Flight Question Need help doing a Direct Reentry from the Moon

[Krishnan]

Hey y'all!
So I have been learning IMFD, as you all know, and today I tried getting back from the moon for the first time!
I am all aligned up with Wideawake and ready to reenter, but I either do some kind of flat-spin into the earth, or bounce off and escape the solar system, as if time warp is enabled (it isn't)
I am holding inverted 40 degree pitch, when speed goes below 7k, I fly rightside up again and do a normal reentry (Which I have done several times) with Aerobrake MFD to Wideawake International.
However, I keep on doing this deadly dive towards the earth, can't hold AoA either.
Could someone please help?
Here is my Scenario File (Needs wideawake int installed).
Thanks so much! Orbiter is very fun, flew to the moon several times, but still haven't got back from there

 

[Urwumpe]

However, I keep on doing this deadly dive towards the earth, can't hold AoA either.

Thats normal for a vanilla Deltaglider, the aerodynamic forces overpower your controls to maintain high AOA.

 

[Arvil]

Even AttitudeMFD has trouble maintaining programmed attitude during reentry.

 

[Krishnan]

Ahh, okay. I can try with the XR2 then?

Just to clarify, how do I get back from the moon using IMFD?

Use Base Approach to plan trajectory, surface launch to get into Orbit using data from base approach, and then use orbit eject using data from base approach?

I was kinda confused because Map Program showed negative periapsis. Dimitris in the tutorial told to use Delta Velocity Program and Map Program for a single, precise burn, but I don't have the time to fiddle around with the variables, and I don't mind making Mid Course Corrections. (He said the same thing with TLI burn.)

IMFD is a great tool, it isn't actually difficult now I think about it, but I have trouble like this when I am practically using it.

Thanks!

 

[Krishnan]

ARGGH! Why does it have to be so difficult? I spent about an hour trying to tweak the variables but I can't get a good ejection burn. I tried adjusting all the variables, bu everything keeps on changng. Longitude, angle, periapsis all change. Please help!! In the videos they did it very easily.

 

[Krishnan]

Is there some specific order that I have to adjust the variables in? It is quite irritating when you spend an hour tweaking variables, Longitude and Periapsis are acceptable, but Angle is bad.

 

[Urwumpe]

Is there some specific order that I have to adjust the variables in? It is quite irritating when you spend an hour tweaking variables, Longitude and Periapsis are acceptable, but Angle is bad.

Well, maybe it helps understanding what you are actually calculating and why it is not possible to "always get the menu you ordered."

You are in a orbit around the moon. The moon revolves around Earth, but not in the equatorial plane. Your ground track forms a sine wave.

You want to fly back to Earth in a direct reentry, which means you want to leave the lunar orbit in a specific hyperbolic orbit. (Luckily there are infinite many such hyperbolic orbits all ending in the right Earth-centric orbit back home, all forming a locus (a virtual circle from which the orbits leave) around the earth pointing side of the moon (Since you want to lower your orbit from the lunar orbit back to Earth). Since you are in one specific orbit, you will ALWAYS find a new escape orbit for each possible earth return trajectory.

And you want to reenter at a specific location on a rotating Earth then. Which again brings us to a locus, the infinite possible options and still just one such option for your current orbit.

Now reverse the order - go from the rotating landing site back to your lunar orbit. Depending on where you want to land, you have to find a specific transfer orbit that reaches your moon orbit at the departure. Everytime your landing parameters change (time, place), you will get a completely new transfer orbit.

If you get a completely new transfer orbit, your way how you need to leave the moon will change, since you are NEVER at the right place for the burn to leave moon on the optimal return trajectory, you will have to pick a return orbit that fits to your position on your lunar orbit at the time of TEI.

As you see, a small change in one of your variables for the escape burn, will mean, that all other variables also have to change to compensate.

IMFD will do most of the work for you, but you have to explain your problem properly and accept that everything it calculates for you is a compromise, a solution to satisfy many variables - and not every solution is optimal.

 

[Arvil]

Meanwhile, while you spent an hour changing variables, your landing location has moved 15 degrees. I’ve done that, too. :sigh

 

[Urwumpe]

Meanwhile, while you spent an hour changing variables, your landing location has moved 15 degrees. I’ve done that, too. :sigh

Shouldn't be a problem as long as your ejection burn starts in the future....

 

[Krishnan]

So I tried the scenario again, came up with this solution. Can I compensate the 16 degrees angle by banking in the atmosphere? I feel like I am spending too much delta v on this burn.

 

[Urwumpe]

View attachment 25856
So I tried the scenario again, came up with this solution. Can I compensate the 16 degrees angle by banking in the atmosphere? I feel like I am spending too much delta v on this burn.

No. Put emphasis on a good entry interface (since you usually have only one chance in your simulated life to do it right) and if you can't get it, delay your return by one orbit and look for a solution about 2 hours later.

With a XR, I would recommend you to really stay between 1.0° and 1.5°, since this CAN overheat and does not like high heat flux - Apollo used 6°.

EDIT: For better explanation there why: The entry angle defines how deep you will dive into the atmosphere even with full positive lift. So, if you dive too deep initially, your heat shield will overheat even while are slowly climbing out of the denser atmosphere again. If you are not diving deep enough, even full negative lift (lift force points towards Earth) will mean that you can't stay in the atmosphere and will skip out uncontrolled. Between the two extremes is the sweet spot where you want to be: Dive deep enough into the atmosphere to have as much control over your trajectory as possible - and stay high enough to prevent overheating.

Yes, it can happen that you can't prevent melting and skipping out at the same time. If you have more excess velocity (the furthest point of the blue orbit is further beyond the moon or is even in infinity), I would recommend you to try a skip reentry. In this case, you are not even attempting to stay inside the atmosphere, you are just trying to delay skipping out long enough to have a second chance for reentry soon.

We have no precise guidance for that outside NASSP, but you should have enough DV left in the XR to move your second entry interface to the right spot for landing - or get into a minimalist parking orbit around Earth.

 

[Krishnan]

No. Put emphasis on a good entry interface (since you usually have only one chance in your simulated life to do it right) and if you can't get it, delay your return by one orbit and look for a solution about 2 hours later.

With a XR, I would recommend you to really stay between 1.0° and 1.5°, since this CAN overheat and does not like high heat flux - Apollo used 6°.

EDIT: For better explanation there why: The entry angle defines how deep you will dive into the atmosphere even with full positive lift. So, if you dive too deep initially, your heat shield will overheat even while are slowly climbing out of the denser atmosphere again. If you are not diving deep enough, even full negative lift (lift force points towards Earth) will mean that you can't stay in the atmosphere and will skip out uncontrolled. Between the two extremes is the sweet spot where you want to be: Dive deep enough into the atmosphere to have as much control over your trajectory as possible - and stay high enough to prevent overheating.

Yes, it can happen that you can't prevent melting and skipping out at the same time. If you have more excess velocity (the furthest point of the blue orbit is further beyond the moon or is even in infinity), I would recommend you to try a skip reentry. In this case, you are not even attempting to stay inside the atmosphere, you are just trying to delay skipping out long enough to have a second chance for reentry soon.

We have no precise guidance for that outside NASSP, but you should have enough DV left in the XR to move your second entry interface to the right spot for landing - or get into a minimalist parking orbit around Earth.

By the angle, what do you mean? Is there a way to view that angle in Map Program? I think the angle displayed in the map program is angle to base, not the angle you are talking about.

Thinking about what you said, I guess I shouldn't deal with this direct reentry thing at all. It's possible and fun, but too complicated. Not worth the trouble.

I should just brake into a parking orbit, use basesync, deorbit, reenter, and land.

 

[Krishnan]

I tried to do the reentry anyways. Kinda messed up with the inverted AoA, overheated. Then I realized that I was coming in basically retrograde, so my plan itself became useless, because I had to do a full orbit around to get to Wideawake.

Better just using main engines I guess. Or is there a way to plan an aerobrake?

 

[Urwumpe]

By the angle, what do you mean? Is there a way to view that angle in Map Program? I think the angle displayed in the map program is angle to base, not the angle you are talking about.

Thinking about what you said, I guess I shouldn't deal with this direct reentry thing at all. It's possible and fun, but too complicated. Not worth the trouble.

I should just brake into a parking orbit, use basesync, deorbit, reenter, and land.

I remember there was a program that displayed the entry interface angle in the plan. I am sure it was IMFD, since I used that quite often to plan entries.

 

[Ripley]

I remember there was a program that displayed the entry interface angle in the plan. I am sure it was IMFD, since I used that quite often to plan entries.

Wasn't it AerobrakeMFD?

 

[Urwumpe]

Wasn't it AerobrakeMFD?

I am not sure, I didn't use it very often, since I was more a fan of ReentryMFD.... but it was a while since I used those.

 

[Krishnan]

Hey y'all!

So... I managed to set up a rough ejection plan and used base approach to do some mid-course corrections. The problem is that base approach kept on messing up my apoapsis, making it lower than what I selected (65 km). Is this normal? I am in Earth's gravitational field, g-metre reads 1.00, so it shouldn't be a problem, right?

Anyways, I did a manual outward burn to raise the periapsis to 65k. Other variables seemed fine. About reentry.....

I basically used surface MFD to hold inverted pitch, varying the AoA to make sure VACC and VS is at a good rate so that I won't burn up in the atmosphere.

When I reached about 7700 m/s, I couldn't control these variables using AoA, I had to roll over and perform a normal reentry using aerobrake.

I used 0.1x timewarp to roll over using aoa, but the hull overheated and I burnt up in the atmosphere.

Where did I go wrong? Was I supposed to roll over earlier? Advice about reentry and the answer to Baseapproach problem would be greatly appreciated.

Thanks!

 

[Urwumpe]

Maybe you should try a higher angle distance for the approach, with a DGXR. Try about 70°-100°.

If provide me your initial scenario before the transearth injection, I can maybe check which numbers work out or if you should delay the return for arriving at the base. I don't have most add-ons you use installed yet in my sandbox, but I can fix this.

 

[Krishnan]

Maybe you should try a higher angle distance for the approach, with a DGXR. Try about 70°-100°.

If provide me your initial scenario before the transearth injection, I can maybe check which numbers work out or if you should delay the return for arriving at the base. I don't have most add-ons you use installed yet in my sandbox, but I can fix this.

Ahh, I figured out the cause of the overheating. I had to do a manual roll-over while trying to not expose the top of the hull to the reentry heating. I managed to do a perfectly good reentry and landing. 45 degrees anticipation angle wasn't enough, causing a high deacceleration and heat near the base. I had to do a 90 degree anticipation angle, which resulted in a reentry with max g force of only 1.5gs. Nice

I am still confused about base approach taking me to a lower periapsis altitude than I wanted. Even with gravitation influence at 1.00, it still kept on lowering my periapsis lower than I needed to. I had to do a manual correction using RCS using map program to fine-tune the altitude. Reentry angle and anticipation angle were fine, it's just the periapsis altitude that Baseapproach messed up. I have seen in a Vimeo video here (Orbiter 2010: Direct / Inverted reentry - XR-2 Moon to Earth on Vimeo ) that the pilot sets the altitude to all the way 145 km for compensation. Is Baseapproach having the same "innacuracy" described with Planet Approach? Setting the periapsis higher than needed?


IMFD is a great tool , really love learning about it. Now, it's time for mars !

Thanks!

 

[Krishnan]

Is it possible because I changed the Anticipation angle mid-flight?