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Flight Question XR2 Aerobrake at Earth coming from the Moon

Cras

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Is it at all possible to aerobrake the XR2 at Earth after returning from the Moon, without having to fly the XR2 inverted?

I have done the inverted direct re-entry with the XR2 to a successful landing, but it is not something I like to do. Mainly because looking at it, I don't think the vehicle should be able to survive an inverted re-entry. The heatshield is after all on the ship's belly.

I was hoping to find a procedure using Aerobrake MFD to instead lower my perigee into the Earth atmosphere and then aerobrake to lower the apogee to something a bit more manageable, then lower it some more via other aerobraking passes until I am eventually in LEO. But I am finding it very hard to do. I have done some succesful aerobraking from MEO down to LEO, but coming in with 10+KM/s is proving to be a bit more difficult.

So if any other Orbinauts out there can pass along some tips on how to do this without destroying the vehicle and crew, it would be much appreciated.

And of course, :hailprobe:
 

boogabooga

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I've done this before. I used Lunar Transfer MFD. I ran a TEI with perigee set to 60 (ish) km. Then on return, I used the XR2 hold attitude for something like an AOA of 50-55. That burned off about 800m/s which will send you back to MEO. At Mach 36, you don't want to get much lower than 60 km, or else you will fry.
 

orbekler

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...I have done the inverted direct re-entry with the XR2 to a successful landing...

You did actually the most difficult kind of reentry, it shouldn't be very difficult to go the classical way.
However, there's a plenty of tutorials and clips.

Some clip of mine ;) (XR-1, but it's the same):
[ame="http://vimeo.com/19787701"]http://vimeo.com/19787701[/ame]

Last minutes of this one
[ame="http://vimeo.com/26377224"]http://vimeo.com/26377224[/ame]
 
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Ripley

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...Mainly because looking at it, I don't think the vehicle should be able to survive an inverted re-entry. The heatshield is after all on the ship's belly...
It sounds something's unclear, but maybe it's me...
Of course heatshield is on the belly, and that's what you have to expose.

I wanted to open a thread about inverted reentries, just to ask "why do we need them?".
I guess its main purpose is not to bounce off the atmosphere, while still generating a great drag and being "pulled" towards the Earth...maybe I'm totally off.

From what I understood so far (and I've never performed ONE single inverted reentry), inverted reentries, like any other reentry, should be performed with "heatshield ahead" anyway, otherwise BOOM!

In this screen I just took from the "XR2 Ravenstar -> 4 - ISS intercept burn complete" scenario, XR2 was flying prograde and I just applied 90° CCW rotation, and 40°-50° "up" rotation.
Don't mind the excessive altitude, it was just a quickie one to show ship's attitude.

MGalleryItem.php
 
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Cras

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The inverted re-entry with the XR-2, while fun to do, I see it as more of exploiting the code of the XR-2 and Orbiter to do something that otherwise cannot be done in the real world.

But you got the idea, the lift from the wings is reversed so while the effect of hitting the atmosphere that fast will want to throw the ship back out into space, flying the XR-2 inverted will allow the pilot to control the AoA and actually cancel out that force and allow you to plow through the upper levels of the atmosphere for a very extended period of time as you slowly get speed down to LEO levels, at which you can flip the ship over and re-enter as normal.

My issue is looking at the XR-2 re-enter the atmosphere this way, the first things I notice is that those huge bug-eye windows for the CDR and PLT are taking the brunt of the re-entry force and believing they would withstand that is just a bit too much to ask.

The XR-2 is already a fantasy machine to begin with, and I have set the config up to tone down that department, while still maintaining the flexibility the XR-2 provides, such as being able to land on the Moon. I launch the XR-2 vertically using velcro rockets, dead stick the landings, since I do not have enough fuel otherwise. Coming from the moon, I also don't have the fuel then to fire a retro grade burn to enter LEO before re-entry.

@boogabooga,
that works great. 60km for the perigee seems to be the magic number. Trying to go lower than that risks loss of vehicle. Using 60km and the very high AOA I was able to lower the apogee down to a managable height in three steps, which I think is the best one can hope for. The first aerobrake gets me into MEO territory, the second down to about 1-2 Mm, then the third is easy peasy to get back into LEO, or if I have enough fuel, just skip the third aerobrake and burn to lower the apogee back down to 280-400km for LEO.
 

orbekler

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...I wanted to open a thread about inverted reentries, just to ask "why do we need them?".
...

Interesting point.
From my point of view, inverted reentry is not needed unless you absolutely need a direct reentry (no LEO).

I also suppose that it's more risky than a standard reentry: if something goes wrong while you're captured by drag you don't have enough room to counter manoeuvre, you can't flip, cant' do anything to improve your situation, just burn.

Also, calculating exactly the "magic number" to sync landing zone it's not that easy, it seems to me that Tommy aleady discussed this in some thread.
 

Cras

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It also goes with the level of realism settings being used for the XR-2. The process I have thus far developed for aerobraking from the Moon requires a few passes, and that adds a bit of time onto the return leg of the mission, and thus requires oxygen. If you have it set up where the ship can't fly the extra day or so it takes to do the aerobrake, then you gotta do what you gotta do.

And I never got a good handle on the whole "magic number" myself. I find that by being able to actually fly inverted and have the v/s at zero for such a long time, that you can enter the atmosphere pretty much anywhere and land at any base, assuming again you have the APU fuel to do it.
 

Grover

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its actually quite easy now the XR2 has an inverted attitude AP (just set it to inverted before you activate the AP)

you just re-enter as normal, but with 180* of roll, giving you the necessary (orbital) centripetal force to hold you roughly in the atmosphere, so you can concentrate on part two:

aim to hold your altitude at 70Km for the entire airbrake, this gives the XR2 plenty of lift and also gives you some room for error (set PeA to 72 or 71km for initial approach, since your life above 70km will lower your PeA down to 70.

i find that 22.5* is about right, but it will change from time to time

just try it out, its not too hard if you know what to look out for
 

Tommy

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First, the inverted re-entry doesn't expose the vessel to different heating than an "upright" re-entry. In both cases, you will have a similar high AoA - so the exposed cross section is the same whether you are inverted or not. The idea that an inverted re-entry exposes the craft to different heat stresses is all in your imagination. Orientation relative to the Earth doesn't determine heating - orientation relative to the velocity vector does - so a 40 degree AoA is a 40 degree AoA no matter what your bank angle is.

Bank angle DOES affect your lift, and thus your VS. Inverted re-entries allow you to stay in the atmosphere longer if you are coming in at hyperbolic velocities. That is the only purpose.

In Real Life, inverted re-entries may not be possible with existing tech. The Shuttle's TPS, for instance, is "insulative", and is only effective for a limited time - after which the heat will have "soaked through" the insulation and into the vessels body and structure. A better insulator, or some form of active cooling, would make inverted re-entries possible in Real Life.

BTW, current heatshields such as the Shuttle's TPS wouldn't fare well with a re-entry profile that involves repeatedly entering and leaving the atmoshpere - the repeated heating/cooling cycle will fatigue the tiles (and the adhesive) and cause failures. Only the ablative heatshields used on the Apollo missions were designed to handle the thermal load of a hyperbolic re-entry.

As for the "magic number", LTMFD makes that obsolete. Just use periapsis mode (rather than re-entry mode), set the PeA to about 65k, and adjust the departure and arrival times to get the periapsis about 90 degrees west of the target. Then, just after entering the Earth's SOI, use IMFD Delta V program to perform a MCC. Use dVp and dVi to reduce the Ang (from Map's second page - if you entered the target base on IMFD's main config page) to zero and the PeA to 65k.

Takes a bit of fiddling to get the TEj and TIn right, but no math and no "time travel" required.
 

orbekler

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...Then, just after entering the Earth's SOI, use IMFD Delta V program to perform a MCC. Use dVp and dVi to reduce the Ang (from Map's second page - if you entered the target base on IMFD's main config page) to zero and the PeA to 65k....

I may have missed, but we're still awaiting your Delta V IMFD tutorial! :tiphat:
 

Grover

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First, the inverted re-entry doesn't expose the vessel to different heating than an "upright" re-entry. In both cases, you will have a similar high AoA - so the exposed cross section is the same whether you are inverted or not. The idea that an inverted re-entry exposes the craft to different heat stresses is all in your imagination. Orientation relative to the Earth doesn't determine heating - orientation relative to the velocity vector does - so a 40 degree AoA is a 40 degree AoA no matter what your bank angle is.

Bank angle DOES affect your lift, and thus your VS. Inverted re-entries allow you to stay in the atmosphere longer if you are coming in at hyperbolic velocities. That is the only purpose.

In Real Life, inverted re-entries may not be possible with existing tech. The Shuttle's TPS, for instance, is "insulative", and is only effective for a limited time - after which the heat will have "soaked through" the insulation and into the vessels body and structure. A better insulator, or some form of active cooling, would make inverted re-entries possible in Real Life.

BTW, current heatshields such as the Shuttle's TPS wouldn't fare well with a re-entry profile that involves repeatedly entering and leaving the atmoshpere - the repeated heating/cooling cycle will fatigue the tiles (and the adhesive) and cause failures. Only the ablative heatshields used on the Apollo missions were designed to handle the thermal load of a hyperbolic re-entry.

As for the "magic number", LTMFD makes that obsolete. Just use periapsis mode (rather than re-entry mode), set the PeA to about 65k, and adjust the departure and arrival times to get the periapsis about 90 degrees west of the target. Then, just after entering the Earth's SOI, use IMFD Delta V program to perform a MCC. Use dVp and dVi to reduce the Ang (from Map's second page - if you entered the target base on IMFD's main config page) to zero and the PeA to 65k.

Takes a bit of fiddling to get the TEj and TIn right, but no math and no "time travel" required.

1) quite correct, you re-enter at 180* roll and roughly 40* AoA, but i find that holding an altitude to just lower your ApA requires an AoA of roughly 22.5*

2)Bank only DIRECTS your lift, not the amount that is generated, so you in my method, as i get close to orbital velocity, any lift then behins to lower my altitude as well, so i begin to roll to one side to lower the amount of VERTICAL lift, and instead just change direction, which doesnt mean much at this stage of my flights

4) you dont need to enter and leave the atmosphere more than once, as long as you use something that generates as much lift as the XR2. you can easily hold an altitude of 70km using your lift to hold yourself in the atmosphere, then you either roll upright and re-enter as normal or roll out of the atmosphere before your ApA gets below 300km then resume orbital operations, and re-enter at a later time
 

dgatsoulis

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I think that we are missing the point here. The XR2 is a near/far future spaceship.
The space-shuttle itself could perform aerobrake maneuvers at banks over 90 degrees. The XR series just take this one step further. I like to call that "realistic imagination".
It sure is fun to apply "realistic" parameters on an imaginary ship and then try to see what it can do. (I do it all the time!)

But (for me) innovation comes when you let you imagination go.
 

Grover

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the only thing stiopping the shuttle doing this is:
A) getting it that high in the first place
b) heat soaking through the tiles to the actual craft, simply: it cant absorb that much heat without problems
 

Tommy

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I may have missed, but we're still awaiting your Delta V IMFD tutorial! :tiphat:

It will be part of an update to IMFD Full Manual - provided I can ever get the time to finish it. Since there aren't any real jobs to be had, I've been doing a lot of "odd-jobs", and the time spent finding these jobs and driving between them has severely eaten into my free-time.

1) quite correct, you re-enter at 180* roll and roughly 40* AoA, but i find that holding an altitude to just lower your ApA requires an AoA of roughly 22.5*

You might be a bit too high. Try a slightly lower PeA, the thicker air will let you generate enough lift at a higher AoA. I usually try for 64k with an initial AoA of 50. As soon as I hit Pe I lower the AoA to maintain a slight negative VS - usually 30 - 32 degrees is fine for me.

2)Bank only DIRECTS your lift, not the amount that is generated, so you in my method, as i get close to orbital velocity, any lift then behins to lower my altitude as well, so i begin to roll to one side to lower the amount of VERTICAL lift, and instead just change direction, which doesnt mean much at this stage of my flights

Changing the direction of your lift IS affecting your lift, but I'll grant that I wasn't very clear on that. I often use Bank angle to "fine tune" my descent rate - it's a bit more precise than AoA adjustments. I rarely go more than 5 degrees from inverted unless I have a crossrange to make up for (and sometimes deliberately leave some crossrange to allow banking without needing reversals) until I am very close to suborbital velocity. By then, I'll have a very high AoA (often 60 degrees) since I don't need lift to stay down - then I roll to vertical and continue a "normal" re-entry. Depending on how well I've timed my re-entry, it's not uncommon for me to climb back up to about 120k if I need to extend my glide (this also gives me a chance to open the radiator and cool down).

These are just minor differences between our methods - both work just fine!
 
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