#I feel like being 99% there understanding Delta-V ... or not

34 messages · Page 1 of 1 (latest)

halcyon prawn
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I have quite a bit of playtime beneath my belt, but up till now I made rockets by feelies and rule of thumb alone.

Ether my rockets failed to get into Orbit around Kerbin, or I oversized them so much that I went to the mun on stage 1 alone.
So I decided that I wanted to understand Delta-V and how to (while still in construction) measure if my rocket can reach Orbit or not.

Yet I struggle.
I build a rocket and measured how much Delta-V i took to reach each level of Atmosphere on Kerbin.
Then I build a larger rocket and made sure that each stage would put out as much Delta V as I measured beforehand.
Also making sure that there lift is greater than 1.
But my construct failed to get anywhere close to my desired goal.

I know the rocket is larger, but should Delta V not be Delta V regardless? Or did is fundamental misunderstood the problem?

agile flume
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Try build rockets that look like real rockets for example if you build a sls from 5 meter parts it should be able to reach all the way to jool

halcyon prawn
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Like I build a Mun-Lander, that I like, aswell as a rocket that will get him to the Kerbin Orbit to the Mun.

But now I need to build a lower Stage to get both into Orbit and I am sick of trial and error

agile flume
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I got you

halcyon prawn
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It be really nice If I find someone here, who might can offer half an hour for some 1 on 1 personal training via Voicecall.

Or lacking that a good explanations or maybe a Youtube Tutorial that explains it properly

agile flume
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How about I just give you a craft which can do It ?

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@halcyon prawn

halcyon prawn
agile flume
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Well just make one craft that can lift everything to low Kerbin orbit atleast

halcyon prawn
errant swallow
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if you read the text on the dv map, it says that all numbers are vacuum dv.

You should generally always measure dv in vacuum mode, and use sea level to check the first stage twr

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50% or more of your ascent/orbit dv should come from vacuum-optimized engines. Above 25km or so they are like 99% efficient

halcyon prawn
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Wait all the DV are Vac-DV? FanKerbdeadinside

... That would have been good to know beforehand.
Thx that allready helps allot

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I experiment a bit more and maybe the rubble will finally role with me.
But should that not be the case I may return in the future hoping for some more knowledge from my superiors.

Should anyone meanwhile feel like he can spare half and hour my DMs are open

errant swallow
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for a simple orbital rocket, you should aim to have a total vacuum dv around 3400 m/s and a sea level twr on the first stage at 1.5-2. A liquid-only first stage can use a slightly lower twr (say down to 1.2) , and a first stage that includes quick-burning SRBs like thumpers or smaller can go higher (say 2.5). If you meet those constraints you should be able to get to orbit pretty easily. If not, you might need to work on your ascent profile

halcyon prawn
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@errant swallow
Thx for the encouragement after some experiments I finally found the Data I needed.

I found that I needed ~1.350m/s to get from the Start Platform into 80k around Kerbin (on 10° after Start) [Kerbin to Space]
And from about 80k Space into a Orbit about ~1.450m/s [Kerbin Space into Kerbin Orbit]

Including some margin for error I got about ~1.500m/s into Space and about ~1.600m/s to get into a Orbit around Kerbin.

From there its just Space Transfer.

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Thx again, I got so frustrated at first, but now I think I got it.

halcyon prawn
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Nope, I failed again

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I build slightly large Rocket and made the last Stage 1.600m/s large and not it fails to get into 80k arround Kerbin ... it goes only to 50k ...

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Its so frustrating

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Wait I confused Atmospheric and Vac-DV again during my calc.

If I use Atmos.-DV it works again.

God, maybe I just need a good sleep and another try at this

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Yeah I forgott that the Building VC returns to Atomspheric Seetings if you return the flight ... yes that was my point of failure

errant swallow
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oh also I forgot to mention: The dv number to reach orbit is not a precise concept (unlike something like a LKO -> mun transfer that can be precisely calculated). It depends on your twr, drag, and ascent profile. A rocket with lower twr or a very draggy one will need more dv to reach orbit, while a streamlined high-twr rocket can get to orbit for less dv (I've seen as low as 2900 though I'm sure it can be less). Keep that in mind, but if you have a launch twr around 1.8 or so and didn't make a flying brick, 3400 is a decent target.

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there's not a "better" approach here, it depends on what you're optimizing for. You can often add more fuel for cheap to increase dv and reduce twr, which might be a better option than adding boosters

agile flume
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How can it be so hard ??😭

errant swallow
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well it's literally rocket science

halcyon prawn
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Now that I know that its this number and none of the others, I will be able to be more efficent

errant swallow
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draggy rockets often benefit from a low-twr, high-dv ascent

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easier to control and lower losses to aero drag. But it will cost extra dv

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also note that the ideal ascent turn depends highly on your twr. A low-twr rocket should turn late and slowly, while a high-twr one shoudl turn early and quickly

halcyon prawn
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There does not happen to exist a handy calculator online somewher does it?

errant swallow
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Calculator for what?