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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.""
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
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.
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