have the formerly leftward control disc facing upward and the formerly rightward engine downward. Wait, I'll draw you a picture, you seem like patient people.

I'll slap some landing legs on it too, make it look like a stick bug. Anyway, I suppose my real question is, how do I determine the amount of fuel I need to stick on a two-kerbal rescue pod to pick-up and go home from the Mün?

So yeah, sorry for going mad-scientist in my first post instead of properly introducing myself.

[Snark]

Hello, and welcome to the forums! 

9 minutes ago, Bakkerbaard said:

Let's face it: Jeb is an idiot. He saw Apollo 18 on his weekend off and promptly took a rocket that was clearly underfueled and headed to the Mün to find Münspiders. But he's my idiot and I want him back, so now I have to send up a two-kerbal rescue lander because he took Bob with him on a dare.

Ah, Jeb, up to shenanigans again.

10 minutes ago, Bakkerbaard said:

This poses two problems: It has become painfully apparent how little I know of space travel and I need to send up a two-kerbal rescue lander.

Welcome to the club of "everyone's been there, my friend"!  Let's see if we can answer some questions. 

11 minutes ago, Bakkerbaard said:

I know PC users have all kinds of mods and things available, like that thing that shows the TWR. The PS4 ("Enhanced" my cheeks) does not seem to have any extra readout available, so I just kinda eyeball it, as in: "This one's about Münar sized." More often than not this results in not being able to return

Okay, a disclaimer about what's to follow:  I play on the PC, and I have never even seen anyone play the console version.  I'm writing this assuming that except for control layout, the UI experience on console is basically the same as on the PC (e.g., the VAB displays the same info, etc.)  So it's possible I could be wrong somewhere, if the two UIs diverge in some way.

On the bright side:  I'm a PC player, yes, but I've never used any "engineering" mod at all for calculating dV and TWR and things.  I like doing stuff the old-fashioned way, with a pocket calculator and a few seconds of my time.  So, even though I'm not quite in the same boat as you, I'm at least paddling across the same pond.    Hopefully the following advice will be helpful.

From an engineering standpoint, there are two main numbers you need to care about:

• TWR:  "Do I have the right amount of oomph to get off the ground?"
• dV:  "Does my rocket have enough fuel to execute the mission?"

Let's go into these:

 

How do I calculate my TWR?

This one's super easy.  You need to know the following:

• Ship mass, specifically the total mass when sitting on the pad.  You get this by reading the little ship-mass readout in the VAB.
• Total thrust at liftoff.  Each of your engines produces a certain thrust.  You get this off the info panel for the engine in the VAB.  Be sure to use the ASL thrust, not the vacuum thrust.  Take these numbers and add 'em all up.  (Only add up the engines that are actually firing at liftoff, of course.)
• Kerbin gravity.  It's 9.81 m/s².

To find your TWR, take your total thrust, then divide it by (mass * gravity).  This gives you your TWR.  There, you're done. 

Example:  I have an 80-ton rocket, powered by a single Mainsail (ASL thrust, 1379 kN).  Its TWR is 1379 / (80 * 9.81) = 1.76.

 

What should my TWR be?

Generally, you'll want it in the range 1.3 - 2.0.

• It physically can't be any lower than 1, or it won't even lift off the pad.
• You generally don't want it lower than 1.3, or else you're wasting way too much fuel on "gravity losses"-- i.e. you waste lots of fuel simply fighting the force of gravity, rather than putting it to productive use by making the ship go faster.
• You generally don't want it higher than 2.0, for a couple of reasons.  First, overpowered rockets go too fast when they're still low in the thick atmospheric soup, which means they waste too much fuel fighting aerodynamic drag, and are at higher risk of overheating and exploding.  Second, you really don't need that much, which means if you've got a super-high TWR, you're carrying more engine than you need, which is bad, because that's dead weight and therefore wastes fuel.

Yes, I realize that "1.3 to 2.0" is a pretty big range.    And the way you design and fly a 1.3 rocket is going to look pretty different from a 2.0 rocket.  It mainly comes down to style and preference.  For example, personally, I like to fly 2.0, and design all my rockets to meet that goal.  And they fly well and work great and I'm happy.  But there are other folks who prefer lower TWR, and their rockets fly well and work great and they're happy, too.

So, my suggestion is not to fret too much about this.  As long as you're in that range, it's workable.  And also, TWR isn't the most critical thing about your rocket.  dV is.  More on that, next.

 

How do I calculate my dV?

This is slightly more complicated than TWR... but not all that much.  You use the Tsiolkovsky rocket equation, as adapted for KSP.

You calculate the dV (in m/s) for a stage thus:

dV = 9.81 * Isp * ln(M_wet/M_dry)

...where Isp is the specific impulse of your engine, M_wet is the total mass of the rocket before the stage burns all its fuel, and M_dry is the total mass of the rocket after the stage burns all its fuel.

(and "ln" means "natural logarithm"; it's a button you'll find on any scientific calculator)

(Yes, that 9.81 is for Kerbin gravity, and yes, you use that same 9.81 regardless of what planet you're on.  That's because it's a universal conversion factor, because all of the engine Isp values that the game tells you are calibrated to that number.)

Example:  You have an upper stage, powered by a Terrier engine (Isp 345), which has 2 tons of LFO fuel.  Total mass of the stage before burning the fuel is 5 tons.  So, naturally, after burning the fuel, it'll be 5 - 2 = 3 tons.

Therefore, dV = 9.81 * 345 * ln(5/3) = 1729 m/s.

If you have a multi-stage rocket, then you find the total dV by just adding up the stages.  Build the top stage first, calculate its dV.  Then add the next stage, calculate its dV, and so forth.  Note that for calculating the dV of each stage, M_wet and M_dry are the total mass of the entire rocket including the upper stages that haven't fired yet.

Caveat:  The above math is simple, and correct, when you're in a vacuum.  Trying to work out dV for the first stage that launches off the pad is seriously mathematically ugly, because of varying atmospheric pressure and changing Isp and all kinds of stuff.  However, bear in mind that by the time you get up to about 10 km of altitude, you're effectively in a vacuum as far as Isp is concerned.  Therefore, my advice would be:  don't bother trying to calculate the Isp of your first stage.  Just make sure it has enough TWR, and make the first stage roughly half the total mass of your rocket, and then go from there.

 

How much dV do I need?

You need a minimum of about 3400 m/s to get to low Kerbin orbit.  From there, you need about 850 m/s to get to the Mun, then around 300 m/s to get to Mun orbit.  Then you'll need 600 m/s to land, 600 m/s to get back to Mun orbit, another 300 m/s to head back to Kerbin.  Total dV budget:  around 6050 m/s.  However, note that that's the absolute best case and assumes you do all your piloting perfectly with optimum efficiency, which is probably not the case if you're new to KSP.    So it would be best to allow yourself at least several hundred m/s of additional safety margin after that point.

Note that these numbers also give you a straightforward way to do a sanity check at various milestones along your mission, so you can tell ahead of time "oh wait, I've borked this, I should quit now and try again because I don't have enough fuel left."  For example, the above numbers show that you need at least 2650 m/s (plus safety margin) of dV after you get to low Kerbin orbit.  So:  when you get to low Kerbin orbit, you can stop there, check your fuel and so forth, and then see how much dV you have left, and if it's less than 2650-plus-margin, you know you need to go back to the drawing board.

Note:  Efficient piloting is really important  (e.g. the trajectory you take to get to Kerbin orbit; the transfer burn to the Mun; the way you land and take off there).  A badly inefficient technique can absolutely kill your performance by eating up scads and scads more dV than it needs to.  And if you're new to the game, it's easy to be inefficient without realizing it.  So... if you're engineering your rockets according to the above advice and they're not working for you, it may be simply that you need to adjust your technique-- in which case c'mon back here, we're happy to help with that, too. 

 

44 minutes ago, Bakkerbaard said:

a control part (usually one of those brownish discs I forget the name of that look like reaction wheels)

The term you're looking for is "probe core".

43 minutes ago, Bakkerbaard said:

I get a notification saying that the *control part* is not facing in the right direction and that might result in a screwy navball.

Yes.  That's correct, and it's a good notification that you should listen to because you'll be making your life a whole lot more difficult if you don't.

(On the PC, there's a "Control From Here" option-- is that available on the console?  If so, you can manually set the control-from-here point to your crew pod so that it will un-hork your navball.  Just... you'll need to remember to do that, and it's easy for the game to reset it when you're not looking, and the likelihood that it will screw up your mission at a crucial point is fairly high.  So you can do it if you want to, but it kinda seems like borrowing trouble to me.)

46 minutes ago, Bakkerbaard said:

I want the control disc facing left, like a wheel, not a pancake.

May I ask why?  It's designed to be used vertically, pancake-style... is there some reason that doesn't work for you?

48 minutes ago, Bakkerbaard said:

As an extra little hurdle, I plan to have an engine (all of this is going to be experimented with tonight, we're in the theoretic stage now) facing right so that once in the vacuum of space this crooked little spaceship will rotated 90 degrees and have the formerly leftward control disc facing upward and the formerly rightward engine downward. Wait, I'll draw you a picture, you seem like patient people.

Nice picture. 

I suppose you can do that if you want to... but just understand you're making life considerably more complicated for yourself than it needs to be.  If that's what you want (this is KSP, after all!), then great!  Just want to make sure that you're doing it because you want to, not because you think you need to, because you don't.  The simplest way would be to just make one vertical stack.

 

By the way:  as long as you're entertaining crazy ideas, one more possibility to consider:

Kerbal EVA thrusters have a ridiculously large amount of dV in them.  It's around 600 m/s.  This means that it is actually possible for a kerbal standing on the Mun to get to low Mun orbit using EVA thrusters alone.

It's hard.  It's only barely possible, and you have to do it just right, and you can only make it to a really low Mun orbit, like 10 km altitude or less.  But it is, in fact, possible (I've done it).

So... there is one option for rescuing, which is to have a ship that doesn't actually land on the Mun.  Instead, your kerbonauts put themselves in low Mun orbit, and you just have to rendezvous with them and pick them up.

Advantages:

• Shaves over 1200 m/s off your ship's dV requirement.
• Simplifies ship design.  Doesn't need landing legs; don't have to worry about it being topheavy and tipping over; etc.
• Saves you from needing to do a pinpoint target vacuum landing while staying efficient, which is really hard to do if you're new to the game.

Disadvantages:

• You need to know how to do an orbital rendezvous (though I contend that this is still a lot easier than efficient pinpoint surface landing).
• Like I said, getting EVA kerbals to Mun orbit is hard, you'll need a lot of tries and you'll need to do it just right.

Just throwing it out there as a possibility. 

 

Another possibility:  Instead of sending a two-kerbal rescue mission, you could send two 1-kerbal rescue missions, if you want.  Yes, that means you have to fly twice as many missions.  On the other hand, it greatly simplifies your ship design, cuts the ship size in half, and gives you two chances to practice.    Again, just a thought, it's up to you.

[bewing]

This being KSP, there are many variations possible. So some more variations on what snark said --

Jeb's current ship may be underfueled, but if it has enough fuel to at least get back to Munar orbit then that will save a whole lot of trouble (and deltaV costs). Even just getting halfway back to orbit makes snark's "EVA and fly the rest of the way to orbit yourself" option a touch less difficult.

To do things specifically the way you envisage, the easiest way is to not build it sideways initially. You start with the probecore facing upwards. Stick the monoprop tank under it. The terrier under that. Then stick the two fuel tanks to the sides of the mess. Rotate them until they are sticking out sideways with the rotate gizmo. Then attach two MK1 pods sideways, one at a time, to the ends of the fuel tanks. Make sure you stick parachutes on it somewhere! Stick on your landing legs. Then grab the probe core, and rotate the entire thing sideways with your Dpad.

However, having a sideways-facing probecore like this really does make it hard to fly your ship straight.

Another pair of alternates goes like this: if you are in career mode, then you are just about to get a bunch of contracts asking you to land tourists on the Mun. So you build your tourist ship with 2 extra seats, and you get paid to rescue Jeb and Bob.

If you are in a Sandbox game, then the question is: do you know how to get into the debug menu? Snark's math does not really allow you to estimate the actual amounts of reserves you need for your actual level of piloting skillz. The debug menu does. You make your theoretical rescue lander, put it on the launchpad all by itself, and use SetOrbit to put it in orbit around the Mün. Then you land it, and get it back into orbit, and fly it back to Kerbin, and see if you can land it there. If you run out of fuel, then it wasn't enough. If it blows up on reentry, then you need to fix that. If it works, then you can start designing the ship you need to use to get the rescue lander to the Mun.

 

 

[Bakkerbaard]

3 hours ago, Snark said:

Hello, and welcome to the forums! 

Ah, Jeb, up to shenanigans again.

Thanks for the thoroughly helpful welcome.

Ofcourse, if Jeb found Münspiders, a one-kerbal pod will suffice...

3 hours ago, Snark said:

Okay, a disclaimer about what's to follow:  I play on the PC, and I have never even seen anyone play the console version.  I'm writing this assuming that except for control layout, the UI experience on console is basically the same as on the PC (e.g., the VAB displays the same info, etc.)  So it's possible I could be wrong somewhere, if the two UIs diverge in some way.

From what internet tells me, just the font differs, for some reason. I can basically translate from PC controls to console controls, I just mentioned it to clarify I don't have access to stuff like MechJeb.

3 hours ago, Snark said:

To find your TWR, take your total thrust, then divide it by (mass * gravity).

Somewhere, right now, a math teacher with a fuzzy grey beard and a cigar is going: "Aw! He's doing math for entertainment now?!"

Anyway. I can work with that equation. This whole explanation has been very helpful. When I checked relevant wiki-pages the place where these equations supposedly were just showed an error. So, yeah. Very helpful, except for this part:

3 hours ago, Snark said:

Caveat:  The above math is simple, and correct, when you're in a vacuum.

Are you, by any chance, a math teacher with a grey fuzzy beard and a cigar?

3 hours ago, Snark said:

On the PC, there's a "Control From Here" option-- is that available on the console?  If so, you can manually set the control-from-here point to your crew pod so that it will un-hork your navball. 

I've seen it. But since I usually have my controls placed like the good boy I am I never noticed anything significant. Plus, I was busy not burning up.

3 hours ago, Snark said:

May I ask why?  It's designed to be used vertically, pancake-style... is there some reason that doesn't work for you?

Nice picture. 

I suppose you can do that if you want to... but just understand you're making life considerably more complicated for yourself than it needs to be.  If that's what you want (this is KSP, after all!), then great!  Just want to make sure that you're doing it because you want to, not because you think you need to, because you don't.  The simplest way would be to just make one vertical stack.

Nice picture, huh. Would you believe I work in graphics?

Anyway, you may ask, yes. But the picture kinda shows it too. For the trip to the Mün I want the rescue pod to be wrapped up in protective fairing, for which it'll need to be upright. Then for the pickup I'd like to send the pod out on it's own, at which point it will rotate 90 degrees so the probe core is right way up. 

It's basically just taking a break from serious attempts. See if it works. And Thunderbirds. I'm almost original-Thunderbirds-old and I loved it when stuff came out of TB2. Hence my fairing obsession. Though I prefer to not build vertical stacks. For no scientifically sound reason I fear tipping, so the lower the better. I told you, mad scientist.

3 hours ago, Snark said:

Another possibility:  Instead of sending a two-kerbal rescue mission, you could send two 1-kerbal rescue missions, if you want.  Yes, that means you have to fly twice as many missions.  On the other hand, it greatly simplifies your ship design, cuts the ship size in half, and gives you two chances to practice.    Again, just a thought, it's up to you.

I'm no bozo, I've watched Apollo 13, like, three times. But I'm not confident or good enough to make two precision landings. Besides, I've seen people land skyscrapers on the Mün, I should be able to pop a two-seater up there. ;o)

Right, thanks for the help so far. I'll have a go at it tonight, for now I'm heading home.

 

@bewing I'm not ignoring you, it just took me really long to write that and now I'm on my way out.

Edited April 21, 2018 by Bakkerbaard

[Snark]

50 minutes ago, Bakkerbaard said:

Are you, by any chance, a math teacher with a grey fuzzy beard and a cigar?

No, but a fairly closely related species, "pencil-neck geek physics major turned software engineer." 

51 minutes ago, Bakkerbaard said:

I've seen it. But since I usually have my controls placed like the good boy I am I never noticed anything significant. Plus, I was busy not burning up.

Good.  Anyway, that option's going to be your pal if you start mounting various stuff pointing in various directions.  Whichever "control from here" part you choose, that's what the navball will fixate on-- i.e. that part's "up" direction will become the direction that the navball is "looking".

So if you do build your contraption with pods going one way and a probe core going the other, you'd want to be sure to set your control-from-here point in one of the "forward-looking" pods while you're flying the rocket in that orientation, then change it to your sideways probe core when you start flying in sideways mode.

53 minutes ago, Bakkerbaard said:

It's basically just taking a break from serious attempts. See if it works.

Fair enough.  That's basically what KSP is all about, right there.    Just be aware that you're making it significantly harder for yourself, this way-- so if you can make it work, great, but if flying proves too awkward, may be worth considering a less contraptiony ship.

54 minutes ago, Bakkerbaard said:

But I'm not confident or good enough to make two precision landings.

Actually, I'd contend it would be the other way around, i.e. doing it in two missions would be easier, not harder.  Precision landings tend not to happen by accident.  It takes a lot of work and repetition and practice and repeated-failed-attempts-with-reverting to figure them out, generally speaking.  But once you've developed the skill to do it, it's a lot easier to repeat.

So, I'd contend that it's actually easier to do it as two landings.  Because the second landing will be a cakewalk, once you've done it once.  And doing it for the first time will probably be easier with a one-kerbal ship than a two-kerbal ship.

But in any case, it's up to you. 

 

[Bakkerbaard]

Status update: There's a new pile of debris on the Mün.

Silver lining: It's really close to the projected landing site!

The two-pod flip-top approach didn't do it for me. Aside from wonky construction, two MK1s are heavier than two of those cheaper lander cans, so never even sent it up. The things I did send up though, mostly never even made it to orbit. Maybe it's the summer heat, but it was like I was a kid throwing firecrackers with wings last night. The only thing I did get up up and away was christened "Overkill". A two-can lander strapped onto a a giant tank of fuel, the 900-and-change engine and four radially mounted fuel/engine combo deals. The big ones with the black stripe and two engines. I'll start paying more attention to the names next time. 

Either way, cartoon-amounts of thrust for a 7-ton lander. If my application of your formula, Snark, was correct I had a TWR of 189-something.

I'd have gotten Jeb and Bob too, were it not for the fact that the spot they're in is either on the dark side of the moon, or in a radio dead zone. I've now put the rescue pod in low orbit and I'm going to use the remaining fuel in their ship to get them in a hopefully close enough orbit and see what that does. Good times.

What amazes me the most about KSP is that I've played other trial-and-error games and one of them even ended up physically broken under the couch, but this one... This one makes me go: "Okay, okay. What if I tweak this bit?"

[bewing]

Hehe!

Honestly, I'm glad that you are getting into the spirit of the game. Imagining a storyline and all the little details that the game just hints at.

If you start to get too frustrated it is possible to turn off CommNet, for one thing -- so that the command blackout on robotic ships stops happening. And you will get better at launching stuff to orbit. SRBs are quite helpful for that. But yeah, keep us updated and maybe we can give you another hint or two that make things work better.

 

 

[Bakkerbaard]

Status update: Jeb and Bob are home safe. Relatively. Jeb found Münspiders, but I went through too much trouble to get him, so I'll handle that when the Kerbal: Alien Invasion FPS comes out.

I thought about turning off the CommNet handicap, but it felt like cheating. I had the rescue lander in low orbit and I figured I could make it if I could just see what I was doing, so timewarp came in handy. Then when I got some light I tried again and I got it within the 1km zone. I've just been forgetting the advice I give my girlfriend all the time: "Damnit woman! Have some patience!"

So, I'll be looking for Minmusspiders now, if you need me. ;o)