Flight Question Aerobrake problem-The Ultimate Quest for the Most Efficient Aerobrake Technique

Nicholas Kang

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Hello, this is my second thread. I am glad to see kind members helping and teaching me in my first thread. Hopefully, the same matter repeats in this thread.

My problem concerns with the aerobrake technique employed in most cases when fuel is limited or maybe we want to utilize the Martian atmosphere for some beneficial uses. I have been to Mars successfully for several times, owing to the guidance of the IMFD. However, IMFD doesn't offer any aerobrake programs. Basically, you (more accurately, I) set the Planet Approach program and ensure your PeA value is some 200 km if you don't plan to utilize Martian atmosphere for aerobraking. I opted for a lower PeA value, some 30-40 km. That is my initial plan, dip down into the atmosphere and watch the atmospheric drag in action. Then, conduct the Orbit Insert program.

However, weird things happen. I actually tried both values; the 200 km PeA first then I reopened the scenario and set PeA to 40 km in the second instance. (I quick-saved the scenario just before setting the PeA value in the Planet Approach program in the IMFD)

Talking about the first case. After setting the PeA value to 200 km in planet approach program. I engaged autoburn (hereinafter abbreviated as AB) and I watched the orbit trajectory in the Orbit MFD slightly changed. Yeap, successful burn. Then, I switched my MFD to Orbit Insert MFD and get prepared for another AB. I accelerated time until reaching the 300 km altitude mark. Then, slowly I watched my altitude dropped and engage AB. I still need to wait until T-value counted zero. However, just before T-value approached zero. My DG turned prograde suddenly (without any autopilot program engaged). Then, I saw it. My altitude dropped below the threshold and I switched to external view.

Aerobrake occured. I saw the bright flames surrounding my DG and switched back to 2-MFD view. The eccentricity indeed dropped. Velocity was decreasing. Not bad, or so I taught. The bad news came when I don't take control of the DG, allowing it to simply hit smack-dab onto the ground. Then, due to DG's high residual speed. It took off again and whosh. It was back off into space. Luckily, I was able to quickly engaged retrograde autopilot and immediately burn to reduce eccentricity to nearly zero. I flew to base manually, with many trials and error because I ignored the numbers, distance and time, so I overshot many times, watching Olympus Base sliding past my side.

The second hard try is equally frustrating. Anyway, it was a whole lot better, since lowered PeA value took me into the atmosphere faster than the previous trial. I became smart by manipulating controls. Once I saw the flame, I quickly engaged horizon autopilot with prograde-facing direction. This time, I turned off RCS and used aileron to pitch down and up occasionally, not wanting to let DG wandered off into space or crashed onto ground once again. I successfully maintained altitude at nearly 15 km. Still watching DG bleeding off energy, I realized that I could at least allow DG to enter a highly eccentric orbit, but definitely captured by Martian gravity.

This time I was successful, albeit further periapsis negative delta-V burn was unavoided. After telling a long story, it is time to ask some questions, directed to the experts of course.

I would like to know what is the optimum height to conduct aerobrake on Mars. Is it as low as 14 km or maybe the atmospheric density may be higher a bit at higher altitudes? Also, is it possible to enter a nearly circular orbit simply by using aerobrake without any periapsis retrograde burn?

Edit: one more question: What about the orientation of DG? I engaged retrograde autopilot as DG has high Lift to Drag Ratio. However, Dg failed to orientate itself properly when it was in the atmosphere, some sort of mysterious forces seem to act on it, causing it to reorientate its position. What is that mysterious force? Any explanation behind?

Please share some of your useful tricks in entering Martian orbit using aerobrake. I plan to demonstrate aerobrake technique to school students using Orbiter. Mars is a good example. (Background: I participated in my school Science and Math Fair. I plan to use Orbiter to demonstrate different orbital maneuvers to them)

Thanks for sharing.

Regards,

Nicholas

---------- Post added at 02:46 PM ---------- Previous post was at 09:48 AM ----------

It seems like nobody cares my thread. I am waiting for your kind sharing of knowledge and experience.

Nicholas.
 
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malisle

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Try completing this. It is completely unpowered landing challenge from some time ago. Answer is yes, you can aerobrake and land without using engines at all. Thread should give you some general pointers.
 

boogabooga

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First of all, you should be aware of this:
[ame="http://www.orbithangar.com/searchid.php?ID=2139"]AeroBrakeMFD[/ame]

I would like to know what is the optimum height to conduct aerobrake on Mars. Is it as low as 14 km or maybe the atmospheric density may be higher a bit at higher altitudes?

It depends on how sophisticated the damage model is of the particular spacecraft you are using. The stock delta glider does not have that sophisticated of a damage model, so I suppose you can go as low as you would like. Just be careful that you don't maneuver violently since very high Gs can damage your spacecraft.

With more sophisticated spacecraft, heating is the main concern. See this thread for example with the XR series:
http://www.orbiter-forum.com/showthread.php?t=34197



Also, is it possible to enter a nearly circular orbit simply by using aerobrake without any periapsis retrograde burn?

Yes, but keep in mind that a natural orbit will return you back into the atmosphere, so you need to conduct a periapsis raising maneuver if you are intending to stay in space.


Edit: one more question: What about the orientation of DG? I engaged retrograde autopilot as DG has high Lift to Drag Ratio. However, Dg failed to orientate itself properly when it was in the atmosphere, some sort of mysterious forces seem to act on it, causing it to reorientate its position. What is that mysterious force? Any explanation behind?

The mysterious force is the aerodynamic force! Same thing that keeps airplanes in the air, etc. Have you ever seen an airplane fly retrograde? That is what you were trying to do.

For the delta glider, use your lift from your wings to control your vertical velocity. Before periapsis, you will have a natural tendency to accelerate toward the planet's surface. After periapsis, you will have a natural tendency to accelerate away from the surface. Use lift to oppose these forces if you need to. Don't be afraid to fly sideways, make S-turns, or even fly upside down. The so-called inverted re-entry can be used to keep you in the atmosphere after you pass periapsis.

Enter the atmosphere at a shallow angle, brake for a bit at about zero vertical velocity, then lift yourself back out of the atmosphere.

With the XR-2, I think the procedure is to maintain a constant angle of attack using the supplied attitude hold autopilot.
 

Thorsten

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I would like to know what is the optimum height to conduct aerobrake on Mars. Is it as low as 14 km or maybe the atmospheric density may be higher a bit at higher altitudes? Also, is it possible to enter a nearly circular orbit simply by using aerobrake without any periapsis retrograde burn?

Disclaimer: While I'm very familiar with atmospheric entry of the real Space Shuttle, I have only passing experience with how Orbiter simulates any of this, as the previous post suggests it seems to be very different for different craft.

You can't treat the question independent on the aerodynamical characteristics of the spacecraft (and in reality on thermal and structural constraints).

Atmosphere density decreases with altitude above a mean level near-exponentially, so going lower will always give you a lot more drag. The deceleration force is roughly proportional to the drag coefficient times the reference area times the air-relative velocity squared, so deceleration will be more efficient at high velocity.

Now, the key is the drag coefficient, because in reality that also depends on angle of attack (AoA or alpha), Mach number and also in a less pronounced way airfoil deflection. Changing AoA for the Space Shuttle from 0 to 40 deg modulates drag a lot.

So assuming you have a guidance that tells you the required drag for a maneuver, by modulating alpha in combination with atmospheric penetration depth you can usually achieve it.

In real life, thermal constraints are a problem and you need to hit the air with the heat shield first - so modulating drag force via AoA is not possible.

The actual entry control scheme for the Shuttle works as follows:

* You know your velocity and range to base - from this, a desired deceleration can be determined.

* Since AoA is fixed by thermal constraints, deceleration control must be via air density, so go get harder braking you go deeper, to reduce force you go higher.

* To control altitude, you need to control vertical speed, and the key to that is vertical acceleration.

* Vertical acceleration is given by the L/D ratio

* L/D ratio is controlled by bank angle, i.e. rolling around the velocity vector - and this can be controlled directly.

So don't think of aerobraking as a simple problem that has one optimal solution - it's a complicated dynamical problem, and it depends on what you want to do and what capabilities your craft has. If the Shuttle had a different heat-shielding, controlling drag via direct modulation of AoA wold be entirely feasible for instance. If there were no structural or thermal constraints, I'm fairly sure you could potentially stop it over a 1000 mile entry trajectory by just plunging into the lower atmosphere fast and flying multiple tens of g deceleration at high AoA.

Finally, echoing the previous poster, an orbit is an orbit and if it is closed and you are in the atmosphere at one point, you will go back - which means the orbit will decay over time.
 

Nicholas Kang

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Thanks Malisle, nice test. Interesting.

Hello boogabooga, hopefully you have forgiven me. Just to repeat, no bad intentions, just simple confirmation. I have tried using the Aerobrake MFD but when I engaged AoA, Alt and Bank autopilot, the values displayed on the upper right corner of the MFD was yellow. Then, I accelerated time, but saw no effect, except constant turning of my DG before acted upon by the aerodynamic force. So, I simply deleted the MFD's files and never use it. I don't know why. Also, talking about aerodynamic force. I experienced the same mysterious self-reorientation phenomenon during reentry to Earth while deorbiting. Do I need to use the same technique as aerobraking to reenter Earth smoothly?

To Thorsten,

I would like to know if trimming up the vessel have any effect on the aerobraking process? Actually, until today, I still don't know the function of trimming (trim here refers to the small slider with the trim value [default is 0.0] on the upper left hand side of the 2-MFD screen which also displays DG's fuel, thrust etc.)

Thanks for all replies. Although my questions are aimed at different people, anyone can answer other questions, not only restricted to the person whom I directed my question(s) to.

Notice: I will be off for about 1 week. Need to focus on my studies. Stay tune for my return.

(I got No.1 out of 300-400 students for my average whole year exams for 2 years consecutively. Need to continue maintain that standard. Hard work and definitely not achieved by replying and asking orbital mechanics here everyday.)

Where there is a will, there is a way.

See you.

Nicholas.
 

Thorsten

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Also, talking about aerodynamic force. I experienced the same mysterious self-reorientation phenomenon during reentry to Earth while deorbiting.

Ever seen a jet fly through the air? :) The pointy end always goes forward in the air (same for a bullet, a spear, an arrow,...). Whenever it deviates from that attitude, the air pushes it back. It's the reason these objects are made pointy - if you'd build a brick-shaped plane, it'd tumble.

Same for the delta-glider - the pointy end wants to be forward in air.

I would like to know if trimming up the vessel have any effect on the aerobraking process?


Dependent on what exactly you trim, the word can mean a shift in center of gravity as well as adjusting gains of your stick. Usually it's done to ease load on the control surfaces or the stick (if you notice you have to pull like mad on the stick to fly straight, you should seriously consider trimming).

So it doesn't affect aerodynamics as such, but as far as weight trim is concerned, it affects the inertia tensor and hence the response to pitching, yawing and rolling moments (what a plane does when you pull 1/2 on the stick depends on weight distribution).
 

boogabooga

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Mars Aerocapture Playback

Hello Nicholas.

Do you know how to manage Orbiter playbacks? Check the manual if not.

I've attached a playback of my own Mars aerocapture effort to demonstrate everything that I discussed. I achieved a low Martian orbit using only a small periapsis raising maneuver at the end. I've also managed the orbital plane such that you will pass almost over Olympus on the NEXT orbit. You can try to de-orbit and land by whatever means you find best, if you wish. When you get back, let me know if you have any questions.

Good luck with your studies.
 

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