Flight Question Mars-Reentry: Next Steps?

[blane]

The Mars reentry gives me a bit of trouble. Not that I don't manage to come down, but it's mostly guesswork. I'm looking for the correct procedure.

The situation is as follows: I am sitting in a DGIV in a circular orbit at 200km. I am nicely aligned with Olympus, a close 3m approach in 1,5 orbits:

My basic plan would be:

- Wait until opposite of side of base.
- Quick deorbit burn that will set the Periapsis to base at about ground level (0k).
- Adjust for an initial 90° AoA, while descending use hover engines to smoothly break, lower AoA as necessary.

If timed correctly this should bring my horizontal speed to 0 at base, hovering slightly over base.

Does this seem like a sane approach?

 

[Cras]

I find you can land on Mars very much like you can on the Moon. The Atmosphere is not much of anything there, and you can just get close to the base, fire your engines to slow your speed, then fall down to the base, slowing your decent with the hover jets.

 

[blane]

Thanks Cras and yes, I noticed that on my first attempt (where I overshot the base by far). Which is why I intend to mostly ignore the atmosphere. I am just not sure at what Altitude/Distance I should start the brake maneuver (assuming an initial AoA of 90° and braking with hover engines only)

 

[Cras]

Almost when you are right on top of the base. I would recommend doing a quick save when you are about half way aside of the planet and then test and see. The DG has some massive main engines for its size and you will be able to stop over a fairly short distance. Get close to it, by watching MAP MFD, and then fire retro grade to the point where it shows your orbit will end at the base, or actually a little bit short of it.

Then level the horizon, then watch as you fall into the atmosphere. Then as you get close, keep the vertical speed no more than say 50 m/s, as then use the VTOL MFD to get over the pad, and land it.

From back when I first tried it, you need to be closer to the base than you think.

 

[blane]

Thanks a lot, okay, I will try this method. Regarding

Get close to it, by watching MAP MFD, and then fire retro grade to the point where it shows your orbit will end at the base, or actually a little bit short of it. Then level the horizon, then watch as you fall into the atmosphere.

what altitude would would I have when reaching 0 horizontal velocity?

 

[Pipcard]

I've seen past threads about Mars atmospheric entry, and they mostly give the same advice: Do as you would on the Moon. So, how does this work?

Tex uses the AerobrakeMFD for entry (no IMFD). I've been wondering how that works. (I can re-enter and land on Earth with the XR2 using the AerobrakeMFD)

The most crucial question is, how far away from the base do you do your de-orbit burn?

Notice how he does a few skips during entry. And how he magically cuts from 1600 m/s to a 40.5 m/s ground speed. Do you use the retro thrusters, too? Obviously, you have to turn on the hovers as well, but at what speed?

I want to do this kind of entry for my WIP video (using the XR2). It's more exciting than a lunar-style landing. And I don't want to use IMFD.

 

[Cras]

Aerobrake is an epic add-on no doubt.

It really gives you a very good idea where you are going to end up, and then adjust accordingly.

The atmosphere on Mars just really is weak sauce. With the DGIV, you probably have enough umph left in the engines to just stop your self and fall in.

If you are flying with more realistic settings with the DG or the XR2, then you may not have enough fuel.

My guess that burning half way across the planet would be over kill with the XR2, but that would be my starting point. Enter the base into Aerobrake MFD, you can use BasesyncMFD to give you a rough idea based on certain criteria you set up, such as angle and anticipation.

But don't stick to the Base Sync's suggestion for a de orbit burn time. Once Aerobrake says you will land close to the base, stop, adjust you attitude to your entry attitude, I would imagine you would need a fairly large AoA to get any sort of quality drag, but, adjust it until it shows you will hit the base close to home, can also adjust the bank angle to get even closer, then sit back and watch the show, making little adjustments as required.

 

[blane]

So, how does this work?

It's at least quite a different situation: Tex enters the atmosphere directly from the orbit injection; note how he hits the atmosphere with an OS of 4.7k. In my case I am sitting in a circular 200k orbit, which gives me an OS of 3.4k -- at that speed the Aerobrake maneuver is already finished.

 

[Pipcard]

It's at least quite a different situation: Tex enters the atmosphere directly from the orbit injection; note how he hits the atmosphere with an OS of 4.7k. In my case I am sitting in a circular 200k orbit, which gives me an OS of 3.4k -- at that speed the Aerobrake maneuver is already finished.

That's the aerocapture maneuver. When he's doing the actual entry (at around the 4:30 mark), his orbit is already (almost) circular, and his speed is about the same as yours. So, it's not a direct entry.

 

[blane]

Hm, sorry then, must have seen that wrong. There were quite some cuts, I will watch carefully again.

 

[Tommy]

It's not that hard to aerobrake on Mars. Sure, the atmo is thinner and you won't get quite as much drag - but your velocity isn't that high to begin with.

I de-orbit a little more than half an orbit before the base, and set the PeA to about 25k. With AerobrakeMFD, I aim about 100k "long". At about 1.5 km/s velocity I kill the AoA hold, engage the airbrakes, and prepare P200S7 (hover Autopilot) - but don't engage it yet. At just over 1 km/s you will find that the DGIV can't hold altitude from lift alone - so I engage the hover AP, adjust the descent rate to put the velocity indicator on the base, and use BurntimeCalcMFD to determine when to engage the Retro Engines. Just adjust the descent rate to keep the velo indicator on the base as you deccellerate and watch the VS at the end so you don't break the gear.

It's a bit harder than just using thrust - but saves a bit of fuel, and I always try to be efficient.

 

[Cras]

I am not infront of my Orbiter install, so I couldn't look myself, but Tommy, could you elaborate on how you use BurnTimeCalcMFD to time the retro Engine burn?

I am with you on always trying to be effiecient, and I often find that I become very wasteful with the fuel when timing the retro burn while in hover. either I slow down too much too early, or fly past and have to stop my forward momentum, and build it up again in the other direction to land on a pad.

 

[orbekler]

I made a pair of clips about it.

This is the easiest method:
[ame="http://vimeo.com/19964095"]http://vimeo.com/19964095[/ame]

With AeroBrake it's a bit trickier (with this I used Hover thrust to flare):
[ame="http://vimeo.com/25592340"]http://vimeo.com/25592340[/ame]

---------- Post added at 09:50 ---------- Previous post was at 09:34 ----------

...Tex uses the AerobrakeMFD for entry (no IMFD). I've been wondering how that works. (I can re-enter and land on Earth with the XR2 using the AerobrakeMFD)

At the end of clip, Jarmonik is credited for AeroBrake MFD, but it seems to me that he's not. It should be GP, an italian developer (no nationalism intended).

 

[blane]

Thanks, orbekler. That really helps as a guideline

Btw: What are these light effects with the XR-2 when you undocked?

 

[orbekler]

Thanks, orbekler. That really helps as a guideline

Btw: What are these light effects with the XR-2 when you undocked?

Just enable Visual effects->Local light sources. It should work for all other vessels as well.

 

[Pipcard]

Just enable Visual effects->Local light sources. It should work for all other vessels as well.

Also, you have to put MaxLights = 8 in Orbiter.cfg.

The atmosphere on Mars just really is weak sauce. With the DGIV, you probably have enough umph left in the engines to just stop your self and fall in.

If you are flying with more realistic settings with the DG or the XR2, then you may not have enough fuel.

But, my XR2 was docked to the Arrow during the Earth-Mars transfer, so fuel is not a problem.

 

[orb]

Also, you have to put MaxLights = 8 in Orbiter.cfg.

You don't have to do that, unless you want or need to set that value manually (e.g. when your graphics card doesn't support T&L).

 

[Napalm42]

Hm, from Tex's flight, I've never thought of circularizing with Mars' atmosphere, do'h:facepalm:

 

[Tommy]

I am not infront of my Orbiter install, so I couldn't look myself, but Tommy, could you elaborate on how you use BurnTimeCalcMFD to time the retro Engine burn?

I am with you on always trying to be effiecient, and I often find that I become very wasteful with the fuel when timing the retro burn while in hover. either I slow down too much too early, or fly past and have to stop my forward momentum, and build it up again in the other direction to land on a pad.

Well, I don't have Orbiter right now either - so I don't remember what button is what, etc. I'll try to walk you through it anyway. It's still not easy, or exact, but it seems to help me be more efficient.

One of the hard parts of landing on Mars (or any airless moon/planet) is knowing how far from base to start the retro burn. You've got to know how far you will travel during the retro burn - and BurnTime can help.

First, of course, comes the planning. I know that I'm going to run out of lift by the time I'm down to about 1 km/s. I know it will take a few seconds to get the descent AP and LevelHorizon going, so I'll probably be down to about 800 m/s by the time I'm ready to retrobrake.

After I de-orbit, I'll open Burntime. I'll set the engine to retros, and set the dV to 800 m/s. Burntime will show a distance (maybe called "range"?). I'll do the same for dV's of 700 m/s and 900 m/s - just to get an idea of braking times. I'll try to manage the re-entry to get my velocity down to 800 m/s at the right distance from target (which Burntime provided). The other two calcs will help me adjust if I'm off. I'll also run a calc using a dV of 100 m/s, and remember that for later.

So, lets say I make my target of 800 m/s at just the right distance. I'll engage the retros. Since I still have drag - I'll come up a bit short. I consider that a safety margin - It's better to be a bit short than to come up long.

I can fine tune near the end by coasting a bit, since I know how far I'll travel during that last 100 m/s of braking - and drag won't be much of a factor at that slow airspeed.

It's not the MOST efficient method, but it's good enough and fairly repeatable.

 

[blane]

Thanks for the hint with the light sources, didn't remember that.

Also, you have to put MaxLights = 8 in Orbiter.cfg.

I tried, but Orbiter.cfg gets re-written and the line removed everytime I load any scenario? (Although I read orb's post so it doesn't seem to matter, just wondering, though).

Also thanks Tommy, that sounds helpful. I will try this method on my next approach