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Space

The Art of Aerobraking 86

Posted by michael
from the flat-rock-on-a-still-pond dept.
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.""
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The Art of Aerobraking

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  • by sprouty76 (523155) <stephen_douglas.yahoo@com> on Thursday October 04, 2001 @06:18AM (#2387629) Homepage
    This is all explained in terms of the Mars Global Surveyor craft that is expected to do its orbital insertion on October 23.

    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.

  • by Iron Sun (227218) on Thursday October 04, 2001 @06:37AM (#2387643)

    Why not design a craft which could always 'right' itself regardless of how it is situated after it lands with a parachute type landing?

    They did. It was called Mars Pathfinder. It bounced around on those big airbags for a while, and after it came to rest sequentially deflated them so the payload ended the right way up.

    What's a 'sustainable' atmosphere?

  • Aerocapture (Score:5, Informative)

    by zardor (452852) on Thursday October 04, 2001 @06:42AM (#2387651)
    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.
  • by trims (10010) on Thursday October 04, 2001 @06:44AM (#2387654) Homepage

    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):

    • MC: probe, prepare for insertion! Here are the initial data parameters, go in 5 seconds! We will talk again in 5 minutes when you get to position X.
    • SP: thanks, MC. <sounds of intersteller number crunching>
    • SP: ok, control vanes, move here; rocket, fire in 4, 3, 2, 1...
    • (3 minutes later, and atmosphere unexpectedly thickens) SP: oh no! Quick, recalculate! <more i.n.c. noises> rocket, give me a 2 second burn then turn 43 degress for a 1 second burn!
    • (SP arives at X 20 seconds late) MC: good job! here's some updated positional data for the next pass...


    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

  • by da5idnetlimit.com (410908) on Thursday October 04, 2001 @07:57AM (#2387740) Journal
    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)

  • Aerosmash (Score:5, Informative)

    by zardor (452852) on Thursday October 04, 2001 @08:37AM (#2387900)
    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 [nasa.gov] website for more details.

  • by BlackGriffen (521856) on Thursday October 04, 2001 @08:45AM (#2387924)
    IIRC, Pathfinder didn't rely (solely) on a baloon deflation sequence to right itself. The craft was a tetrahedron (pyramid with 4 triangular faces), and no matter how it landed it would right itself as it opened.

    BlackGriffen
  • by treyb (9452) on Thursday October 04, 2001 @09:15AM (#2388053)
    The problem with this approach is the time lag between Earth and wherever they are (which is measured in light-minutes).

    Light-minutes and light-years measure distance, not time.

  • by Mishra42 (526400) on Thursday October 04, 2001 @09:42AM (#2388164)
    At school we're actually designing a Mars Sample return Mission. As a result I've been researching many different methods for orbital insertion. AeroBraking is an excellent method to change your orbit. As an Example the Magellan spacecraft was orbiting venus and wished to enter a more circular orbit. It was calculated that 900 kg of fuel would be needed to do this traditionally, more than the spacecraft had. Using aerobraking only 38 kg of fuel were used. Of course the downside is the manuver took 3 months to perform.

    For atmospheric entry however parachutes alone are not enough. Mars's atmosphere is 1000 times less dense than Earths. What parachutes are used for is to give a huge reduction in speed, from about 900 MP to about 160 MPH. A parachute that does this weighs about 10 KG. A parachute that would bring pathfinder to a survivable landing would have a 250 ft diameter and weigh 420 kg, more than the lander itself! It's much better to slow down some with a parachute then use retro rockets in one form or another.

    -Mishra
  • by code_rage (130128) on Thursday October 04, 2001 @12:36PM (#2388681)
    Things are getting better every day and by the minute ...


    Bzzzt! Thanks for playing. I wish I had your confidence in NASA's management. Unfortunately, instead of emulating successful missions such as Mars Pathfinder, NASA is planning to spread the model for a failed mission (Mars 98) throughout the agency.


    Mars Pathfinder was managed and built in-house by JPL -- this was the lander / rover mission which succeeded wildly (albeit with modest goals) in 1997.


    Mars 98 was the combined missions of Mars Climate Orbiter, Mars Polar Lander, and the experimental Deep Space 2 impact samplers. The loss of the two primary spacecraft was attributed to poor coordination between JPL and the contractor who built them (Lockheed Martin Astronautics).



    Yet, the "plan" being advanced to contain the massive cost overruns on Space Station, is based on outsourcing as much work as possible. Hmm. I'm not saying that outsourcing is always a bad thing, but NASA had better tell the taxpayers what's going to be different in the new formula.


    Here's NASA's "Commercialization" plan:
    http://www.spaceref.com/news/viewsr.html?pid=3730 [spaceref.com]

  • by JoeRobe (207552) on Thursday October 04, 2001 @02:49PM (#2389397) Homepage
    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

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