TWR? Delta V? WTH?

[GoSlash27]

Why does everybody keep saying that calculating DV is "hard"?

There's nothing hard about it. It's a straightforward equation.

I've never used anything to calculate it for me other than spreadsheets, and in those cases I programmed them myself.

I'm not a math genius or anything, it's just really not complicated. Other than a single natural log, it's literally *all* multiplication and division. Not even much of that.

a/b. Take the log of that. Multiply the answer by c and d. That's it.

Best,

-Slashy

[numerobis]

Indeed. It's easy -- it's just tedious.

[GoSlash27]

Indeed. It's easy -- it's just tedious.

Heck, not even all that tedious, really. It's just 4 numbers and one of those is a constant. It's just barely more time consuming than figuring out a tip at the restaurant. If there's anything that makes it tedious IMO, it's that you have to do it so many times... not that it takes much work (or brains) to do in a single iteration.

Best,

-Slashy

[Red Iron Crown]

I found summing the part masses for m0/m1 to be the tedious part, running the actual equation is the easy part. Staging makes it even more so.

[dbmorpher]

The amount of time spent installing the mod is much less than the time it will take to do all calculations by hand. Unless you are a strict purist I would go with a mod. KSP is about having fun, not repetitive math.

[GoSlash27]

I found summing the part masses for m0/m1 to be the tedious part, running the actual equation is the easy part. Staging makes it even more so.

'Zackly. And I must stress that this isn't shaolin monk level math. Not like I'm tooting my own horn over my uber math genius skillz. It's just really not difficult.

Now having said that, understanding how the math works and actually working it by hand (at least for a while) can help you gain insights into the enginerding by answering questions that MechJeb can't answer for you, such as which engine is optimal for a given application or which staging scheme will get you the most DV for the least stage mass.

Spreadsheets are invaluable for these answers and being competent (if not stellar) at algebra is very useful in building a better rocketship.

[Specialist290]

As GoSlash has already made a new, separate topic for the discussion on why some users may find delta-v calculations to be difficult, tedious, or counterintuitive, I'd recommend that that particular discussion be continued there, to keep this thread from getting too far off-topic

[capi3101]

I agree that calculating delta-V by hand is tedious - but only in as much as that you have to take the time to figure up the wet and dry masses; the actual calculation is only difficult if you don't have ready access to a calculator (I mean, can anybody tell me what ln(3.28864685689) is without punching that into a calculator/spreadsheet? Because that would be impressive).

I do think that anybody genuinely interested in the math behind the game should try to do it by hand a time or two before installing KER or Mechjeb. If for no other reason, it heightens ones appreciation for piloting assistance mods like those two in particular.

My two cents. Now back to your regularly scheduled thread...

[LethalDose]

(I mean, can anybody tell me what ln(3.28864685689) is without punching that into a calculator/spreadsheet?

Okay, I'm not old enough to have needed to do this, but students in science and engineering students in the not too distant past (~ 30-40 years ago) would have been able to do this off the cuff with little difficulty due to a property of logs which states ln (xy) = ln(x) + ln(y)

3.28 yaddayaddayadda is very close to 10/3 ~ 3.33333, therefore ln(3.288...etc) ~ ln(10/3) = ln (10) - ln (3) = ln (2) + ln (5) + ln (3). The values for ln (2), ln (3), and ln (5) would have been memorized by said students (probably to 4 or 5 decimal places. I don't), and then it's simple addition. Similarly, you could use a slide rule to get those values, or approximate ln(3.28) directly, but at that point, you're basically using a calculator or spreadsheet.

wrapping up: ln(2) + ln(5) - ln(3) = 0.69315 + 1.60944 - 1.09861 = 1.20398

compared to ln(3.28864685689) = 1.19047

which is just a hair over 1% difference for easy math. Certainly within tolerances for KSP or an exam.

I'm not saying that players should be knocking this out by hand every time they want to calculate dV (that would be 31 flavors of stupid), I just thought it would good chance to demonstrate an old method for calculating logs that lots of younger students may not have been exposed to.

EDIT: Just a note, the method isn't restricted to natural logs, it works with base 10 and any other base.

Edited December 11, 2014 by LethalDose

[RocketScientistsSon]

Thanks for the answers everyone! I have been browsing some other threads and I have another question that may or may not be related.

What is a gravity turn? I have a feeling I have been doing this all along and just didn't know it. But, its always nice for things to be explained out.

[Tweety]

Without too much confusion (i hope) its basically your ascent profile into orbital insertion (how you get to orbit). Typically most people start them at 10k altitude. It's where you start gaining horizontal velocity so you don't just fall back down due to gravity clawing at your craft.

I may have overcomplicated or explained incorrectly

Tweety

[LethalDose]

Thanks for the answers everyone! I have been browsing some other threads and I have another question that may or may not be related.

What is a gravity turn? I have a feeling I have been doing this all along and just didn't know it. But, its always nice for things to be explained out.

In KSP parlance, 'gravity turn' refers to the maneuver where a rocket changes it's orientation from a vertical profile to a more horizontal trajectory for orbital insertion. 99 times out of 100 the "go to 10km and turn 45 degrees" equals 'gravity turn' in KSP.

In reality, 'gravity turn' refers to using the earth's gravity to pitch the rocket over slowly due to the offset torque of gravity on the rocket's CoM when it leaves a vertical profile (the thrust vector is offset from the weight vector, and a slight net rotational force is created). This happens more when you're launching with FAR, and you need a more gradual turn.

So, the way "gravity turn" gets used in the community may not be technically correct, but there is absolutely no reason to try and correct it. We all know what we mean when we say it.

[Red Iron Crown]

So, the way "gravity turn" gets used in the community may not be technically correct, but there is absolutely no reason to try and correct it. We all know what we mean when we say it.

Hard to believe, but I agree with this (I'm often a stickler for correct terminology, but it doesn't matter in this case).

The KSP gravity turn just refers to the change from vertical acceleration to horizontal acceleration. Some do it more gradually or abruptly than others, but the effect is the same (reduce rate of raising of apoapsis, increase rate of raising periapsis).

As an aside, I think "proper" gravity turns will become more popular in 0.90, as the ability to keep pointed prograde automatically will make it much easier. Players with less experience or piloting skill are going to be able to do much more efficient launches, and they'll become less tedious for skilled players.

[capi3101]

45 degrees at 10k is fine in stock aerodynamics as a rule. Use NEAR or FAR and you'll nose over uncontrollably if you don't do it gradually enough (that can happens in stock too, it's just not as common).

[Temeter]

Okay, I'm not old enough to have needed to do this, but students in science and engineering students in the not too distant past (~ 30-40 years ago) would have been able to do this off the cuff with little difficulty due to a property of logs which states ln (xy) = ln(x) + ln(y)

3.28 yaddayaddayadda is very close to 10/3 ~ 3.33333, therefore ln(3.288...etc) ~ ln(10/3) = ln (10) - ln (3) = ln (2) + ln (5) + ln (3). The values for ln (2), ln (3), and ln (5) would have been memorized by said students (probably to 4 or 5 decimal places. I don't), and then it's simple addition. Similarly, you could use a slide rule to get those values, or approximate ln(3.28) directly, but at that point, you're basically using a calculator or spreadsheet.

wrapping up: ln(2) + ln(5) - ln(3) = 0.69315 + 1.60944 - 1.09861 = 1.20398

compared to ln(3.28864685689) = 1.19047

which is just a hair over 1% difference for easy math. Certainly within tolerances for KSP or an exam.

I'm not saying that players should be knocking this out by hand every time they want to calculate dV (that would be 31 flavors of stupid), I just thought it would good chance to demonstrate an old method for calculating logs that lots of younger students may not have been exposed to.

EDIT: Just a note, the method isn't restricted to natural logs, it works with base 10 and any other base.

Yeah, the as a whole is less the mathematics, more the 'add one battery and redo DV for all stages'. You also shouldn't underestimate the barrier before getting an understanding of a specific calculation. People aren't made for math, so that can be a very tedious process and look very hard from the outside.

It's certainly a bit nerdy - there is a neat german term for it, namely fachidiot^^ - to see the simple logic from the inside and then wonder why other people can't do those easy tasks.

[GoSlash27]

Thanks for the answers everyone! I have been browsing some other threads and I have another question that may or may not be related.

What is a gravity turn? I have a feeling I have been doing this all along and just didn't know it. But, its always nice for things to be explained out.

The term "gravity turn" has a different meaning in the KSP community than it does in the real world.

1) "Gravity turn" when you hear it on this forum, refers to a gradual reduction in pitch over the duration of the flight from vertical to horizontal, thus yielding the most efficient balance between gravity losses and drag losses.

2) You may also see the term "gravity turn" or "gravity kick" used to describe the point at which a rocket transitions from the vertical boost phase into the gradual prograde curve.

3) Occasionally, somebody will use the term "gravity turn" as it's actually used in the real world; a rocket that slowly "falls over" during launch at a rate sufficient to generate an efficient profile without needing active vectoring. A true "gravity turn" launch will continue to rotate beyond horizontal to generate a radial-in burn to circularize all in one shot without throttling or restarting the engine.

These are very rare in KSP, so when you hear this term, it's almost always in the first or second context.

Best,

-Slashy

Edited December 13, 2014 by GoSlash27

[LethalDose]

Yeah, the as a whole is less the mathematics, more the 'add one battery and redo DV for all stages'. You also shouldn't underestimate the barrier before getting an understanding of a specific calculation. People aren't made for math, so that can be a very tedious process and look very hard from the outside.

Yes, I agree that this kind of math should not be expected of players. You've quoted me stating that would be "31 flavors of stupid". The purpose of that post was to demonstrate it's not that hard, not that it should be expected. I thought I made that pretty clear...

It's certainly a bit nerdy - there is a neat german term for it, namely fachidiot^^ - to see the simple logic from the inside and then wonder why other people can't do those easy tasks.

Edited December 13, 2014 by Vanamonde

[RocketScientistsSon]

Cool. So more questions then,

1. Is the 10k up and turn 45 degrees the most efficient way of doing it? or is it just the "standard"?

The tutorial i watched when I first started playing was at 15k to 30 degrees and burn until your periapsis is in the LKO range. Then set a burn to create an orbit and circulize. Is that a stupid way of doing it?

2. Does your gravity turn profile (is that proper Kerbal terminology?) depend on your rocket? Or can you get the same profile with any properly built rocket? In other words does your delta V or TWR affect how you do your gravity turn?

3. Do you guys have a typical gravity turn profile you use? If so, what is it?

thanks.

[Red Iron Crown]

1. It is not the most efficient, an efficient gravity turn starts a bit sooner and is much more gradual, not staying at one pitch angle. This takes a bit more piloting skill and a lot more attention.

2. Delta-V doesn't affect gravity turn shape. TWR does, greatly. The higher your TWR, the sooner you can start pitching over (to a point, no advantage to making your trajectory cross too much atmo). This reduces gravity losses. On an airless body, it's best to start the turn almost immediately.

3. My rockets are usually staged and have an initial TWR of 1.3-1.4. I go straight up until 7-8km, then start pitching over gradually until I'm horizontal at about 60km or so. Engine gets cut when my apoapsis is at the altitude I want (100km is my standard parking orbit), then coast to apoapsis and burn to circularize.

All this is in stock aerodynamics, gravity turns are a good bit different in FAR/NEAR as I understand it.

Edited December 13, 2014 by Red Iron Crown
"Altitude", not "latitude".

[Starman4308]

Cool. So more questions then,

1. Is the 10k up and turn 45 degrees the most efficient way of doing it? or is it just the "standard"?

The tutorial i watched when I first started playing was at 15k to 30 degrees and burn until your periapsis is in the LKO range. Then set a burn to create an orbit and circulize. Is that a stupid way of doing it?

2. Does your gravity turn profile (is that proper Kerbal terminology?) depend on your rocket? Or can you get the same profile with any properly built rocket? In other words does your delta V or TWR affect how you do your gravity turn?

3. Do you guys have a typical gravity turn profile you use? If so, what is it?

thanks.

1. No, it's just the simplest, easiest, and laziest way to do it manually. While using MechJeb is even lazier, I still recommend watching it go once or twice, because it'll do a reasonably good turn, and helps show you how to do it right.

2. The technical term is "pitch profile"*. And yes, TWR profile very strongly affects how you pull your ascent, either in stock or FAR/NEAR. If, for example, you have a low TWR second stage, you need to get more upwards kick on the first stage: if you have a high TWR second stage, you can go more horizontal on your first stage. This assumes the presence of atmosphere: if there is no atmosphere, the only​ reason to go up is to not crash into terrain.

3. It depends on the rocket, particularly as I use FAR/NEAR, which is really sensitive to TWR profile. In stock, I usually start tipping over at 7 km, and gradually work my way to 10-20 degrees on the way up, doing my best to imitate what I've seen MechJeb do.

*Technically, a true gravity turn requires realistic aerodynamics, because a true gravity turn is "make one slight adjustment close to the ground, abuse aerodynamics to keep it on course all the way up". That only works with FAR/NEAR.

[LethalDose]

All this is in stock aerodynamics, gravity turns are a good bit different in FAR/NEAR as I understand it.

Truth.

When using FAR, Ascending 10 km , then making a hard 45 degree turn will ruin your day in FAR. every. time. In FAR, you need to turn over gradually, and sometimes the rocket does it for you creating a real gravity turn. I usually start about 5 degrees right off the pad, and increase the angle as fast as I safely can. It takes a some practice though.

The tutorial i watched when I first started playing was at 15k to 30 degrees and burn until your periapsis is in the LKO range. Then set a burn to create an orbit and circulize. Is that a stupid way of doing it?

I haven't seen the tutorial, but it doesn't sound "stupid". However, it does sound less efficient than 10km & 45 degrees. The reasoning behind why it's a less efficient ascent is left as an exercise for the reader.

RIC is spot on re: dV, TWR, and ascent shape. Nothing else needs to be said.

Finally, to contrast ascent profiles slightly to RIC, since I use FAR, my rockets usually have a TWR of 1.1 to 1.25, and as mentioned, I start pitching over immediately, albeit less dramatically.

I can get away with a lower TWR because launches with FAR tend to experience less atmospheric resistance (not sure if FAR changes the atmosphere or the physics, but that's well beyond this discussion). There also seems to be a consensus that lower TWR's off the pad slightly improve launch efficiency, likely due to the slower speeds in the lower atmosphere reduce gravity and drag losses more than leaving the lower atmosphere quickly.

Really, as long as your first stage has a TWR between 1 and 2 off the pad, you're probably fine. If you want to know more, I'd recommend watching some of Scott Manley's videos, or spending some time watching DasValdez on Twitch.

EDIT: Freaking Ninjas...

[GoSlash27]

^ I agree with RIC on all his points.

T/W is the most important thing on bodies with atmospheres. You want the velocity to match the increase in terminal velocity so that you are generating constant drag (in newtons) throughout the ascent, and this will naturally give you a huge variation based on t/w.

On airless bodies OTOH, the characteristics of the ship don't matter. A gravity turn is all about balancing gravity losses against drag losses, but since there's no drag, there's only one ideal profile; get it going horizontal as fast as you can without hitting the terrain.

For a generic profile on Kerbin, my profile very closely matches RICs.

Boost phase: realistically, 2 Gs would be perfect, but since you naturally gain acceleration as the fuel burns off that's more engines than you need. It is more efficient to start with just enough engine to get 1 G and let the acceleration build. Vertical to 7KM.

Transstage phase:

This is a gradual reduction in thrust from 1G to .75G as the nose falls prograde from vertical to 45*.

altitude/ pitch/ acceleration

7KM/ 90 / 1G

15KM/70 / .95G

25KM/ 45 / .75G

Injection phase:

25KM/ 45 / .75G

35KM/ 25 / .5G

Circularization phase: From here, we will maintain .5G acceleration and pitch to establish our orbit. There are 2 ways to do this:

1) The KSP way: rotate prograde. Establish the apoapsis, then throttle just enough to keep it until you get within 15 seconds of it. Throttle as necessary to keep the apoapsis 15 seconds ahead until periapsis is at orbital height.

2) The real world way: Establish the apoapsis, then pitch as necessary to maintain the apoapsis without throttling. When periapsis matches orbital height, cut the engine. This is an on-the fly piloting thing, so there's no standard profile for it. It's just a gradual transition from prograde to normal.

Burning prograde causes your apoapsis to get higher and closer, while burning radial in makes it go lower and move farther away.

I use the KSP method because... frankly, I'm lazy and why not make it as easy for myself as possible, since the equipment lets me do it.

HTHs,

-Slashy

Edited December 13, 2014 by GoSlash27
correction

[LethalDose]

Slashy indirectly makes an interesting point about upper launch and injection stages: You don't need to maintain TWR > 1 during the entire launch (in fact you only really need it off the pad). Kicker stages (stages used after the first stage, but before finalizing orbital insertion) can use engines with lower thrust, lower mass and higher ISP_vac and only achieve TWRs around 0.5 or 0.7 without any problem. They just tend to require longer burn times, but they save mass and typically generate more dV.

It's just a pet peeve of mine when Das Valdez teaches launcher design and he says "TWR 1.5 for the kicker stage". I know there's a reason he does it (standardization and StageRecovery) but he never mentions those reasons during his streams and I think it's leading to the misconception that you have to maintain a TWR > 1 until you hit orbit, which is just wrong.

[RocketScientistsSon]

Thanks guys. I got in to orbit today! I have played before and gotten craft around the mun and minus. But, I think that just had more to do with luck. I feel like I am starting to grasp some of the broader points to rocket design and this game has taken on a WHOLE new aspect that I am soaking up and loving. My next goal is to try and refine my profile a little more while messing around with new rocket designs until I find one I like and think I can use when needed.

Perhaps I went to school for the wrong engineering degree...

[GoSlash27]

Thanks guys. I got in to orbit today! I have played before and gotten craft around the mun and minus. But, I think that just had more to do with luck. I feel like I am starting to grasp some of the broader points to rocket design and this game has taken on a WHOLE new aspect that I am soaking up and loving. My next goal is to try and refine my profile a little more while messing around with new rocket designs until I find one I like and think I can use when needed.

Perhaps I went to school for the wrong engineering degree...

Love to hear this sort of thing!

We're always here to help, so feel free to hit us up.

Best,

-Slashy