Burn times

[hhatch]

 

I think this is right now.

[Starchaser]

5 hours ago, Snark said:

Actually, it is right. 

Egad.

Embarrassing.

My sleep hasn't been right. My wife was in the hospital thursday and friday. I claim fatigue!

2 hours ago, hhatch said:

Well I hope these pics make it. This looks wrong too. It takes you to Imgur. Launch 1 is the angle I've been using to launch. Launch 2 is the angle suggested in Scott Manley's video. Both angles were executed starting at 10,000 m. The Mun orbit shows me in mun orbit with a maneuver added to go to Kerbin. Scott Manley suggested placing the maneuver at a 45 degree angle. It appeared from the video that the angle was in relation to Kerbin. The Kerbin encounter is the orbit I created to get to Kerbin. Sorry for the mess I made

In the modern model you want to be at or around 45° at 10km, yes., but you want to keep lowering your angle as you climb. You should be close to horizontal by 50km or so. Like 10 to 15°

Let me post a couple screenshots of my own to answer your return to Kerbin question.

So, here's my system. Excuse the clutter.

 

I wound up, actually, using the outer station as my burn example as it's inclination was slightly less. Inclination will have a large efffect on the delta v needed.

So, I started with a node in about the same place you did, and as with yours, it took about 400 delta v for the return. I did, however, forget to grab a screenshot.

Oddly enough, when I dragged the node closer to where I thought the proper burn spot should be, it created a kerbin exit trajectory.

(In both of these two pics, it actually gave me an intersection with kerbin and I dialed back the delta v to give a visible periapsis. Even though I forgot to highlight it, this one was 83km And you can see, the burn needed was 250 delta-v

Sigh. And I saved the second image twice. Can't even work Paint today.

Anyway, the last one would have shown a node needing 191 delta-v with a periapsis of 84 km. The point I was trying to make is that the recommended starting point is something of a guideline. You should grab the node in the middle ring and slide it backwards and forwards on the orbit and see how it improves. Also. I proved multiple times that I'm an idiot!

  

[Snark]

7 hours ago, OhioBob said:

@Snark, that old constant scale height method gets you in the ballpark, but it can be in error by up to as much as about 19%.  The problem is that scale height is a function of temperature, so it varies by altitude.  It can change by as much as 2 km.

Yep, which is why I went with an actual "go measure it with the barometer" approach to verify. 

[Plusck]

13 hours ago, hhatch said:

Well I hope these pics make it. This looks wrong too. It takes you to Imgur. Launch 1 is the angle I've been using to launch. Launch 2 is the angle suggested in Scott Manley's video. Both angles were executed starting at 10,000 m. The Mun orbit shows me in mun orbit with a maneuver added to go to Kerbin. Scott Manley suggested placing the maneuver at a 45 degree angle. It appeared from the video that the angle was in relation to Kerbin. The Kerbin encounter is the orbit I created to get to Kerbin. Sorry for the mess I made.

12 hours ago, hhatch said:

 

I think this is right now.

 

OK, that works now

Well, you're right about the lower angle to get to orbit. With the current atmospheric model, you basically want to use as shallow an ascent angle as possible without burning up or running out of time to get your speed up.

However, with a ship like that, to be as efficient as possibe you should be initiating the turn off the launchpad. When you start building bigger and draggier vessels, you might go back to starting vertically for better control, but for now it is just costing you (not much... but still), viz.:

 

This is massively overpowered, so 65° off the launchpad works well. With a more reasonable vessel (or with a Swivel instead of a Reliant), 75-85° would probably be better.

It looks scary later but it's fine as long as it keeps rising:

The main worry is science instruments burning up, so you've got to make sure it is still rising quite quickly at the 30-35km altitude.

The other advantage of initiating your turn immediately is you don't need much control authority - the shape of the rocket keeps it flying straight, and "follow prograde" SAS doesn't actually have to do any work, meaning virtually no steering losses either.

 

As for the return burn from the Mun, the reason why your best-placed node is there is two-fold:

- you're already on an eliptic orbit, so if the return burn happens anywhere other than at Pe then it'll need to compensate for the current orbit. Your return burn actually has to happen at Ap, so it'll first raise Pe up from the Mun's surface before adding enough velocity to break out of the Mun's SOI. Therefore it is actually adding most velocity in exactly the true direction you want to go in.... if you see what I mean.

- since there is a reasonable amount of time before you reach that node, the Mun will have shifted along its orbit by the time you get there. Therefore the burn won't be quite as close to the midpoint between the Mun and Kerbin as it looks in your picture.

But whatever the reasons, your resulting orbit around Kerbin looks absolutely fine. It really doesn't look like you're doing anything wrong.

If you had been in a circular low orbit around the Mun, it would have looked like this:

Notice that Ap on my future orbit around Kerbin is very close to my current orbit around the Mun. By the time I start the burn (37 minutes later) future Ap will be right next to where my ship is. When I come out of the Mun's SOI, I'll actually be part of my way down from Ap towards Kerbin Pe.

Like this:

One of the hard things to do in KSP is visualise where you'll be in your various moving frames of reference. But your pics show that you're getting the right answers, so you're fine.

Edited November 12, 2017 by Plusck

[Kryxal]

Since you can drag a maneuver node, set up the node you THINK will send you where you want to go, and move it and watch your endpoint (in this case the Kerbin periapsis, which you can make constantly display by clicking).  The other alternative is to have your PE high enough that you don't plow into the surface, and wait for the Mun's orbit to take you to a better ejection point.