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Akatsuki Venus Climate Orbiter
pandaneko
post Sep 8 2011, 10:02 AM
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JAXA HP is saying;

OME was fired for 2 seconds at 11:50 (JST) on 7 September as planned in order to establish quantitatively external disturbances (such as lateral propulsion) and the telemetry data is now being analysed.

The 20 seconds firing planned for 14th will be used to verify the attitue control logic system.

P

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Paolo
post Sep 9 2011, 11:43 AM
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Pandaneko, can you confirm the Google translation of today's JAXA release that the measured acceleration was less than expected?


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pandaneko
post Sep 9 2011, 02:00 PM
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QUOTE (Paolo @ Sep 9 2011, 08:43 PM) *
Pandaneko, can you confirm the Google translation of today's JAXA release that the measured acceleration was less than expected?


Yes, I confirm that the measued acceleration was less than expected. As a result the planned firing of 20 seconds on 14th will be shortened to 4 seconds in order to re-check the status of OME. Apparently, there is no change to Akatsuki after the first firing.

What does all this mean? Someting fell off again before the first firing? Oxidiser leak? Perhaps, helium, less of it remaining?

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Paolo
post Sep 9 2011, 02:31 PM
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if the "bell" of the thruster is physically damaged, gases would not expand the way they should, which could explain the "loss of acceleration"


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Paolo
post Sep 10 2011, 12:34 PM
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according to the NASAspaceflight forum the engine provided only 13 p.c. of the expected thrust. very bad news...


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pandaneko
post Sep 10 2011, 12:53 PM
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QUOTE (pandaneko @ Sep 9 2011, 11:00 PM) *
Yes, I confirm that the measued acceleration was less than expected. As a result the planned firing of 20 seconds on 14th will be shortened to 4 seconds in order to re-check the status of OME.


My apologies. Next firing will be "about" 5 seconds, not 4 seconds.

P
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pandaneko
post Sep 11 2011, 02:18 PM
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I am no longer sure if Akatsuki will be able to be of any use because today's Yomiuri newspaper says;

"the propulsion measued at the first firing was one ninth of the expected value"

I do not know how much fuel there still is left, but my gut feeling is that Akatsuki will run out of fuel. Sad...

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Paolo
post Sep 11 2011, 03:46 PM
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they can still make it to venus using the RCS thrusters, but the mission will be shorter


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pandaneko
post Sep 15 2011, 01:24 PM
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re second firing test on 14th, same as the 1st, propulsion less than expected (no value given in today's JAXA release). JAXA will think about what to do next based on these lower values.

They may announce something more concrete.

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nprev
post Sep 16 2011, 02:33 AM
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sad.gif ...thanks for the update, Pandaneko.

The (possibly) good news here is that the engine is still capable of generating at least SOME delta-V, so instead of merely jettisoning it & relying entirely on the RCS some productive maneuvers seem possible at first glance.

Big unknown here is whether the thrust vector is still aligned properly (thinking nozzle damage here) or perhaps even stable & predictable.


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tanjent
post Sep 16 2011, 05:20 AM
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I recall contingency plans to jettison excess oxidizer, but I believe that fuel not burned in the main engine can be burned in the RCS. (The engineering details are far from clear to me, because chemically speaking oxidizer should either be required or not required, regardless of what engine is being used.) If however, the excess fuel is somehow usable by the RCS then the question becomes where it can be used most efficiently in terms of changing the course of the spacecraft. If the OME is 87% less efficient that originally planned, then unless the RCS is even less efficient that that, it probably doesn't make sense to burn any of the remaining fuel in the OME.
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Paolo
post Sep 16 2011, 05:31 AM
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the OME is a bi-propellant thruster, i.e. it generates gas by the spontaneous combustion of two liquids (hydrazine fuel and nitrogen tetroxide oxidizer).
RCS thrusters are mono-propellant thrusters, i.e. they generate gas by decomposing hydrazine on a catalyst bead. Think of your car's catalyser: that is a remote relative of this technology.
This is why RCS thrusters need no oxidizer.

Edit: the OME nominally provides 500 N of thrust, and should now provide about 1/9th of that, i.e. about 55 N.
There are two families of RCS: one providing 23 N and the other 3 N (for roll attitude control only, probably not usable for trajectory control).
I am not sure that using the OME would be a good idea. it would probably use too much hydrazine to provide too little thrust (I don't have info on the fuel consumption of the different thrusters but I assume that that of the OME would be larger than that of the RCS)


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Paolo
post Sep 16 2011, 12:39 PM
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A Mainichi Daily News release: Venus probe unlikely to enter orbit fit for atmospheric observation


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James Van Allen
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Explorer1
post Sep 16 2011, 10:41 PM
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Am I right to assume that aerobraking, Magellan style, isn't plausible, if worst comes to worst and the engine is kaput?
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Holder of the Tw...
post Sep 16 2011, 11:06 PM
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Magellan was already in orbit, and very gradually lowered that orbit a bit at a time with atmospheric friction that was within tolerable limits. To bleed off enough speed all at once to make orbital insertion at the rate Akasuki is going - no, you would be fried. BUT ... if you could manage to get into an orbit with the thrusters, then I don't know, you might look into using aerobraking to get into a lower and more favorable orbit.
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