General Question Altitude and Entry Angle

[PaulG]

The relationship between the altitude you are starting the deorbit process and the entry angle is clear; the higher you are the more shallow you need to be to avoid overheating.

But, given a vessel, such as the DGIV, is there a table or chart available that shows the "safe" range?

Of course, this can become multivariate, with distance from base, and anticipated angle, but, one step at a time.

Thanks.

 

[Cras]

I use a mix of DVTools, Basesync, Aerobrake, IMFD to figure this all out.

Essentially, enter basesync the deorbit I want (the angle, anticipation), get the dV required for the altitude I am at, put that into Aerobrake, get that info and throw it into DVTools to get a burn time, use IMFD to get that burn.

So far, I have been able to burn and get good de-orbits from as low as 220 km to as high as 600 km. Haven't tried higher yet, but I imagine I will at some point.

Not a table, but a fluid procedure that will get you a quality de-orbit burn solution every time.

And this is not an original idea on my part. Check out DVTools and follow the de-orbit procedure for an aerobrake Earth deorbit and you will see how to do it.

 

[Furet]

But, given a vessel, such as the DGIV, is there a table or chart available that shows the "safe" range?

Maybe this chart could help you: [ame="http://www.orbithangar.com/searchid.php?ID=5529"]DGIV Reentry Chart[/ame]

 

[Urwumpe]

Actually, "the higher you are, the more shallow you should be" is not absolutely correct. If you for example return from the moon or further, you actually need to be steeper, so you don't skip from the atmosphere before slowing to sub-orbital speeds. How fast you can turn to control your spacecraft and the lift available dictates this mostly, together with the limits on heat flux and dynamic pressure. Being too slow at an altitude is as bad as being too fast - because being too slow turns into being too fast quickly, when your descent rate increases.

Such more accurate plots can be calculated, but are not simple "for x do y" plots. You better understand them as "How can I get from the way to a hot and fiery death to a controlled descent?". You use the plots to steer into a trajectory that brings you home, while entry interface is just the reference for giving your vessel the best possible conditions to get on such a safe trajectory. You still need to fly home.

You for example need to remember that different inclinations also result in different velocities relative to the atmosphere. At EI, you still measure in inertial velocity, not surface relative (since it makes no sense at all there, the surface is far away). Instead of having a trajectory for every orbit and inclination, you have safe corridors, that you try to reach as soon as possible.

This little bit of theory also explains the reentry displays of Shuttle or Apollo pretty well: Some lines there are limits, on the Shuttle you also have the good trajectories as lines in the displays, in Apollo, you have the really bad lines (trajectories that will kill you if you don't leave them manually ASAP). On Buran, you had been seeing the optimal corridor between the limits.

A safe range does not exist. The minimum and maximum range you can have to a base depends on your total energy and the cross range to that base from your initial orbit plane. Once you are on a good trajectory, you will reach the base, that much is promised.

 

[Ripley]

...put that into Aerobrake...

How do you exactly do this?

 

[Cras]

How do you exactly do this?

The "HDv" button. You can ener a hypothetical Dv and Aerobrake will apply it to its prediction and you can see where you will end up if you had that much applied. Also takes into account your attitude as well. So the way I go about it (in the XR-2 anyway), is take the Dv that Basesync tells me I am going to need for that re-entry profile, and enter that in the HDv for Aerorbrake, then activate the AoA hold (40 degrees for me, but which ever you want to use), then see where aerobrake predicts I will end up. Plus the other things, such as what the deorbit will look like, will I skip on the atmosphere (while not an end game occurance in the XR-2, but something I tried very much to avoid regardless), and check if the G's or heat will be too much to handle.

 

[PaulG]

Yeah, I figured this would be the case, and I understand why. What I don't understand is how does one "pick" the path to take or how does one "know" a path will not work.

For example, 350k altitude 180° from base. Lots of options here to deorbit and get to the base. But, as the distance to the bases decreases, the options become more restricted. For example, 350k altitude and 90° from base I believe has very few or "on the edge" options.

How does one go about planning it out? Or, is it simply done by experience and simulation?

A safe range does not exist. The minimum and maximum range you can have to a base depends on your total energy and the cross range to that base from your initial orbit plane. Once you are on a good trajectory, you will reach the base, that much is promised.

 

[Urwumpe]

How does one go about planning it out? Or, is it simply done by experience and simulation?

In first place, it is possible to provide limit plots of the vehicle. How much speed is tolerable at which altitude, how fast must you be for not descending too fast. You can calculate in advance, how much energy you have and how far you can glide with it.

Then, for the actual deorbit, it is a bit of simplified simulation. There are many algorithms for deciding the optimal reentry trajectory and deorbit burn. You can put EI at a set altitude and distance to the base and then look for a good trajectory, or backtrack the trajectory among many reference trajectories that bring you home best. It is never a full simulation needed, but you never have a simple function "Landing site L and Orbit O results in Deorbit maneuver X".

 

[PaulG]

Thanks. Given the available tools for Orbiter, what would you suggest to use and how?

In first place, it is possible to provide limit plots of the vehicle. How much speed is tolerable at which altitude, how fast must you be for not descending too fast. You can calculate in advance, how much energy you have and how far you can glide with it.

Then, for the actual deorbit, it is a bit of simplified simulation. There are many algorithms for deciding the optimal reentry trajectory and deorbit burn. You can put EI at a set altitude and distance to the base and then look for a good trajectory, or backtrack the trajectory among many reference trajectories that bring you home best. It is never a full simulation needed, but you never have a simple function "Landing site L and Orbit O results in Deorbit maneuver X".

 

[Urwumpe]

Thanks. Given the available tools for Orbiter, what would you suggest to use and how?

Excel. Sounds maybe strange, but you can already get a lot of the data plotted for flight planning that way. For the rest...well, I prefer Reentry MFD, but also look at doing my own MFD there.

 

[PaulG]

Do you have a reference for equations I can use?

Excel. Sounds maybe strange, but you can already get a lot of the data plotted for flight planning that way. For the rest...well, I prefer Reentry MFD, but also look at doing my own MFD there.

 

[PaulG]

So, a follow up question here. BaseSync MFD seems to be doing something very wrong.
My altitude is 306 km and I'm going to KSC. My Inc and LAN are 66.25 and 61.44 @ MJD 56221.7449. PeA is 303k and ApA is 310k.

I set in BaseSync the following parameters:
ReA = 0.8, ANT = 10, ALT=80k.

When I do the deorbit burn, my PeA is around 40k, but over the base its more like 60-70k over the base after I applied the specified burn, in the DGIV, and therefore I miss it.

Why? Is there some limitation? What works better?

 

[Tommy]

Where did you come up with 10 for the ANT? This is FAR too low. ANT is the distance the vessel will travel AFTER Re-entry Interface (ie, crossing below 80k for the first time) and before it actually lands. It is Angular distance, and measured in degrees.

So step one is to increase the ANT to something more reasonable - try around 45 - 60 (depending on how "hot" you want to get).

Second, while you saw a predicted PeA of 40k, this is a purely ballistic calculation and does NOT account for atmospheric effects. Once you get into the atmosphere aerodynamics has a major influence on your flight. Your PeA is higher because of aerodynamic lift. You need lift, because the vessel needs time to shed velocity before it sinks too deep into the air.

On a re-entry from LEO, you have a velocity of about 8,000 meters per second. That's a lot of energy that you have to convert from kinetic to heat. To keep the hull temp survivable, you have to convert that energy in a slow and controlled manner.

I suggest learning more about the principles involved in an atmospheric entry, and here's a good start:
[ame="http://www.orbithangar.com/searchid.php?ID=3428"]Tutorial: Concepts for atmospheric reentry[/ame]

 

[PaulG]

10? Random. How do you pick it? What is the guidance?

The tutorial you posted isn't all the helpful. I can easily deorbit 180 degrees off, low angle and get there no problem.

My questions involve more of how to figure out what the correct parameters are....

Where did you come up with 10 for the ANT? This is FAR too low. ANT is the distance the vessel will travel AFTER Re-entry Interface (ie, crossing below 80k for the first time) and before it actually lands. It is Angular distance, and measured in degrees.

So step one is to increase the ANT to something more reasonable - try around 45 - 60 (depending on how "hot" you want to get).

 

[sorindafabico]

How do you pick it? What is the guidance?

I use trial and error. I found Ant = 36 as a very good value to deorbit the XR5 from the ISS altitude, per example, but this value doesn't work so good for other vessels or a very different altitude.

 

[Ripley]

I set it at 38-39 for DGIV from ISS altitude.

 

[Tommy]

You are making a common mistake among people who are somewhat new to re-entry. You place FAR too much importance on the specifics of the de-orbit. Even for a specific vessel, there is not a single specific answer. There is a range of possible answers. How diverse that range is depends on the vessel. A DG has a MUCH larger range than an Apollo capsule because the DG can produce more lift.

For instance, with a DGIV leaving the ISS, I can de-orbit above my target and land one orbit later. I can also de-orbit as close as 7M away from the target and still make a deadstick landing. There is NO perfect answer that will get you to the target - there is a range of answers that will allow you to hit the target. Whether or not a re-entry is successful (ie, safe and on-target) depends MUCH more on how you manage your energy AFTER you hit the atmosphere.

Urwumpe already mentioned "corridors". He also mentioned that there is a minimum altitude you can safely have at any given velocity (and the altitude increases with velocity). In a DGIV, coming in from the ISS, you will reach around 8 km/s velocity, and a DGIV can safely handle that velocity at altitudes above around 65k. So, for a DGIV, the re-entry corridor starts at around 65k. In a high AoA flight configuration (AoA 30 - 60) the DGIV creates enough drag that you can safely descend at around 80 m/s. In a low AoA configuration (15 degrees or less) you can't descend much faster than 15 m/s. This varies with the mass of the vessel - an empty vessel can descend faster than a fully loaded one (since it will loose velocity faster).

It doesn't matter how you get to that corridor - only that you enter it and stay inside of it. You can de-orbit early, and have a low ReA, or you can de-orbit later and have a higher ReA. As long as you can have the descent rate down to about 80 m/s by the time you are down to 65k altitude you will be fine. There is a limit based on the lift the vessel can produce. For instance, to de-orbit from a mere 7M from target, you will have to have a ReA of about 6.5 degrees (around -1000k PeA). Then you will set the AP to hold a 15 degree AoA until your VS is about 100 m/s, then increase the pitch to attain a steady descent of around 80 m/s (this usually means an AoA of 50 degrees or more initially to overcome the "bounce" and avoid climbing out of the Atmosphere). How much lift a vessel can produce (and it's current mass) determine how steep the ReA can be and still be able to pull out of the dive in time to avoid burning up. The more lift, the steeper the ReA.

So, the goal during the de-orbit burn isn't to hit the base - it's to hit the start of the re-entry corridor on a vector that allows you to remain in the corridor. This, of course, begs the questions "Where is the start of the re-entry corridor? And how big is it?"

The answer, of course, is that "it depends". Depends on the vessel's lift, drag, it's current mass, it's heat tolerance, etc. To calculate the corridor, we would need specific info on how the vessel calculates hull temps - and we don't have that. By watching the full-auto re-entry AP, we can see that the DGIV can handle an 80 m/s descent rate, and can also guesstimate an average decelleration rate. We know we have a max Vel of about 8k m/s at the start of the aerobrake, and use the average decelleration to calculate the time and distance traveled during the aerobrake. That tells us where the corridor starts. To use this with the MFD's, figure the distance traveled as an angular measure - not linear. Use this distance in the ANT setting.

The easiest way, of course, is a bit of trial and error. Start with an ANT of 45 (for winged vessels) and see what descent rate you need to reach the target. Then you can watch the hull temps - if the temps stay low you can decrease the ANT, if you have to reduce the descent rate to avoid burning than you need to increase the ANT.

Every re-entry will be a bit different. Different starting orbit, different fuel load, or different vessel entirely, etc. Once you grasp the concept, you'll be able to adjust for the variations without really needing to think about if.

The main point is to consider the re-entry as TWO separate stages. Like most space ops, plan from "last" to "first". Plan the second stage (the atmospheric stage, or ANT) first, depending on the vehicle. Then plan the de-orbit based on that target - and adjusted for the starting orbit. Keep in mind that EI (or ALT) is related to ReA. For instance, the DGIV specifies an ReA of 1.2 degrees based on the built in Flight Control Computer Display - which assumes an ALT of 120k. If you use a lower ALT you will need to reduce the ReA to hit the same corridor. I suggest picking an ALT and sticking with it - adjust the ReA but leave the ALT the same for all vessels (for that planet, at least). The standard for Earth is 120k, but you will probably get better results with a lower EI, the 80k that you are using is fine for LEO, but a bit low for higher orbits or lunar returns. I use 90km, which works for interplanetary returns as well.

 

[PaulG]

Thanks. I am aware of the overall concepts, how altitude (speed) at the time of deorbit, distance from base at the time of deorbit, and amount of dV applied affects the reentry angle and the distance traveled once entering the atmosphere (ANT).

But, there does come a point where picking reentry parameters just won't work. In the example I gave, I don't believe there is a way to bleed off all the speed prior to reaching the base. I've tried a couple approaches but ultimately I can't balance the decent rate and deceleration properly with those parameters. Hence, it led me to ask how one could know that the parameters chosen aren't going to work or will be on the edge of any reasonable way.

I think you did answer it though by saying there is no MFD available to fully predict and it seems to me much of that is done by experience. I still think though there is a way to combine the three input parameters to provide a probability of success. Yes, I know that it does depend on how you fly down, but that is where the probability would come in. I think there is a minimal solution to every combination provided a fixed weight, AoA, and minimal flight path. From that basic solution it should be easier to factor in the more dynamic variables.

It also seems to me that IMFD may be able to predict the basic plan. Is that true? If so, how?

 

[Urwumpe]

But, there does come a point where picking reentry parameters just won't work. In the example I gave, I don't believe there is a way to bleed off all the speed prior to reaching the base.

That is where ANT gets important. Like you can't stop your car or bike in any randomly short distance, there are minimums that apply to spacecraft as well. This distance is called ANT in most MFDs.

I don't calculate in my car, how many meters I need to stop by kicking the brake pedal to the firewall. I start braking some distance further away from the goal and then reduce or increase the brake force if needed, especially in the winter months, when I don't know how know how the road is like.

Nothing else you do in a spacecraft. The most popular strategy there (I don't know of any spacecraft since Gemini, that does not use it for gliding reentry) is starting with a full positive lift (wings level, preselected AOA) until you reached zero vertical velocity (you fly at constant altitude AKA level flight). This initial reentry phase has a few advantages: You begin piloting with clearly defined parameters (level flight) and by the time you needed from EI to level flight and the accelerations you experienced, you can tell how close you are to BBQ. Too fast and too hot? raise a bit. Too slow? increase descent rate until you get the proper deceleration for reaching your base.

Remember the part with the road condition above? Same for spacecraft. You could have a hotter atmosphere with higher density at higher altitudes than usual. Your spacecraft could be slightly heavier than planned. Maybe you are a few tenths of a degree off your reference trajectory. Maybe a thruster failed. Maybe you will run out of RCS fuel before a hot reentry is over.

45° ANT works great for most winged vehicles, this means you need to fly at about 16 m/s² deceleration most of the time. For capsules, you can fly with about 10°-15° ANT, since you fly at about 3-4 times higher deceleration with them.

PS: AOA is a function of Mach number for most spacecraft, Sideslip is zero ALWAYS.

Can't really remember when I last failed to land on a base. Usually only when I develop a new vessel and have not yet proper flight data to plan reentry.

 

[PaulG]

Thanks. The past 2 posts have been some of the best explanations given.

---------- Post added at 04:13 PM ---------- Previous post was at 01:10 PM ----------

Actually, I have a follow up question for Urwumpe.

You had stated in a couple previous posts on this same topic that you use IMFD for deorbit planning.

Two questions:

1) Why IMFD?
2) How do you use it?

Thanks.