Flight Question Direct reentry calculation

[Marijn]

One of the things I like most doing in Orbiter is reentering earth's atmosphere for landing on a base. My goal is to plan a burn while in a parking orbit around the moon in such a way that no additional corrections are needed in order to fly all the way to the base, like Dimitri and Tex show in their video's using the IMFD Course Delta Velocity Program in combination with the Map program.

To take this to the next level, I want to avoid the need to make last-minute corrections by banking while flying through the atmosphere. The wings should be level all the way (except for rolling over) and I want to end up within 1 kilometer or so.

The Map programs displays an Ang value. My question: What does this value mean exactly and how can I calculate the value which will bring me all the way home?

I have done a lot of testing lately. My current understanding of this Ang value is that it is no angle, but the amount of degrees longitude to the base at periapsis-time relative to the orbital plane.

Here comes the problem: Because @periapsis, you're not home yet. There's still some distance to cover. In my particular scenario, I know a value of 1.33 will bring me exactly over the base without any banking. So my question is: How can I calculate this value outside of earth's SOI?

I noticed that the stock Map MFD in Oribit-Plane mode shows the line exactly over the base at periapsis time when 0.0 is used. When switched to Ground Track, the line will exactly predict the actual offset to the base at the time of arrival.

So what's the difference between the Orbit Plane and Ground Track? I know Ground Track also predict future orbits. But I struggle to understand why there is a little difference between these lines for the same orbit. I feel that the difference between those lines is exactly the value I am trying to calculate.

In my test flights, I did set the periapsis at 120km to avoid any braking. I do expect that actually braking requires another offset which needs to be calculated so it can be part of the Ang value, but I'll worry about that later.

Thanks for any help on this.

 

[Urwumpe]

Remember about the ground track, that ground rotates with Earth. The orbit plane on the other hand is almost fixed in space.

Also quickly forget "keeping wings level" or "reentry without banking" as something that is good to achieve. It is not. Banking is control. Not banking is an uncontrolled reentry. See below why.

For solving your navigation problem, you should know approximately how much distance you need for slowing down after passing EI. If you do this from outside Earths SOI, you will likely arrive fast - so plan some distance more there.

This value of "1.33" or else can't be calculated without flying your reentry first - see it as a replacement for reentry simulations done on a number cruncher like NASA does before a reentry (and actually during a reentry as well). What you need to know aside of how you begin the reentry at EI is knowing when your reentry is over and your landing approach begins - which conditions do you need to make it safe?

To begin such a simulation, create a scenario with your approximate orbit before reentry and note the geographic coordinates of EI in the first tests of this scenario. Place a minimal base on where EI should be.

The most accurate way is flying three reentries first. First, maximum lift. Simply keep wings level. Like your favorite "wings level reentry" should be. Do that simply until you are low and slow enough for landing. Note the distance from your marker base to where you ended up by looking at Map MFD. Now in Orbiter 2016, terrain can be more annoying, so you now have to get another number by plain flight-testing: How much distance do you need to simply land in a straight-in approach from the altitude and velocity where you ended reentry. Using approximately Altitude/sin(20°) as guesstimate works for the DG.

Add the distance to your full-lift reentry distance and you know how far you can get maximal. Now you could simply take this distance, but if you do, any difference in a reentry during your missions resulting in you gliding a shorter distance at maximum lift means you will not make it to the base. You assumed the maximum range possible and got less. Also, a full lift reentry means that you will get something like a high-speed stall at the end - you will be very high when you run out of speed and can't control your spacecraft without RCS. Which is bad, because you might already have depleted your RCS fuel for keeping AOA controlled during such a long reentry.

So, bad number. Next test flight. repeat the scenario of the first test reentry, but now do the opposite: How fast can you slow down without burning up. This is a test flight you can repeat a few times because as you can imagine: There is always a way to make it better. Bank as far as you can during this reentry, but try to reverse your bank often enough - ideally have a base near your reentry ground track but far away that you can aim for.

Don't add any landing distance to this one. This assumes you will fly over the base and do a HAC to align. This minimal distance can't be made any shorter - if you come home from a mission and try to use this distance for planning the reentry, you will likely overshoot.

Now you have minimum and maximum range possible. What you need last is a good middle. I prefer taking a distance that is about 30% the way between minimum and maximum. This is a good first guess then to test in the next flight, if I also have optimal control over the spacecraft when flying towards this goal.

The good news: You only need to do these test flights only once per spacecraft and in the best case, somebody else already did it for you.

But yes - the only way to get such numbers is by flying simulations. Many of them. And ideally, after each mission, you refine your numbers by doing a retrospective: How can you do it better next time?

 

[Marijn]

Thanks for your extensive reply Urwumpe. You did help me a lot by stating that the Orbit Plane is fixed in space. I just ran a simulation and I think I can calulate the number now. Let me clarify a few things though:

"reentry without braking" is just for testing purposes. Of course you need to decelerate to finally land. I did set a periapsis of 120km for a bunch of test-flights to make sure that the very fact of braking is not causing the need to aim for anything other than Ang=0.0. But apperently, even without entering the atmosphere, aiming at Ang=0.0 does not bring me over the base. I expect braking to introduce another value which will end up being part of the required Ang (because earth's rotation is not slowing down and heading and base lattitude also influence this) and I want to keep those problems seperate for now.

"keeping wings level" however, is something I not only consider elegant, but I think it serves a purpose: Each amount of bank requires some pitching up as well in order to maintain a given AoA. That means there is less room left for actually increasing AoA when needed. I want to reenter without using thrusters by using trim and COG balance only. I hate it when thrusters go like poof-poof-poof to maintain a steep AoA. Maybe I am wrong on this, but if you notice that you always end up west of the base, it seems natural to me to aim for an equal amount east of the base and fly straight-in without banking. I fail to understand why that would be a bad idea.

As soon as I enter earth's SOI, so at G=0.5, BaseSync can calulate the neccessary plane change. If you burn until RIn=0, IMFD's Map display will show the Ang value I am after. But plane changes are costly and I believe it can be prevented if I could calculate this value ahead of time while being in the moon's orbit.

For now, you helped me understanding that the amount of seconds between periapsis and arrival is to be multiplied with the rotation speed at the lattitude of the base, 408.4 m/s for KSC. This explains the offset.

Since IMFD's Map display gives the coordinates of periapsis, the amount of seconds to arrival at the base can be calculated. I will post the calculation if I am able to solve it.

 

[Urwumpe]

"keeping wings level" however, is something I not only consider elegant, but I think it serves a purpose: Each amount of bank requires some pitching up as well in order to maintain a given AoA.

You mistake Angle of Attack with Pitch angle there. That is actually wrong. You must understand AoA like this:

Its the angle between longitudinal axis and air velocity vector, measured in the plane of your spacecrafts vertical axis and the spacecrafts longitudinal axis (As if you are looking at your spacecraft from the wing tip). If you look at the spacecraft from the top of it, you measure the sideslip angle between the two vectors.

When you bank, the coordinate system for the AoA rotates with you. All you need to do is rotate around the velocity vector. Your longitudinal axis plots a cone around the velocity vector.

The Shuttle can always fly well at its 37° AOA, even if it is banking 110°, just by this simple geometry.

 

[Marijn]

Those are some really useful insights. I need to think it over for a bit.

Banking is control. Not banking is an uncontrolled reentry.

But why is banking considered a best practice? If I set an inclination which brings me right over the base, at a distance which allows for braking and reaching the base with some margin towards both ends to prevend an under- and overshoot. What's wrong with that?

 

[Urwumpe]

But why is banking considered a best practice? If I set an inclination which brings me right over the base, at a distance which allows for braking and reaching the base with some margin towards both ends to prevend an under- and overshoot. What's wrong with that?

Well, because its plain luck if you get close to your base that way. A tiny miscalculation, a sluggish reaction of the attitude control and you will accumulate distance. The longer the reentry, the more distance you can get. In the worst case, your energy will not be enough to glide to the base, you might need fuel that you don't have.

And now the REALLY bad news in your case: If you try that with a hyperbolic orbit (ecc > 1), chances are very high that you simply skip out of the atmosphere and coast into oblivion. You very often need to bank heads-down in such reentries to STAY inside the atmosphere until you are at sub-orbital velocities.

 

[Marijn]

Ah, ok. But my desire to calculate these things is exactly fot that very reason: To eliminate the luck factor as much as possible. And I am using Aerobrake MFD to adjust the AoA as needed to cover the distance flown until running out of total energy (I assume Aerobrake end of line indicates that. It could be potential energy only?).

This is rather easy. The only problem which can occur is when both trim and COG balance reach their extreme ends. Especially a XR5 full of cargo rapidly reaches this point. I feel that banking reduces the range left in this regard. Because if you don't pull the nose up when banking (whether it's called pitch or AoA is rather academic I would say) the negative vertical speed will increase and therefore you end up burning. I do have the habit of extending the aerobrake during the entire reentry. Because you do gain some margin when you retract them. I wish the XR's could extend them halfway, or even at quaters.

 

[Urwumpe]

(whether it's called pitch or AoA is rather academic I would say)

Its not... the difference is huge. Pitch is relative to the surface of the planet. AOA is relative to your spacecraft. You can have 0° AOA and fly straight down in a dive. Just try it.

Also I can assure you: Trying to calculate everything in advance fails more often than it helps. Even NASA doesn't do it. NASA has better simulation models and more CPUs than we can ever get at home, but still they never plan further than they can actually see. They always plan for corrections because measurements are never accurate, simulation models always simplified, etc. And sometimes somebody enters ft/s instead of m/s....

Guess how NASA did get to the moon at a time, when all computers at NASA did less calculations per second and had less memory than a cheap chinese smartphone. And when the real-time computing center had less CPUs cores than a Raspberry Pi....

 

[Marijn]

You can have 0° AOA and fly straight down in a dive. Just try it.

You are exactly right. My point is that banking requires the trim and or COG balance to be pulled, leaving less room to pull them even further when required and therefore putting restraints on margins. As I said, I am not using thrusters so this is important. That's why I called it academic. You end up adjusting the trim and COG in the same amount and same direction anyways, whether this adjustment is expressed in terms of degrees pitch or AoA.

And I certainly don't want to calculate everything. But asking what Ang value is optimal in IMFD isn't a very advanced question I would say. Aiming for a track which brings you right over the base seems a very good initial point to aim for, even when doing sharp banking manouvres afterwards for whatever reason.

 

[Urwumpe]

Strange... I never needed that for the XR-vessels. I used trim and COG for setting the coarse settings and left the fine corrections to the AOA-hold function. I simply watched the accelerometer and set the bank angle accordingly.

 

[Marijn]

It's no real need for the XR vessels. I just want to understand the Ang value of IMFD's Map program and be able to leverage this understanding so it allows for a burn right into the best inclination calculated from outside the earth's SOI. This will allow for serious delta-v savings, maybe even the best delta-v solution possible. Leveled wings allow for the maximum glide distance. Banking ruins maximum glide distance isn't it?

The XR vessels's attitude-hold autopilot in- or decreases bank amount in steps of five degrees. I think that really rough. Also the pitch (or AoA) minimal adjustments are in steps of 0.5. That's also rough. The alternative is Hover MFD's autopilot, which can be set to three decimal places. But Hover MFD isn't controlling the COG balance. I do get the best results by not using any autopilot and manually set trim and COG.

 

[boogabooga]

Also quickly forget "keeping wings level" or "reentry without banking" as something that is good to achieve. It is not. Banking is control. Not banking is an uncontrolled reentry.

I understand Apollo capsules could generate some lift, but they were certainly not delta gliders. I think the goal of planning a very accurate initial orbital plane so as to minimize off-plane maneuvers during re-entry is worthy.

I guess I would add that the IP author might find this thread useful regarding how hard to push re-entry: https://www.orbiter-forum.com/showthread.php?t=34197

Too bad Photobucket decided to take a dump on the internet, but you should get the idea

Also, my SOP for lunar returns in an XR has been to use an anticipation angle of 90 degrees.

 

[Urwumpe]

I understand Apollo capsules could generate some lift, but they were certainly not delta gliders. I think the goal of planning a very accurate initial orbital plane so as to minimize off-plane maneuvers during re-entry is worthy.

Still Apollo capsules had to reverse bank a bit more aggressively than a Space Shuttle later did. Lunar speeds are fast. :lol:

 

[Tommy]

One of the things I like most doing in Orbiter is reentering earth's atmosphere for landing on a base. My goal is to plan a burn while in a parking orbit around the moon in such a way that no additional corrections are needed in order to fly all the way to the base, like Dimitri and Tex show in their video's using the IMFD Course Delta Velocity Program in combination with the Map program.

To take this to the next level, I want to avoid the need to make last-minute corrections by banking while flying through the atmosphere. The wings should be level all the way (except for rolling over) and I want to end up within 1 kilometer or so.

The Map programs displays an Ang value. My question: What does this value mean exactly and how can I calculate the value which will bring me all the way home.

IIRC Ang is the angle of incidence between your current (or planned) course and the target. It assumes no changes will be made. This is impossible. Even if you don't make any MCC's or braking burns, the second you hit the atmosphere things will change. Unless you canreproduce your reentry with absolute perfection you can't accurately predict the Ang you would need to avoid some corrective banking.


BTW, increasing the bank angle will increase your negative VS, so you need to pitch down, not up to maintain the decent rate. If maintaining "wings level" was a desirable as you think, NASA would have done that with the shuttle. Instead, the entire reentry is flown at fairly high bank levels for two reasons. Its easier to make precise adjustments to the bank angle that it is to make precise adjustments to the AoA, and in the real world atmospheric pressure is weather dependant and impossible to predict accurately. Since the OMS engines can't be used to extend the flight if you come up short, the shuttle deliberately reenters with an excess of energy and uses the S-turns to bleed that energy off as needed for that particular flight.


In short, there is no advantage to a "wings level" re-entry, and in real life there would be a serious disadvantage to it.

 

[Linguofreak]

This thread was almost three years old...

 

[Marijn]

Meanwhile, instead of trying to understand all the theory, I did a lot of practicing and I think I've perfected the technique:

This is the final stage of a mission which started on earth. The 'unused' cargo slots were holding FUEL and LOX consumables and were deployed along the way to maximize the fuel efficiency. The fuel warning sounds exactly at the moment of touchdown which was no coincidence. I've programmed an application to sort out all the math to set up the scenarios.

If anybody can do it even better, I'd love to see a video!