[1.7.3] Community Delta-V Map 2.7

[Kowgan]

I see the problem.

TL;DR: "Mix" is 95% atmospheric, 5% vacuum.

During typical ascents, we usually burn almost the entire ascent maneuver while in the atmosphere, and then a small burn comes at the apoapsis to circularize, am I right? That's what the mix can be translated into.

To develop further more, "4-5k" was an erroneous guess, up there. My bad. I should have written "5700" atmospheric.

If you fill up a vessel with exact 6000m/s of atmospheric delta-V for Eve (and have a reasonable TWR), and reach orbit there, you will, most likely, end up in a low circular orbit with a few hundreds of m/s left.

I understand that people want more precise values, and want to exclude as much of estimative as they can.

If you see fit, I can rephrase that on the map, so "Atmospheric maneuvers use atmospheric Delta-V. Vaccum maneuvers use vacuum Delta-V.", and adjust the values for that. Although those values would only decrease from their current.

[RocketBlam]

It's not that they want more precise values, it's that they want values they can use. Simple, easy to understand values that reliably work.

[Warzouz]

That would also explain 3200dV off of Kerbin. My absolute best I've ever done was about 3300 (vacuum) dV.

I think I did a 3150 (vac) once, with a streamlined test payload to 75km (by comparing the dV left in orbit to the VAC value at launchpad). I got more success by crossing 45° at 7/8km. On the otherhand I failed to orbit a non-streamlined payload with 3600m/s...

But I agree that this 3200m/s value had to be tempered. It's more 3200 to 3700m/s

I think MJ has some mesurement feature to calculate dV usage (like a stopwatch).

[mhoram]

Regarding dV measurement I can see two problems:

1. In Pre-V1.0 times 4500 m/s was the ballpark that fit nearly every rocket, if you had a reasonable engine configuration (low-TWR rockets needed more).

Now in Post-V1.0 times the form of the rocket has a much higher influence on the needed dV amount.

An aerodynamic craft can reach top dV values while a non-aerodynamic craft will require a few hundred m/s more.

Since it should be good style to create aerodynamic rockets, I would find it better to include a value that is targeted at aerodynamic crafts.

2. Measuring the amount of dV needed for the ascent can be ambiguous. Should it be the atmospheric, vacuum, a mixture or MJ-readout value?

Using the atmospheric value has the advantage that it is in any case more than enough to reach orbit, but has the disadvantage that you can reach orbit with less dV.

For the vauum value it is just the other way round and has the additional advantage that it is consistent with prior dV-maps.

Furthermore these two methods are easily explainable.

MJ readout has the advantage that it is just right but the disadvantage that you need to have MJ installed to get this value.

The notion of a mixture between atmospheric and vacuum dV does not seem far fetched from my point of view. It is a better approximation than either of them, but has the disadvantage that it requires an explanation on the dV-map.

A 95%-5% distribution seems to be a bit off. The isp of the engines changes gradually with air density and I would guess that at an altitude of 10-30km the vacuum value is a better fit than the atmospheric one.

Usually I eyeball this value at 50%-50%.

Another option would be to state a range with the additional text "3200-3700 depending on ..." but this would most likely confuse the readers.

For reasonable engine configurations I have the impression that the atmospheric and vacuum dV values have the same ratio to each other, so I would suggest to use the vacuum value for dV-maps.

[Warzouz]

I think it's less than 50/50. ISP rises very fast with altitude. Even at 10000m, you're near VAC value. In the end VAC value is largely predominant.

I would say 20-30% atmo / 80-70% vac. But again, it depends on the ascent profile we use. Recently, I target 45° at 8km, not 10km. I stay longer in lower atmo. But the end result is positive. This ascent profile is not efficient with low ISP engine or non streamlined payloads though.

[Kowgan]

Regarding the first problem, the values in the chart are already set for optimal conditions. This includes aerodynamic-friendly rockets.

I understand the 50/50 point of view, and I agree with it when thinking logically. But for some reason, in practical experiences, the values seem to just fit the 95%-5% results, when comparing initial dV amounts with final dV amounts.

Maybe those are just two different ways to measure it, and we're getting confused as such.

By the 95% thing, what I'm trying to say is, if you fill your ascent stage with given value into Atmospheric dV (and considering proper rocket and conditions), you should have just a bit more than enough dV to circularize a low orbit.

In any case, this is some problem. I'm afraid the great disparity between rockets and results post V1.0 won't be softened by changing the current "mix" value to "vacuum". If the current value isn't enough to make someone's non-aerodynamic rocket reach space, switching values to Vacuum won't help much.

Either way, I'd like to read more inputs. Based on the statement above, would you prefer the values as vacuum anyway, or should we keep it as mix/atmo?

[Warzouz]

I think it doesn't matter. Those aren't precise values, they vary more than in 0.9 old atmo. Further more, the vac/atmo difference depends on the engine you use.

Personally, I always use VAC value, it's more simple. I usually use 3300 (or 3400 if I want to return parts of the stage). I use 3600 for non streamlined payloads (should even be more)

Focusing on the 3400m/s and streamlined payload, on launchpad, the atmo KER value is even 3100m/s (5% vac wouldn't change much)

For example, this rocket (click to enlarge) is set for 3400m/s VAC. It's a SSTO which is suppose to have 200m/s left before staging at LKO. So, I'm support to get there for 3200m/s VAC or 2900m/s atmo (with no safety at all)

In other words : your 3200m/s value is fine. Don't even mention it's a mix, use the VAC value.

Edited July 23, 2015 by Warzouz

[Björn Eberhardt]

I was looking at the highlighted numbers of Galahir950's version of the Delta-V map and don't understand why Kerbin-Eve is not marked as the least Delta-V requirement, but has instead the whole 11750 m/s highlighted. From LKO, you only need to scratch the athmosphere of Eve to get from a hyperbolic to an elliptical, a circularized orbit and then slowly descend into the athmosphere. With 4340 m/s, that is even slightly less than a landing on Minmus, if I am not mistaken.

[Galahir950]

I was looking at the highlighted numbers of Galahir950's version of the Delta-V map and don't understand why Kerbin-Eve is not marked as the least Delta-V requirement, but has instead the whole 11750 m/s highlighted. From LKO, you only need to scratch the athmosphere of Eve to get from a hyperbolic to an elliptical, a circularized orbit and then slowly descend into the athmosphere. With 4340 m/s, that is even slightly less than a landing on Minmus, if I am not mistaken.

I actually have a few reasons for this:

1. When I decided to make the chart, I decided that "Kowgan's word was law" and I would not put anything on the summary chart that contradicts his or could cause confusion.

2. On the chart I have 2 entries for all the high D/v bodies.

3. I chose to use all the landing D/v for the bodies that have atmosphere because the maneuver needed to do that is too risky for new players to successfully try. If you have even a slightly too deep "scraping" altitude, you will burn up when you hit it at interplanetary velocity.

[Kowgan]

@Warzouz: Thank you for the input.

The problem is: The 3200m/s may be fine, as you said, but that's for Kerbin only. For other planets, like Eve, the difference between Vac/Atmo is huge.

[mhoram]

I just saw that metaphor updated his dV map and he solved the problem by specifying a range for atmospheric ascent.

http://forum.kerbalspaceprogram.com/threads/120030-1-0-Delta-V-Maps-and-Atmospheric-Charts?p=1960225&viewfull=1#post1960225

[Kowgan]

Thanks for the link.

I'm personally not a fan of putting a range (especially big ones) of possible dV's on a chart. As explained previously, it can be confusing to the reader, and we're trying to simplify it at its best.

That said, if there's no objections, I'll update the chart, getting rid of the "mix" stuff, and replacing it with Atmospheric dV for ascent nodes (surface <-> low orbit), for bodies with atmosphere.

Edited July 24, 2015 by Kowgan
NO

[Kowgan]

After testing even more, atmospheric values shown themselves unreliable, once none of the most popular mods who show dV calculations will show usable data, depending on body, altitude, and vessel configuration. I don't blame these mods. I blame physics.

Here's two very distinct vessel examples that can reach Eve low orbit from sea level, and have a comfortable amount of fuel left:

5m Rocket

Lander

As you can see, both vessels show low or zero m/s for sea-level dV, and this deviates vastly from the practical results, as the vessels go up. Especially in bodies like Eve or Jool.

For this reason, I'm gonna be a good son and follow the tradition: I'll abandon mix/atmo values, and convert all the map values into Vacuum. That should make everyone's lifes easier.

[Björn Eberhardt]

I'll abandon mix/atmo values, and convert all the map values into Vacuum. That should make everyone's lifes easier.

I think, 10k Vacuum dV is sufficient to go to LEO from surface for an average Eve-optimized craft, as long as the TWR remains at some magic value above 1 during flight (which is the case for the two crafts from the screenshots).

[jofwu]

Found an error. All of the lines leading TO "Kerbin SOI edge" need an aerobraking arrow, in that direction.

When you're going away from Kerbin, these values represent the amount (from LKO) needed to lower/raise your solar apoapsis from Kerbin's altitude to the target planet's altitude. When you're coming BACK, they represent the amount needed for "Elliptical Kerbin Orbit" (which is what the Kerbin SOI edge node really means).

Come to think of it, wouldn't it also make sense to replace "Kerbin SOI edge" with "Elliptical Kerbin Orbit"? That uses consistent terminology. And... why is the "Elliptical Orbit" left off for the planet's without moons? It makes sense considering it's not practical. But again, it would be more consistent. And I feel like consistency helps with comprehension for those who are new to these.

Just my two cents.

[Kowgan]

Jofwu: Good eye! I'll put the aerobraking arrows there on the next update.

As for the "Kerbin SOI Edge" name, I left it like that due to space restrictions. I think both terms can be applied as they are, without generating confusion.

The Elliptical orbit for planets without moons isn't as practical as the rest, and for that, I left it out of the design. The original design didn't even have these Elliptical nodes. I've added them to save some dV in some cases where the player's target is a moon, not its planet.

For "Orbit Target Altitude" finetuning, I recommend Alexmoon's Launch Window Planner. For elliptical orbit data on every planet and moon, I highly recommend Metaphor's dV Maps.

[Kowgan]

Updated to v1.2.1. All values are now measured as vacuum delta-V.

Changelog:

[B]1.2.1[/B] (06/08/2015)
- Converted values to be measured as vacuum delta-V
- Added aerobraking arrows towards Kerbin SOI (Thanks Jofwu!)

[Tatonf]

What does that mean ? Why do I suddenly need 10 000 dV instead of 6000 dV for orbiting Eve ?

[Kowgan]

@Tatonf: Easy way to understand it: A certain amount of fuel will provide you 6000 Atmospheric Delta-V in Eve. The same amount of fuel will provide you 10000dV in Vacuum.

No practical values were changed. You don't actually need more delta-V now. Only the unity of measurement was changed.

It's like converting Miles per hour to Kilometers per hour. 87mph = 140km/h, but both are the same speed.

[Tatonf]

But that's totally ignoring the Isp modification during ascent.

[Kowgan]

As it always have been.

[swashlebucky]

Hi,

I am working on a version of your map that is a bit more printer-friendly and better suited to hang on the wall beside your screen (mainly larger text and more contrast).

The license you have is essentially a Creative Commons BY-NC-SA license. Would it be ok if I just put that on the map instead of the whole license text you included in your first post?

[Tatonf]

So if I understand correctly, getting 3400 m/s in vacuum with my rocket does not guarantee that I'll be able to go in LKO ?

Well of course it won't. I'd just like to point out that I never know what to do with the numbers given for atmospheric bodies.

It's like "Well it's around those values but you'll have to test it yourself anyway".

[Warzouz]

To fail at 3400m/s means you have a very non streamlined payload or a messy ascent profile. Sure if you go straight up and circularize then, you'll need around 4000m/s (may be more).

A reasonable rocket would be 3300m/s (VAC) with safety margin for a regular payload.

My first stage usually have 3350 to 3400m/s but it needs to deorbit itself to land near KSC (powered landing).

I think I get to LKO (75km) for 3150m/s (VAC - no safety) that means the real dV (including atmo loss to ISP) should be around 2900 to 3100m/s to LKO.

[Kowgan]

Hi,

I am working on a version of your map that is a bit more printer-friendly and better suited to hang on the wall beside your screen (mainly larger text and more contrast).

The license you have is essentially a Creative Commons BY-NC-SA license. Would it be ok if I just put that on the map instead of the whole license text you included in your first post?

All the necessary credits (apart from yours for remixing) are already contained in the image itself, on those upper-left notes. As long as you leave those there, you're free to rock'n'roll.

The license doesn't need to be in the map itself. If you post the map somewhere else, you can just specify the license name on the description.

So if I understand correctly, getting 3400 m/s in vacuum with my rocket does not guarantee that I'll be able to go in LKO ?

Well of course it won't. I'd just like to point out that I never know what to do with the numbers given for atmospheric bodies.

It's like "Well it's around those values but you'll have to test it yourself anyway".

You'll be looking at the 'Vaacum Delta-V stats", but that doesn't mean you'll have to be guessing anything. That's why it's called a conversion from atmospheric Delta-V.

Here's a visual reference on what you should be looking at when building your vessel, if you use either MechJeb, KER, or any other mod that shows Delta-V stats:

[ÃŽâ€V stats]

Notice that both mods can be set up to show Atmo and Vacuum Delta-V. When building your vessel, you should be looking at the Vacuum Delta-V indicator.

This example vessel have a total of 4238m/s vacuum dV, more than enough to put us in a LKO.

The first stage has 2477m/s, not enough to put us into orbit, but enough to oomf our way up. (actually, enough to set our Apoapsis at 80Km and still have some fuel left)

The second stage will burn the remaining 923m/s (3400-2477=923) to make sure we're in a circular orbit, and still have plenty of dV left.

I hope this have clarified something. Let me know if there's any more questions.