Re-entry questions

[sujiong]

I'm having trouble learning how to re-enter earth's atmosphere (Delta Glider). When preparing for re-entry from LEO, how much should you reduce your velocity by so that you don't either bounce or burn up? How far around the globe is it from the retrograde burn to when you really start to feel the heat? Should you approach re-entry with the nose up or on your side relative to the horizon? Also, I can never maintain the required angle of attack. The nose always just drops again, whether I'm using RCS or aerodynamic surfaces. I've tried in the standard issue DG and also Dansteph's add-on DGIV, and in this particular ship I have not had one attempt where it has not exclaimed DESTROYED! at me somewhere in the upper atmosphere. I'm sure this is pretty realistic. I'm not interested in any of the add-on re-entry MFDs or autopilot modes at this stage, nor do I want to learn how to land at a specific base - I'll get round to that later. I just want to know how to fly in manually. Any help would be appreciated - cheers.

 

[destinos]

Yeah I'd like to second this. There seems to be a lack of a good re-entry tutorial. Can anyone recommend one? Not to be too nitpicky about something that's already kind of slim, but a tutorial that can be followed on paper always really helps, as opposed to just a standalone playback.

 

[Hmuda]

No idea about tutorials, but I usually burn retro until my perigee just barely touches the other side of the Earth. After that, maintaining a 40° AOA seem to do the trick.

Two extremely handy MFDs for re-entry are the AeroBrakeMFD and the BaseSyncMFD. Using these you'll not only land safely, but where you want to.

 

[destinos]

Two extremely handy MFDs for re-entry are the AeroBrakeMFD and the BaseSyncMFD.

Yeah, it may be more specific to ask for an AerobrakeMFD tutorial, since knowing how to use that program seems like it would give an understanding of the re-entry process as well.

 

[Urwumpe]

A good start is here:

http://www.orbiterwiki.org/wiki/Intuitive_Atmospheric_Entry

Basically you need to pay attention on two factors:

1. Your reentry angle has to be low enough to reduce the first heat spike. -1.0°-1.25° is a good angle for the DG-type vehicles. The reentry angle is measured at Entry Interface (EI), a virtual window in space, which you use in navigation for "lining up" with your landing site. I use the following IMFD parameters for EI on DG-type vehicles: altitude 120 km, anticipation angle of 45° (45° away from the landing site on a great circle ~ 5000 km distance on Earth). You don't need IMFD and it is no optimal tool for deorbiting... but works. I could also create a simple plot for you to calculate the PeR on Earth for getting optimal reentry angle for a circular orbit altitude...it is no rocket science.
2. You need to manage your deceleration during reentry by controlling AOA and Bank. You start at 40° AOA and usually zero bank. This is called "full lift". Reenter and watch the vertical velocity. Stay in full lift until you are flying level (which means: Vertical velocity is close to zero).
3. Heat is a critical problem at the beginning. To control it on all vehicles which display heat (DG-IV, XR-series), I allow the vehicle to gain a small bit of vertical velocity again and stay at full lift. When I get close to +25 m/s, I start to limit the upwards acceleration. For getting this, I bank towards the base. The more I bank, the higher will get my descent rate, and the higher my descent rate, the higher will be heating and deceleration.
4. Now, you start the energy management: I aim for 16 m/s² deceleration on all DG style vehicles, so I slow down pretty constantly until I am in visible distance of the base. I let my ground track sweep over the base, until it is a small bit of distance away (if you have a DeltaAz display: 10.5°) and then do a roll reversal: 60° bank to the left becomes 60° bank to the right. And repeat it again when I sweeped over the base again...
5. Keep this up until you are down to Mach 8. Now you lower the nose slowly (to avoid heat spikes) so you get down to a good glide. You should be close to the base now and can use HSI for final targeting.

 

[sujiong]

Thanks a lot, this is a great help. Just managed to get the DGIV down. Came within a few degrees of burning up at one stage, the alarms were sounding. Wasn't aiming for anywhere in particularm, but ended up levelling out over Hawaii funilly enough. Thanks a lot!

 

[Tommy]

I recommend the "Delta Tweak" add-on if you fly the stock DG - this allows a much more realistic entry profile than the "vanilla" DG.

For when to de-orbit, it depends on your altitude. From the ISS's altitude, 17.2 M from the base works well. If in doubt, de-orbit early - it's far easier to extend a re-entry than it is to shorten one. PeA will vary a bit with the ReA you want, usually from 0 to 40k alt. Lower PeA's will shorten the re-entry by getting you down to Re-entry Interface sooner.

For DG type vessels, including the DGIV and XR series, you will want to have stopped your vertical speed by the time you are down to 65k to 70k. Higher than that and you may not lose enough velocity and start ascending again. Lower than that and you'll fry. From there, maintain around -80 m/s vertical speed (50 m/s if heavily loaded) to get good decelleration without overheating.

To understand the physics better, check out ar81's tutorial "Concepts for Atmospheric ReEntry (at OH)

There is a lot of "wiggle room". A DGIV can be deorbited over the target and complete a full orbit during re-entry, or you can de-orbit (from ISS alt) only 6M from the base if it's not carrying cargo and has very low fuel (6 degree REA, PeA -1000k, initial AoA 20 degrees/ zero bank).

 

[halcyon]

I could also create a simple plot for you to calculate the PeR on Earth for getting optimal reentry angle for a circular orbit altitude...

Is there a formula for this?
Where could I read about the mechanics of this? I'd like to run my own calculations on this one specific thing, or if you do it I'd like to see the math you use.

 

[Andy44]

For DG type vessels, including the DGIV and XR series, you will want to have stopped your vertical speed by the time you are down to 65k to 70k.

If you watch how AutoFCS flies a Shuttle Fleet reentry, you'll notice that this altitude is also the altitude at which the shuttle orbiter tends to level off at and fly for quite a long time as it bleeds off energy. Also, the X-20 Dynasoar levels off here for a while when using Reentry MFD. It seems the 65-70km altitude is the best for most lifteing bidy reentries to bleed off energy at.

 

[Oceanic]

some shameless self advertisement http://www.orbiter-forum.com/showthread.php?t=8425

 

[Urwumpe]

Is there a formula for this?
Where could I read about the mechanics of this? I'd like to run my own calculations on this one specific thing, or if you do it I'd like to see the math you use.

I recommend the "Fundamentals of Astrodynamics" (The BMW book), if you need it in compressed form and are short on money (it is cheaper than most modern books on the topic).

You basically need the flight path angle equation and solve it for periapsis radius. The periapsis radius only depends on the apoapsis radius for a given target angle.

 

[PriestLizard]

Basic formula for calculating de-orbit burns:

d = h / tan(alpha)

d : distance from base
h : altitude
alpha : desired reentry angle

So usually you want to keep an angle of about 1.0°-1.5°. Now you devide the altitude by the tan of your desired angle and you have the distance from base, where you need to do your deorbit burn.

Check the distance from base on your MapMFD. This process is more likely to work on a rather circular orbit. If your initial orbit has an high ecc, try to make it circular first. You will know how to deorbit from high ecc orbits later.

Example:
From an ISS orbit of ~380km and a desired reentry angle of 1.2°, I calculate: ~18000km. So I'd do my deorbit burn when I'm abut 18Mm away from my target base (align with BaseSync before). Aim for a perigee of 0km (altitude).

Honestly I never figured out how to use AeroBreak (I only had messy results with it) so I cannot tell anything about it - but BaseSync is def a great tool to aim for your target base.

Good luck

 

[halcyon]

I recommend the "Fundamentals of Astrodynamics" (The BMW book), if you need it in compressed form and are short on money (it is cheaper than most modern books on the topic).

You basically need the flight path angle equation and solve it for periapsis radius. The periapsis radius only depends on the apoapsis radius for a given target angle.

I don't have extra cash to spend on astrophysics books right now.
You said earlier you could create a plot for someone to do these calculations. Could you?
Or just post the formula for PeA vs entry angle vs starting alt?

 

[PhantomCruiser]

Not shameless at all Oceanic, that's a damn fine tutorial you made there... Thanks to you I can hit the mark like never before.

 

[halcyon]

Urwumpe,

Can you post the plot/formula you mentioned?

 

[mjessick]

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[Brycesv1]

i usually do the de-orbit burn when im right over my intended landing spot. burn until your going to reach about 50-60km on the exact opposite side. point the nose 5 degrees up and you should maintain about a 70km altitude until your velocity drops enough that you can enter the denser atmosphere without instantly burning up. id say about mach 8-10 is a good speed to actually point the nose down and use atmospheric flight rules to drop your speed and altitude. bank 80 degrees to the left or right and pull up as hard as u can. if u get too hot, lower the bank a little bit so you drop slower but always pull up as much as possible unless you get a warning about wing load. once your at 45 degrees from your target base, do a reverse roll and repeat. this bleeds off enough speed quickly enough that you should have a few hundred Km to line up a landing with your base. using this method (correctly) i have never crashed, burned up, or missed my base. also unless i have lots of cargo im bringing down or if i still have all my scram and RCS fuel (you should have dropped about 90% of it during reentry) i dont even get hot enough for the heat shield to activate

 

[sujiong]

some shameless self advertisement http://www.orbiter-forum.com/showthread.php?t=8425

Oceanic - NICE ONE on the tutorial video - I'd given up on calculating my own path in and started to use AeroBrake MFD, but I really didn't get how to use it until I saw your video, thanks a lot!