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The Art of Aerobraking
gizmo_mathboy writes: "Yahoo! Dailynews has the following Space.com article about the risk of using aerobraking for orbital insertion of spacecraft versus the certainty of using conventional propulsion systems. This is all explained in terms of the Mars Global Surveyor craft that is expected to do its orbital insertion on October 23. Skip the wimpy aerobraking and as a prophead trapped in a code monkey's job I say, "In Thrust We Trust.""
Depends on what you mean by 'thin'. Compared to the vacuum of space, even Mars' atmosphere is thick as pea soup and would work fine for aerobraking.
The danger lies in using aerobraking to rid yourself of all your velocity. If you use if to get rid of a nice portion of it and then thrust your way to a complete stop (relatively speaking, of course), you get the use of the atmosphere with less risk.
Dyolf Knip
Actually, it's the Mars Oddessy craft that's about to perform aerobraking, the Global Surveyor has been in Mars orbit for several years now.
No, I don't want a free iPod
A really nice demonstration of this can be seen in the movie 2010 (the sequel to the famous 2001). The Russian spacecraft uses this technique to slow down for an orbit around Jupiter. It's really fun to watch. If the science only partially intrigues you, then watch the movie just because it has John Lithgow. I like that actor more than Keanu Reeves.
"It's the little touches that make a future solid enough to be destroyed" --William S. Bourroughs
I am sure in the interest of reducing size, reducing weight, reducing cost, and increasing the amount of available instrument and sensory device space onboard an interplanetary craft which is designed to land on an alien planet within a certain *limited* budget, exploring as many alternative kinds of landings mechanisms is well withing the realm of understandable and highly saught after.
Packing a couple of large parachutes into a space craft for landing on a planet with a sustainable atmosphere would make a lot of sense if the means of adding an entire rocket/fuel powered landing/propulsion system onto the same craft would not produce greater yield/results within the intended mission/budget. Why not design a craft which could always 'right' itself regardless of how it is situated after it lands with a parachute type landing? That would/should not be a very difficult task given the amount of incredible talent that is at NASA's disposal.
As a *very* sincere and heartfelt sidenote...Policy usually destroys or f*cks up NASA missions. That is a *certain* gaurantee...NOT the entire staff of Ph.D. Physists and Engineers working on the projects. It always makes me sad/unhappy when people blame NASA engineers for NASA's recent terrible public mistakes. Blame policy, politics, and administration (be it impossible deadlines, mismanagement, etc.) for NASA's recent unfortunate public image. Things are getting better every day and by the minute and all it will take is just one immensely and incredibly perfect mission for NASA's public image to be returned to it's former 'moon landing' era confidence. Thank You for listening. Please support NASA and it's mission to keeping dreams and imagination alive despite the rest of the world's problems.
Using aerobraking to rid yourself of 'all your velocity' (interplanetery velocity, relative to the orbital motion of the target planet) is called aerocapture. This has never been attempted before, and would require prescise atmospheric targetting (to within a few kms), precise details of the atmospheric density parameters, and perfect understanding of the spacecraft's atmospheric behaviour, all on the first, and only 'deep' pass through the atmosphere. Since we are 'burning off' all our interplanetery velocity in one go, the heat load would be quite extreme, probably needing a dedicated heat shield, which could be discarded after the aerocapture pass. Of course, the spacecraft wouldn't need an Mars orbital insertion (MOI) rocket engine, and the ton or so of fuel that would go with it. (A smaller rocket burn however would be required 1/2 an orbit after the 'deep' aerocapture pass, to raise the spacecraft enough so it wouldn't pass through the atmosphere a second time). Once aerocapture has been achieved, and the spacecraft had been checked out (and allowed to cool down!), a gentler aerobraking phase can then be used over time to reduce the orbital velocity of the space craft, and lower the resulting eliptical capture orbit into a circular one suitable for science studies.
It has been proposed to use areocapture for some of the later mars orbiter missions, but it was deemed too risky, particularly after Mars Surveyor98's areobraking phase showed how unpredictable the Martian upper atmosphere really is.
If you remember the film 2010 (I think), the Russian exploration ship used areocapture at Jupiter, by inflating huge baluttes (balloons) around the craft and plowing through Jupiter's atmosphere.
-- We don't understand software, and sometimes we don't understand hardware, but we can *see* the blinking lights
No, I'm not going to talk about V-ger or anything like that.
The article mentions that one of the major problems with aerobraking is the fluctuation in density of the admosphere causes problems with calculations for the aerobraking. That got me to thinking...
Now, recently, we've started to build the landers with a reasonable amount of autonomous intelligence, so they can cope with some problems without requiring instruction. However, from all that I've read, all the space-borne probes we've send are dumb as a rock: that is, they can't do anything that Mission Control doesn't tell them to. They're a true remote-controlled vehicle.
The problem with this approach is the time lag between Earth and wherever they are (which is measured in light-minutes). I realize that adding some sort of intelligent processing to a probe causes an additional weight to be carried (and power consumed), but for christ sakes, I can get a Lego Mindstorms to run around my livingroom by itself; one would hope that we might be able to build a semi-autonomous space probe.
Basically, we should be able to build something that does this (MC=Mission Control, SP=Space Probe):
Basically, what I'm suggesting is that we break the mentality of requiring absolute control over the probe at all times, and allow them a degree of adaptability and flexibility by providing them with some reasonable programming. That's no happening now. And as the maneuvers we attempt grow in complexity, we're going to find it almost impossible to completely pre-calculate everything. If we keep trying, we're going to fail.
Adaptable and intelligent semi-autonomous probes are the long-term solution.
-Erik
There are always four sides to every story: your side, their side, the truth, and what really happened.
Hmm. You mean trust like they did with a previous Mars probe that accidentally reentered (may it rest in its pieces) due to a miscalculation over the size of the thruster?
-WolfWithoutAClause
"Gravity is only a theory, not a fact!"Must we always discuss everything twice?
They devellop hardened cpu for space exploration.
I seem to remember Hubble is using *hum* 486 dx2-66 with cosmic ray shield for stability calculation...(those got replaced after they fixed the lens the first time, they got an upgrade to Pentium Pro, I think)
Don't know about your Mindstorm CPU, but that should be twice its power...
BTW If it can find its way in the Living Room, did you manage to get it to open the fridge yet ? 8)
It takes 40+ muscles to frown, but only four to extend your arm and bitchslap the motherfucker
MC: Bomb #20, please return to the bomb bay.
SP: But I received the drop order.
MC: The drop order was in error. Please return to the bomb bay.
SP: OK, but this is the last time...
...
Captain: Talk to the bomb... Teach it phenomenology...
...
SP: In the beginning there was darkness, and me...
(Rusty memories of AI hardware in "Dark Star")
The living have better things to do than to continue hating the dead.
Economically, aerobraking is definitely the way to go. Putting large amounts of propellant on board would make the mission more expensive, or take the place of instruments, radio gear, computers, etc.
Until we get really advanced propulsion technologies that are both powererful and economical (high thrust, high specific impulse), we're going to need to use methods like this.
"Open the pod by doors, Hal" > "I'm afraid I can't do that, Dave" sudo "Open the pod bay doors, Hal" > alright
If the shuttle is a prototype, then it has to be the most advanced prototype ever. The SSME is the most amazing engine in the world. To increase the Isp even a tenth of a percent would be an amazing accomplishment.
... well, they're working on that. But _that_ is where their money problems are coming from ;)
To add to this, there is an upgrade in the works. Several parts of the orbiter are being upgraded. The Avionics system is one of those. They've already put new displays and computer hardware in the cockpit and after the avionics is upgraded in a few years, it will be "up-to-date".
I agree that we need the money and public support that we had in the Space Race days of Mercury through Apollo. Unfortunately the public sees no reason for this. Back then we had the Cold War to fuel our need to advance in space technology. Now most people could care less. I don't think any wars in the next 10 years will show a demand for a TIE-fighter, unfortunately.
As for the space station
That's Mr. Eradicator to you.
trance-port
Eek! It would probably not be possible to enter and land through mars atmosphere 'perpendicularly'. For 'entry' purposes, assume mars atmosphere to be 125Km high. The spacecraft is travelling at interplanetery velocity, say 7.5Km per sec. If we decide not to slow down, we will hit the surface in 17 seconds with a *big* bang.
The time is too short to run the entry sequence (jetesson heatshield, deploy parachutes, fire retros etc)
The deceleration G forces required to slow down in the limited time would be massive, (>100 Gs, causing structural engineering design issues)
The total integrated heat load on the heatshield would be the same, but the peak loads would be much higher (up to half a gigawatt. Thats a lot of asbestos)
And since you are going 'straight down', once you jetesson your heat shield (and its stored thermal energy), you will probably land on it a few seconds later and melt.
The ideal solution (as demonstrated by mars pathfinder) is to come in at an shallow angle of about 15 degrees, and in this case the whole entry sequence takes a good few minutes, the peak deceleration is about 20Gs and the peak heat load is about 100Megawatts.
See the Mars PathfinderEntry Descent and Landing website for more details.
-- We don't understand software, and sometimes we don't understand hardware, but we can *see* the blinking lights
Now if only the Martian defense force does not get the probe.
;-)
"It is a greater offense to steal men's labor, than their clothes"
Just a comment: I'm reading some comments from people saying that this is the first time aerobraking has been used. This is not true. Mars Global Surveyor (http://mars.jpl.nasa.gov/mgs/) used aerobraking to do it's orbital insertion several years ago. This is said in the article, so I'm surprised that people are saying that's it's a new technique. In any case, MGS's aerobraking phase was extremely successful. There is of course this fear of the atmosphere suddenly thickening, but this wouldn't happen in a matter of seconds, it would take quite awhile, enough time for the spacecraft to respond.
The story says that the MGS had some problems with aerobraking. Yes, it had some problems, and they said it took longer than it should have, which it did, but the way that they did it was much safer than direct orbital insertion with conventional propulsion systems. The primary source of the problems was (and I know this from following its news DURING it's aerobraking phase) that they didn't want to hurt an already damaged solar panel, so they were being very conservative because if they lost that panel, the mission was over. They normally could have easily handled the inconsistencies, but that in combination with the solar panel problem made them reevaluate some things:
To make sure the panel would be alright, they needed the pressure on the panel to be less that 0.2 N/m^2. They could only do this by extending the aerobraking phase. The major reason for breaking it up into two phases was because there would be a solar conjunction in June, 1998 in which we would not be able to talk to MGS for awhile. Thus we got it out of aerobraking mode before we were going to lose communication. It began phase two so late because a major part of the mission was to map Mars, and to do this required the spacecraft to be in certain places at certain times. To achieve this, they needed to wait awhile before restarting aerobraking.
There was not a fear of "crashing" the spacecraft here - they wanted to keep that solar panel intact, so they lengthened the aerobraking phase, which made them rearrange the mission slightly. It really wasn't a big deal.
Also, "labor-intensive" is a bit of a stretch - the orbits at the beginning of the first aerobraking phase were on the order of a couple days, and only a fraction of that time was spent going through the atmosphere, which gave them a very large amount of time to figure out where the spacecraft was and where it was heading. The phase 2 aerobraking orbit (much easier than phase 1) to begin with was about 12 hours. It definitely wasn't a scramble. They also fail to mention that a lot of science was done both during and in between the aerobraking phases - it wasn't a wasted year.
Also, it seems to me that now that we have the information (density data, etc.) from the MGS aerobraking, the Odyssey aerobraking predictions will be much better. In addition, if the MGS predicted atmospheric densities and such were so far off for the MGS mission, and MGS still survived, then Odyssey will do fine. It's just a matter of being conservative.
Let's remember that the spacecraft doesn't just go flying into the atmosphere, it gets itself into a very large, very elliptical, "rough" orbit, after which it begins aerobraking to lower the orbit and slows itself down. I'm sure somewhere on the MGS website you can see how it lowered its orbit with each pass, making it more circular. It's really slow, and from what I've seen from MGS, quite safe procedure, assuming you're careful.
I don't know if this helps anyone out. But really, the aerobraking phase isn't all that dangerous, and using the MGS as an example of how difficult it is is definitely a mistake.
JoeRobe
The best way to predict the future is to invent it.
Aerobraking is an elegant solution, making possible missions that aren't otherwise possible. Applying the lessons learned from Mars Global Surveyor, just make sure your structural design is sound, and go about your aerobraking conservatively and patiently.
.00000001% chance (or whatever level of risk you're willing to assume) of overheating or overstressing the spacecraft on any given pass.
In other words, you do the statistics and you just dip far enough into the atmosphere that there is only a
Maybe someday thrust will be so cheap we don't need to spend weeks in an aerobraking phase, but until then, I hope we get very good at it.
That that is is that that that that is not is not.
Ahhh, but we want it to *stay* in orbit, that's why this problem is hard :)
I knew I should do a quadruple check of anything I submit/post after 3am. Not to mention the fact it was a dupe (but I can sorta blame Michael for that. Ain't due diligence a bithc? ;-)