Flight Question Moon - Earth: Transfer & direct entry

[kirog]

Ahoy!

This is my first post, and I have several questions regarding direct entry apollo-style, which I am completely and utterly unable to do without missing terribly. I've tried and tried, and of course the problem is the lack of understanding the basic physics behind the most important factor, timing.

The problem is that the only Tutorial out there seems to be Tommy's IMFD tutorial: it's great and it works on this specific scenario, but my problem is that I can't derive any general conclusions because it's like "press this button and now press that one, and you're fine" instead of presenting a common solution.. when I want to do my own missions, from launch to splashdown, this tutorial doesn't really help (I don't want to offend the author though!)

Imagine it like this: I want to do a simple earth to moon transfer, do a LOI, and a subsequent TEI in order to perform a direct entry and land in the vicinity of Wideawake with a capsule.

I can do the TLI and orbit the moon, not a problem.
Problems emerge when I want to find out how to do the correct TEI because:

1) PlanetApporach Program (IMFD) lets you select your inclination (Eq.) upon arrival. hmm.. is this.. important? Ah well let's just do that TEI anyways.. but:

2) Then I open BaseApproach and this is where the chaos starts. I think I don't have to tell you that this method of "let's just do a TEI without a plan and try to correct things later" is an awful, dirty approach to the problem.... sometimes it works, but, well.

3) LunartransferMFD seems to be a great tool, extremely high accuracy, but its TEI or re-entry options don't allow selection of LAT/LON or a specific base, so I guess the whole direct entry-thing is entirely timing-based and can be done with any MFD if one understands the basics correctly?

Here is the question:

Apparently getting the MAGIC NUMBER (GET) is vital. This makes perfect sense. But I think the method of GETTING the correct magic number needs better explanation, or I might need a brain transplant so ah.. well.. It would be great if you could give some hints or tips so I can understand what I should be doing.. I'd rather be able to do a direct entry with LunarTransfer MFD instead of IMFD because the latter is too advanced and intelligent to help a newbie understand what he is doing

Many thanks in advance!!

 

[agentgonzo]

A direct reentry is a complicated thing. You're going from lunar orbit into Earth's orbit and you want to approach so that you're coming in at exactly the right angle to not miss and also to not burn up if you come down too steeply. Add to this the fact that the Earth is turning below you and you need to get all the times lined up so that you not only don't die, but you also land when the correct part of the Earth is below your flight path (target landing zone).

My advice: Get used to some of the simpler things in orbiter (intercept and dock with the ISS, launch from the Earth to the moon and land) first. This will give you a much greater understanding of what's happening so that when you do attempt the direct reentry, you know what it all means (you'll still die the first time thougH)

 

[kirog]

The thing is, I can do TLI, TEI with earth orbit insertion, go to ISS, and re-entry from LEO is no longer a problem (capsules and winged alike, without thrust), so I really want to understand the direct entry thing it's my personal nightmare, and my personal challenge.. but I understand that this topic is not an easy one and it'll be hard and lenghty to explain things in a reasonable way...

 

[Tommy]

The problem is that the only Tutorial out there seems to be Tommy's IMFD tutorial: it's great and it works on this specific scenario, but my problem is that I can't derive any general conclusions because it's like "press this button and now press that one, and you're fine" instead of presenting a common solution.. when I want to do my own missions, from launch to splashdown, this tutorial doesn't really help (I don't want to offend the author though!)

No offense taken. I admit that the "Magic Number" is confusing. I hate using them in tutorials, but that tutorial was focused on the inverted re-entry technique - the IMFD part was sort of an afterthought so it didn't really get the attention it should have.

I'll try to explain the "Magic Number" a bit better. The whole idea is to leave the Moon at the right time so that you enter the atmosphere at the right place. Since the Earth is rotating, timing is everything. In a capsule, even more so since you have so little range (a glider can travel for a long ways in the atmo - but a capsule, well, not so much!).

BTW, the "Magic Number" is explained in some detail in the PDF. That method, while quite crude, will work in other scenarios - but you have to do the whole re-start the sim thing every time.

There is an easier way, but it doesn't always work. When it doesn't work you just have to wait a bit, perhaps try to set up again on the next (lunar) orbit, or a couple hours, etc.

Base Approach, in "Re-Entry" mode, can perform this flight. You will need to enter in the Latitude and Longitude of the desired landing area. Then set the ReA (re-entry angle - about 6.2 degrees for a capsule). Set Alt (re-entry interface altitude), the default should be 120k, anything from there down to 80k will work - just be consistent.

Set the Ant (re-entry anticipation - how far (in radial degrees) you will travel AFTER you cross the re-entry interface). Not sure what's best here - a little trial and error should help you figure it out - if you overshoot reduce the Ant, if you come up short increase the Ant. This will vary with the vessel, and even with different pilots (especially in winged vessels) so you'll have to experiment a bit.

Set the TEj - set at least 6400 to give you time to get into orbit. Next, increase the HInt. This is basically the time of flight. If IMFD can't find a solution, increase the HInt until it does, then tweak it a bit for lowest dV. You can alternately adjust TEj and HInt to find the lowest dV if you have the patience for it. Surface Launch can be used to enter a suitable orbit, then Orbit Eject is used to the TEI (don't forget to set them to use Base Approach, not Course). MCC's are made with Base Approach.

It can be tricky to get an efficient plan using this method - don't be afraid to to experiment with the TEj and HInt. The secret to efficiency is getting the TEj optimized more than the HInt.

 

[kirog]

Aye, thanks Tommy!

Yes, the BaseApproach Program did the trick. Looking at the basics of this program, I now begin understand what it does. Of course this is one of the mejor problems in Orbiter, you don't have a team of scientists calculating the whole mission on the blackboard before you even set foot in your capsule.. your Method of getting the Magic Number is graceful, and using the BaseApproach as you explained it helps a lot in understanding what's happening there...

 

[Tommy]

your Method of getting the Magic Number is graceful

"graceful" is not the word I'd use! It's crude. But it works, and I guess that's what counts.

BaseApproach is a better way almost all of the time, but can be a little more confusing to set up at first. Once you get it, it's much easier and doesn't require the "test run" and "time travel" that the "Magic Number" uses. I wish I had understood Base Approach when I made that tutorial!

Thanks for the kind words, and glad I could help.

 

[RAF Blackace]

I have an additional question. I have the same problem as Kirog so I will try the base approach method. But does this method allow you to check what time of day on Earth the reentry will be at ?

Its always nice to land in the daytime and I always seem to re enter at night in AMSO.

 

[Tommy]

You'll have to calculate the time of day (in UT) yourself. You can figure out when you'll be landing either from TIn, or PeT (which would be shown in IMFD Map, in seconds. You'll need to convert that to days, hours, minutes, etc, and add it to the launch time). That will allow you to figure out if it will be "daytime" where and when you land.

Incidently, I've been using a different approach to the Moon-Earth direct re-entry. I now use LTMFD for planning, launch heading and TEI burn, then use IMFD's Delta Velocity for a MCC/base alignment burn just after entering the Earth's SOI. Results have been quite good - more efficient and better accuracy. Still a bit of refinement until I've got is down to as simple a process as possible - and there's still more of an alignment burn than I'd like (that seems to depend entirely on the Moons latitude when you leave).

If I get that down to a repeatable process, I'll try to put out a "Home Direct V 2.0", but it won't be anytime soon. At least with this method there's no need for a "magic number".

 

[orbekler]

...At least with this method there's no need for a "magic number".

Question: is "Magic Number" required to have correct timing just for "Direct Reentry", not for "Orbit Insert" right?

 

[Tommy]

Question: is "Magic Number" required to have correct timing just for "Direct Reentry", not for "Orbit Insert" right?

Correct, for an Orbit Insert at LEO, there is no "magic number", which is only needed to determing the time of arrival so you can enter the correct distance from the target.

For an insert, just ensure that your earth inclination will be high enough - or use IMFD's Base Approach program set to Insert - you can adjust how many full orbits you want to make before landing. This can help lower dV by letting the planets rotation aid alignment.