Akatsuki Venus Climate Orbiter Options

[Paolo]

I thought it was time to start a separate thread on this mission, launching soon
some good medium-resolution images of the spacecraft are available on JAXA digital archives
http://jda.jaxa.jp/jda/p3_e.php?time=N&...mp;mission=4066

[monty python]

I've always wondered why the cassini people would risk a stuck closed engine cover. The risk for such an impact must have been thought to be well above zero or just compleatly unknown.

[rlorenz]

I've always wondered why the cassini people would risk a stuck closed engine cover. The risk for such an impact must have been thought to be well above zero or just compleatly unknown.

Where dust hazard is high, I think Cassini tends to fly through HGA-first

I was amazed at the IEEE Aerospace Conference this year to see a whole talk devoted to the engine cover (quite fascinating)

I believe Cassini ops consider the pros and cons of closing it after each burn (i.e. if another burn is coming up in a
couple of weeks, and we're at apoapsis away from ring particle hazards, leave it open as the (small) risk of sticking
is judged more than the (small) risk of impact). Obviously for long periods, and those close to rings etc, better to close
it. Worst case, the whole thing can be jetissonned by pyro.

(Really worst case, I guess you could blast through it with the engine, though you might get some small thrust
asymmetries)

[pandaneko]

There is an interesting article in today's Asahi newspaper. I will translate this first, because it is informative to lesser knowleged people like me. This article makes me think... It goes like this:

Akatsuki's failure is now known to have been caused by a mechanical valve mulfunction. Mechanical valve failures have been notorious with space probes. Fundamental solutions require desing alterations and it is feared that Hayabusa 2 might also be affected by this.

These mechanical valves are placed in the piping system which pumps fuel into the combustion chamber. JAXA said that test firing after launch did not show mulfunction, but during the retro firing the valve only opened only 1 to 10% of expected gap. As a result not enough fuel was supplied.

Burn temperature, if perfectly mixed fuel and oxidant is used, will go up to 2000 degrees and the nozzle will start melting. Therefore, typically, more fuel than required is supplied to cause in-efficient burn so that burn tem stays low.

However, with Akatsuki valve mulfunction meant more violent burn. Akatsuki was equipped with a ceramic nozzle, which is 200 degrees more resistant than alloy nozzles at 1500 degrees. However, this limit is thought to have been exceeded after two minutes of burn.

1993 NASA Mars probe had the same problem, the valve not closing and fuel flow reversed and it is thought that the probe exploded in mid space. In view of this JAXA added a second valve in a series. This would decrease the probabilit of reverse flow, but it doubles the probability of valve mulfunction. Nozomi went up in 1998 and the valves mulfunctioned and Nozomi was lost.

Therefore, JAXA went back to one valve system with Akatsuki and conducted extensive ground testing, but still the valve failed yet again.

With HTV that will be going up to ISS in a few weeks' time is equipped with two mechanical valves in a series and tow of this are placed in pararell (4 mechanical valves, that is) because HTV is made to manned specifications. Its computing system is 6 fold redundant.

On the other hand ISAS/JAXA probes are typically at around 0.5 ton, compared with NASA/ESA's tons.

Akatsuki's failure is right now even driving JAXA into re-checking HTV components and even Hayabusa 2's launch may be affected by all this. Substantial design alterations may mean that Hayabusa 2 may not go in 2014.

Pandaneko

I would have thought that larger and heavier probes are easier to make and cheaper...because materials cost must be less of a problem, and lots of space between components meaning easier design, no?

[pandaneko]

Q1: Design philosophy defference between fuel tank and oxidant tank

Q1 content: Fuel tank side has a diaphgram and the oxidant tank has redundant latching valves. What is the difference between these arrangements? 【

Answer by JAXA: page 9 of investigation 1-2

Answer contents by JAXA:

1. RCS: 1 liquid propulsion system

1.1 We adopted 1 liquid system RCS (attitude control thruster system) because it has been used extensively.

1.2 As a result, RCS does not need oxidant and receives fuel only from fuel system.

1.3 For this reason, our design is such that either one of the two systems will allow 3 axis attitude control and delicate orbital insertion firing. Thus, with each thruster and LV-F they are made into a redundant system.

1.4 With pipings etc, which cannot fail without external forces, no redundancy has been employed.

2. OME: 2 liquid system

2.1 We adopted 2 liquid system for OME (orbit change thruster)

2.2 For that reason, there are 2 supply systems for OME, fuel supply system and oxidant supply system

2.3 We adopted, basically, mono (or uni) system for reasons of resource restriction, use frequency, reliability by trading off.

2.4 With 2 liquid system the wrong mixture of fuel and oxidant will lead to an explosion in the worst case (*), 2 fold redundant system has been adopted to prevent this happening.


2.5 With the fuel tank side, we use a rubber surface to achieve fuel vapour and fuel liquid seperation and prevent fuel reverse flow upstream by a CV-F valve.

2.6 With the oxidant tank side, since there is no rubber surface resistant enough to oxidant we used CV-O and GLV-1 to prevent vapour going upstream.

(*) With Mars Observer, it is thought that it exploded with the wrong mixture just before reaching Mars.

#: It is possible to do liquid and vapour seperation using a metal plate. However, we did not adopt it for Akatsuki because of its low efficiency in extracting out oxidant, and repeated operation is not possible, and therefore, ground testing not possible.

2.7 With the one way mechanical valve, it is true that we did not incorporate redundancy, but all other space agencies adopt the same strategy and it is a generally accepted practice.

As will be shown in 2.2 of this report, we either do not adopt redundancy, or alternatively, we can use a simple series of valves. The only system of pararell and series redundancy (4 valves in all, against both leak and blockage) has been adopted only with HTV in this country which is designed for manned specification.

With GLV-1 and GLV-2, they have benn made into a pararell system to ensure redundancy against blockage.

3. High pressure gas system

3.1 Redundancy has been incorporated both against open/close mulfunctions with regulater valve, latching valve. This policy has been adopted with Hayabusa (1)

Pandaneko

This section has a continuation and I will work on that tommorrow. P

[rlorenz]

2.5 With the fuel tank side, we use a rubber surface to achieve fuel vapour and fuel liquid seperation and prevent fuel reverse flow upstream by a CV-F valve.

2.6 With the oxidant tank side, since there is no rubber surface resistant enough to oxidant we used CV-O and GLV-1 to prevent vapour going upstream.

Very interesting, Pandanenko - thanks again for your hard work on these translations.

Something I do not understand myself is why CV-F is necessary, if its purpose is to prevent
mixing of fuel and oxidizer in the upstream pipework (one theory for the Mars Observer failure -
although only a theory - another holds that the regulator failed..)

In principle the polymer bladder that keeps the fuel at the outlet end shouldnt let any fuel into
the upstream pipework anyway. Is CV-F there to guard against diffusion through the bladder ?

I can see why a valve is necessary if using spin (a la Giotto) to keep propellant at the outlet side
of the tank, or using a shaped metal disk (surface tension propellant management device - which
I think is what the text says JAXA decided not to use on the oxidizer side) but not so much with a
bladder. Tough call, balancing one thing that is not supposed to happen (valve sticking shut, as
here) against another thing that is not supposed to happen (fuel leaking upstream through bladder)...

Maybe some propulsion experts out there can comment on how common practice it is to install
these valves

[pandaneko]

What follows is the continuation from the earlier section re design philosophy

For information:

Philosophy regarding domestic satellites 2 liquid system mechanical valve redundancy

Single: (single system)

*: same philosophy as with Akatsuki and examples include

Engineering test satellite, type VIII, such as KIku 8 (ETS-VIII)
Lunar satellite, Kaguya (SELENE)
Ultra high speed internet satellite, Kizuna (WINDS)
Mars probe, Nozomi (PLANET-
Hayabusa (MUSES-C)
Infrared astronomical satellite, AKARI (ASTRO-F)

Series redundancy:

Reliability against vapour mixing is better, but blockage risk is increased

Examples include:

Communication engineering satellite, Kakehashi (COMETS)
Data relay satellite, Kodama (DRTS)
Multi-purpose transport satellite, Himawari 7 (MTSAS-2)
Quasi something (I cannot find the right translation here, P) satellite, Michibiki
(This satellite is meant to cover GPS hungry areas)

Parerell series redundancy:

This ensures redundancy agaist both vapour mixture and blockage

An example is HTV engineering test vehicle, Kounotori 1, a supply ship for ISS


Pandaneko