Question Transx - how to actually fly an eject burn?

[johan]

So, maybe I'm being mildly daft or maybe I'm just skipping over the right tutorial. Somebody please point me in the right direction?

Yesterday, I watched flytandem's Voyager slingshot** tutorial, then set it up myself. No problem there, it even felt like I (mostly) understood what I was doing.

Then I thought "hey, let's see if my setup is worth anything!" So I launched, got into a parking orbit - and realized that I have no clue how to now actually do the eject burn. So I played around a bit. One time I tried using maneuver mode and "copied" all the values for prograde, outward, plane change, and eject date, into the maneuver setup (this is how the Earth to Mars tutorial works)... pointed the nose and did the burn... only to find myself in some completely off-plane silly orbit that wasn't going to get me anywhere except orbiting around the sun. :facepalm:

What I expected to happen, is for TransX to allow this sequence:

1. Plan my journey
2. Launch using the heading calculated, to the height I planned
3. Use the plan I created on the surface to burn as appropriate to eject to the next stage

For the last step surely there must be some way to show me which way to point the nose and when to open the throttle.
Yet no matter what button I play with or which piece in the manual I read, this functionality does not seem to exist.

What good is it if I can plan a wonderfully complex trajectory and never have the means to execute it?

What am I missing?

**The example is for a slingshot, but the same problem exists for normal plans too. Unless it's a purely prograde burn, I don't see how TransX will get me to the next stage as per the plan.

 

[BruceJohnJennerLawso]

So, maybe I'm being mildly daft or maybe I'm just skipping over the right tutorial. Somebody please point me in the right direction?

Yesterday, I watched flytandem's Voyager slingshot** tutorial, then set it up myself. No problem there, it even felt like I (mostly) understood what I was doing.

Then I thought "hey, let's see if my setup is worth anything!" So I launched, got into a parking orbit - and realized that I have no clue how to now actually do the eject burn. So I played around a bit. One time I tried using maneuver mode and "copied" all the values for prograde, outward, plane change, and eject date, into the maneuver setup (this is how the Earth to Mars tutorial works)... pointed the nose and did the burn... only to find myself in some completely off-plane silly orbit that wasn't going to get me anywhere except orbiting around the sun. :facepalm:

What I expected to happen, is for TransX to allow this sequence:

1. Plan my journey
2. Launch using the heading calculated, to the height I planned
3. Use the plan I created on the surface to burn as appropriate to eject to the next stage

For the last step surely there must be some way to show me which way to point the nose and when to open the throttle.
Yet no matter what button I play with or which piece in the manual I read, this functionality does not seem to exist.

What good is it if I can plan a wonderfully complex trajectory and never have the means to execute it?

What am I missing?

**The example is for a slingshot, but the same problem exists for normal plans too. Unless it's a purely prograde burn, I don't see how TransX will get me to the next stage as per the plan.

TransX isnt really planning software. For that sort of thing you'll need IMFD by jarmonik, whose website I can get for you if you like. To do an interplanetary eject burn, the easiest way to do it is to set the sim date to a launch window for whatever your target is, then open transx in 2 windows. Adjust one of these so that it displays your trajectory relative to the sun and your intended target (target eject-next view-target your destination). Open the maneuver mode in the other mfd, and add about 3900 m/s of prograde delta-vee. As you swing the ejection direction around with the date for the maneuver, you will see the green ellipse expand in size or decrease, based on whether your orientation for the maneuver is prograde or retrograde (respectively). Line up the trajectory so that the ellipse high/low point matches that of your target orbit (in blue), and that should get you reasonably close. An MCC in deep space (at low speeds) can be used to correct any drift and get you right to your target.

 

[statickid]

To be clear, you are not talking about pressing the view button until you see the attitude target with ejection burn time?

---------- Post added at 08:44 AM ---------- Previous post was at 08:41 AM ----------

When I use TransX, it switches to the next stage in the plan after some time has elapsed from the original burn. Don't know when exactly, maybe it is based on SOI of source planet.

 

[BruceJohnJennerLawso]

To be clear, you are not talking about pressing the view button until you see the attitude target with ejection burn time?

---------- Post added at 08:44 AM ---------- Previous post was at 08:41 AM ----------

When I use TransX, it switches to the next stage in the plan after some time has elapsed from the original burn. Don't know when exactly, maybe it is based on SOI of source planet.

I find it easiest to just use the maneuver mode to get things done in TransX, although it tends to be a bit rough around the edges in terms of delta-vee efficiency. I think the switch is based on SOI change, but I usually dont fret too much about it. If you have your trajectory arriving in the general area of your target when you eject, MCCs can easily get you right on the bullseye when you arrive.

 

[dgatsoulis]

A couple of weeks ago, blixel and I were discussing about when is the best time and how to perform an MCC for an Earth to Mars journey. Blixel sent me a video on how he did it and I sent him the video you see below as a response. It also covered the TMI burn, so it is relevant to the question here.

[ame="http://www.youtube.com/watch?v=FFQYTPyag-E"]DG to Mars (TransX and IMFD) - YouTube[/ame]

David has made and will upload (AFAIK) a much better video than this, covering all the ways a "typical" orbinaut performs an ejection burn with TransX and which way is the best.

 

[blixel]

So I launched, got into a parking orbit - and realized that I have no clue how to now actually do the eject burn.

Interesting timing on this question. I was just discussing this with dgatsoulis. I recorded a video a couple of weeks ago explaining the usual methods for handling the Eject burns using TransX. I try to upload my videos on a schedule, but since this question came up here, I'll go ahead and make it available now in hopes that it will help you.

 

[johan]

...the video you see below as a response. It also covered the TMI burn, so it is relevant to the question here.

Thank you! This answers a crucial question for me: "how do others do it?"

So, what happens if, for whatever reason, the orbit I start in isn't well aligned with Mars? So when I do the TMI burn, I have to have some plane ch.....

...wait...

You're going to tell me to fix that at the MCC burn, right? And to make sure that my starting orbit is at least somewhat aligned with where I'm going (say, less than 10 degrees RInc)

Right?

 

[dgatsoulis]

Thank you! This answers a crucial question for me: "how do others do it?"

So, what happens if, for whatever reason, the orbit I start in isn't well aligned with Mars? So when I do the TMI burn, I have to have some plane ch.....

...wait...

You're going to tell me to fix that at the MCC burn, right? And to make sure that my starting orbit is at least somewhat aligned with where I'm going (say, less than 10 degrees RInc)

Right?

It's pretty hard to answer that without knowing exactly what you are doing. Are you setting up the ejection burn as a maneuver, are you simply burning prograde when the Time to burn reaches 0, are you doing something else?

In TransX, an eject burn with 10° of R.Inc (even with 1°) is not a good eject burn. You will be way off course. There is nothing that MCCs can't fix, but if you make an accurate eject burn, you will need a lot less ΔV for corrections.

Now, let's say that you somehow managed to mess up the launch and you ended up in an orbit with a lot of R.Inc to the ejection trajectory. The first thing you need to do is to adjust the eject orientation variable until the R.Inc gets to a minimum. If it's more that 0.5° then don't fly that ejection burn.

There are many ways that you can "save" a bad launch and still make the ejection burn with minimum (extra) cost in ΔV but it would take a very long post to cover them.
The most usual is to change the eject orientation until the white line passes over the periapsis of the ejection trajectory.(without caring about the R.Inc).
Then make a burn at periapsis, going to a high ApA (~2/3 to the Moon's alt). At apoapsis make the plane change to get the R.Inc to 0 and fall back to periapsis. Once there, complete the burn.

I've been meaning to make a video explaining what is exactly going on during a "planet to planet" TransX plan, so I think I'll start making it.

 

[johan]

It's pretty hard to answer that without knowing exactly what you are doing.

Well, you guessed right - I messed up my launch (probably because I'm misunderstanding something about how TransX works in that phase), and now I'm in earth orbit trying to launch to (say) Mars, and when the time comes to burn, I turn pro-grade using the autopilot, hit the throttle, and keep the pro-grade autopilot on all the way.

Of course, depending on how badly messed up the RInc is... well... lets just say it didn't end well for the crew, poor sods.

I've been meaning to make a video explaining what is exactly going on during a "planet to planet" TransX plan, so I think I'll start making it.

I think this would help explain a lot to me! One thing especially sticks out like a sore thumb on e.g. flytandem's tutorials, I can see him adjusting the eject orientation during launch so it lines up with his position on the planet. However, apparently I have a different version of TransX, since in that view, for me, the planet's sphere is shaded white, so I can't see the green line representing my "orbit" while I'm sitting on the surface.

---------- Post added at 21:35 ---------- Previous post was at 21:30 ----------

Interesting timing on this question....since this question came up here, I'll go ahead and make it available now in hopes that it will help you.

Great timing as far as I'm concerned! Thank you sir, this has helped me a lot - perhaps predictably, I like the simplest method that produces fairly accurate results, best: turning prograde on, then turning it off again very early on and letting the natural left yaw take its course. I might help it just a touch, but not as actively as I saw you doing - simpler is better as far as I'm concerned.

Of course, now I have to figure out how to fix my launches!

 

[dgatsoulis]

One thing especially sticks out like a sore thumb on e.g. flytandem's tutorials, I can see him adjusting the eject orientation during launch so it lines up with his position on the planet. However, apparently I have a different version of TransX, since in that view, for me, the planet's sphere is shaded white, so I can't see the green line representing my "orbit" while I'm sitting on the surface.

Ah yes the white sphere bug. There are at least two versions of TransX that fix that. One is the 3.13.1.2 version which you can get here

The one I'm using is atomicdryad's which you can get from here. It also has an "ENT" button, where you can enter the value you want for a variable.

 

[blixel]

Of course, now I have to figure out how to fix my launches!

I would have to see your launch to know what's wrong with it. Without seeing it, my guess is that you are not setting up your Eject orientation before taking off ... or you aren't doing it quite right ... or you are setting it up right, but then you aren't flying the right heading when you take off.

I have a whole Earth to Mars video series (2 actually) on my youtube channel. In those videos I show how to set up the Eject orientation while you're still on the ground. You can browse my youtube channel to find those videos, or send me a private message on here and I'll send you the direct link.

 

[statickid]

Well, you guessed right - I messed up my launch (probably because I'm misunderstanding something about how TransX works in that phase), and now I'm in earth orbit trying to launch to (say) Mars, and when the time comes to burn, I turn pro-grade using the autopilot, hit the throttle, and keep the pro-grade autopilot on all the way.

Of course, depending on how badly messed up the RInc is... well... lets just say it didn't end well for the crew, poor sods.



I think this would help explain a lot to me! One thing especially sticks out like a sore thumb on e.g. flytandem's tutorials, I can see him adjusting the eject orientation during launch so it lines up with his position on the planet. However, apparently I have a different version of TransX, since in that view, for me, the planet's sphere is shaded white, so I can't see the green line representing my "orbit" while I'm sitting on the surface.

---------- Post added at 21:35 ---------- Previous post was at 21:30 ----------



Great timing as far as I'm concerned! Thank you sir, this has helped me a lot - perhaps predictably, I like the simplest method that produces fairly accurate results, best: turning prograde on, then turning it off again very early on and letting the natural left yaw take its course. I might help it just a touch, but not as actively as I saw you doing - simpler is better as far as I'm concerned.

Of course, now I have to figure out how to fix my launches!

This is what i'm talking about, you say you are using the prograde autopilot, i'm assuming you mean the button on the bottom of the screen.

Why aren't you using the attitude target view of TransX? It is much more accurate, not to mention, there is no other way to actually USE the parameters you set such as 5 DV outward and 200 DV of plane change.

 

[johan]

This is what i'm talking about, you say you are using the prograde autopilot, i'm assuming you mean the button on the bottom of the screen.

Yes, that's exactly what I mean.

Why aren't you using the attitude target view of TransX? It is much more accurate, not to mention, there is no other way to actually USE the parameters you set such as 5 DV outward and 200 DV of plane change.

I wish I knew how!! This would intuitively make most sense to me... but I can't figure out how to make TransX show me that view, except when I'm doing a Manoeuvre. A Manoeuvre isn't what I want to do, I want to use the plan variables I spent several minutes setting up!

---------- Post added at 22:28 ---------- Previous post was at 22:25 ----------

Ah yes the white sphere bug. There are at least two versions of TransX that fix that. One is the 3.13.1.2 version which you can get here

The one I'm using is atomicdryad's which you can get from here. It also has an "ENT" button, where you can enter the value you want for a variable.

Thankyou! :tiphat:

I'm getting the one with the ent button.

 

[Weirdo Earthtorch]

+1 thumbs up for David's / Blixel's videos. I've learnt more about TransX from those videos than from any others. His 'learn with me' series and 'man on the street' style make it very easy to follow what's going on. Highly recommended.

Cheers WE.

 

[blixel]

+1 thumbs up for David's / Blixel's videos. I've learnt more about TransX from those videos than from any others. His 'learn with me' series and 'man on the street' style make it very easy to follow what's going on. Highly recommended.

Thank you for the encouraging feedback. It is very much appreciated.

 

[ADSWNJ]

I wish I knew how!! This would intuitively make most sense to me... but I can't figure out how to make TransX show me that view, except when I'm doing a Manoeuvre. A Manoeuvre isn't what I want to do, I want to use the plan variables I spent several minutes setting up!

Hi Johan,

Here's how I think about it. Your work on the ground is very important, to get you to launch into an accurate parking orbit, ready for a prograde burn to your destination. (p.s. & note ... you burn prograde to escape Earth's gravity, on the dark side if you are going to Mars or beyond (Prograde to Earth, Prograde to Sun), and on the sunny side if going to Venus or Mercury (Prograde to Earth, Retrograde to Sun))

I like to think that the hard work getting the plane and outward velocity right is "burnt" into the parking orbit, so essentially a prograde burn at the precise point will get you approximately in the right escape course to Mars.

Setting up for the launch, you want to play with the launch time and the eject orientation to get as close to a 90º launch as possible. Make a note of the launch azimuth and get on that bearing as soon as possible. On the ride up to orbit, continually have a gentle bank to keep driving the RInc towards zero. The white line gets more and more sensitive until it flips to 90º perpendicular to your ship (i.e. like the Align Planes end of burn RInc 0.00 position). From here, tiny adjustments up to your parking altitude, and you should be well under 1º of RInc error.

Back to the eject ... we COULD continue just to burn prograde at the eject time. HOWEVER ... we want to be much more precise than that, so hence we use a maneuver burn. Have both MFD's up on TransX, with one doing the maneuver parameters, and the other showing the plan (until we kill it and then it becomes the maneuver outcome). Set up the eject date to match the plan, and add prograde to match the plan's eject delta V. You should have a hypothetical burn that looks similar shape to the plan, but it will be rotated say 30-40 degrees off. Correct this orientation with a tiny adjustment to the escape time, to lay the maneuver right on top of the plan.

At this point, flip to the other MFD (in escape plan, NOT maneuver mode), and reset all the variables. This kills the plan (which was prioritizing over the maneuver) and then the maneuver will be shown. You can now go back to your maneuver planing and keep tweaking it to get your min distance to where you want it (e.g. 20Mm). You should find that if your parking orbit was cool, you don't need tons of outwards or plane change.

Finally - you can switch view to maneuver burn mode, align it and burn it. (Try to burn to a difference of xxxE-05, rather than leaving say 0.1 or 0.001 of error).

Have a play with this and see how you get on.

-ADSWNJ

---------- Post added at 02:59 AM ---------- Previous post was at 02:30 AM ----------

By the way ... check out this old thread: http://www.orbiter-forum.com/showthread.php?t=27739

 

[johan]

By the way ... check out this old thread: http://www.orbiter-forum.com/showthread.php?t=27739

:tiphat:

This is exactly my next set of questions! I figure I get the eject part of it now... the reason I'm still having trouble is because my parking orbit is screwed. The reason my parking orbit is screwed, is because I don't understand what I'm doing when I'm sitting on the runway, setting up my launch. I'll have a read through there.

---------- Post added 31st Jan 2013 at 00:15 ---------- Previous post was 30th Jan 2013 at 23:44 ----------

So THAT is how it works? Geez... that thread should be published as part of Orbiter!

 

[ADSWNJ]

So THAT is how it works? Geez... that thread should be published as part of Orbiter!

Yeah - there was a TON of cool info in there. I loved the pitcher's analogy too. Many people will just revert to IMFD because it has more bells and whistles, but I'm attracted to TransX simply because you have to work harder to understand it!

 

[johan]

Yeah - there was a TON of cool info in there. I loved the pitcher's analogy too. Many people will just revert to IMFD because it has more bells and whistles, but I'm attracted to TransX simply because you have to work harder to understand it!

Yes, IMFD does not force you to understand what you're doing. I know because I've made many successful interplanetary trips using IMFD, and this only just clicked for me.