Satellite Launch Vehicle

Ahhh, I would tentatively have to say yes, hydraulics are an old and proven technology but they do have their disadvantages compared to electric actuators (weight?, etc) even when the high voltages are taken into account. As for the increasing in voltage, remember as voltage increases current decreases for the same amount of power. This means that you can reduce the amount of copper or other conductors, thereby reducing the overall weight (and size). Given the fact that payload's are (likely) getting more power hungry this is (probably) becoming more and more important. There is a similar push in the automotive industry to up the voltage from 12V for these reasons.

I think this qualifies as one of my most vaguest posts ever . Hope this helps.

As for the increasing in voltage, remember as voltage increases current decreases for the same amount of power.

please correct me if i'm wrong, I thought that high voltage and low amperage was safer than low voltage high amperage??

They say it's the amps that kill you, right? And from what I've been told, the more amps, the more it will "hold" you to it.

Yes it is the amps that kill you but it is the volts that push them. It is just like hydraulics. It is the juce that kills you but it is the pressure that pushes it. High current at low voltage is safer that high voltage at low current (above 10mA).

If the voltage used is DC, the risk of damages from arcing increases quickly with the voltage. Once ignited, at 270VDC, you will easily draw an arc of few cm if a few amps are available. This is not much of a problem in the vacuum of space but is dangerous while conducting tests on earth. Make sure you have a good isolation and creapage distances between connections and forget about using standard switches and relays. You need special interrupters. Semiconductors are OK since they don't produce any arc (unless they are exploding). Look at what is being done in industrial power electronics devices such as motor drives. They use high voltage DC current and get big bangs when things go wrong especially with a good amount of capacitance on the DC bus.

It takes about .003 amps to kill you, provided it flows through your chest. Ordinarily, because of the high resistance in your body, it takes voltage over about 50 volts to have any likelihood of killing you. The amperage flowing in the working circuit (let's say the 300 amps flowing in the 48 volt circuit of a fork lift truck) has nothing to do with the amperage that will flow through you it you touch the + and - leads.

At high voltages, "skin puncture" can occur, which reduces resistance dramatically. Sometimes the local current in a part of your body is enough to cause serious burns -- or in the most dramatic cases explosions. Sometimes, relatively high amperages can throw you clear of a voltage source, saving you; sometimes your heart stops entirely rather than fibrillating -- which can also make the event more survivable. There are tables you can find online that describe the effects of different amounts of voltage and amps.

But in any case, yes it's amps that kills you, but you cannot have amps without volts, and your resistance is very high.

As far as hydraulics vs 270 V -- I'd vote for hydraulics being a little safer, because simple contact with piping terminations will not hurt you. But 480 service is used safely in industrial settings all the time, and in the environment of a space craft, I'd expect things to be well-thought-out for safety.

For a satellite it probably doesn't matter. The procedures and rules governing operations and assembly of these space vehicles is a little beyond the normal consumer white goods that say "NO USER SERVICEABLE PARTS" on them.

With respect to the hydraulics, the colder the temperature the more likely the fluid will not flow properly, even with the oil used for airplanes.

I would vote for elecric operated actuators without any lubricant.

Check the viscosity ratings of different oils at - 50 degrees F.

Jack, that sounds reasonable to me. Weight savings are always a big factor. I've worked on several payloads, but don't know anything specific about this launch vehicle design change. I imagine speed of response, weight, reliability, cost, etc were all factors.

As far as safety and reliability, more difficult things are worked around than high voltage.

Bijuprasadb, one issue is that grounding must be done correctly for satellites and launch vehicles - the possible results of an errant spark are catastrophic. Same deal for the fuel tanks on airliners - bad grounding will eventually result in a very bad day (this and/or a short was probably what caused the TWA flight 800 explosion). That goes for handling low voltages, high voltages, or no voltage. Larger voltages than 270 are handled regularly in many industries. Launch vehicles, also like airliners, are designed to withstand lightning strikes, and frequently do.

ddk, if you're talking about lubrication for a part exposed in space, outgassing in vacuum is a bigger deal than cold for oils - most oils would basically turn into asphalt as they boil off and leave some longer hydrocarbon chains behind. Various special greases and dry lubricants are used on spacecraft mechanisms, or, like you suggested, they are designed to not need any lubrication.

The other type of actuator I can remember being used is like a closed capsule of, I think, petroleum jelly which expands when heated & contracts again when allowed to cool. I can't remember the name of these as it's been a while since I've worked on satellite design but they were used as door actuators & similar applications.

For separating a satellite from the launch vehicle, and other release mechanisms, explosive bolts are the tried and true method, but are largely being replaced by NEA's (non-explosive actuators, creative name), especially for things like doors, like you're talking about. I've heard of the jelly thing, but have never seen it. Frangibolts (http://www.tiniaerospace.com/frangibolt.html) are widely used. They are basically a notched bolt surrounded by a cylinder of NiTi shape memory allow, like a long washer. A heater causes the NiTi to expand, adding tension to the bolt until it breaks at the notch. There are several other arrangements, but that is the basic release one. Very cool, very reliable. It sounds like the petroleum jelly was similar, maybe the predecessor of frangibolts.

Just remembered - you might be talking about paraffin actuators. Those are used for smaller, more delicate mechanisms and are often re-usable.

The 'jelly' actuator was not really like the separation bolts you mentioned. It does not give a high strength hold but is reversible. Where I remember using it was to open & close the door protecting a telescope, the door was lightweight & had a separate latch to hold it closed.

Just edited my last post - I bet you're talking about paraffin actuators. I haven't used them, but I know they are used quite a bit.

That's the one, paraffin actuator.

I just stumbled across the information I had on paraffin actuators whilst looking for something completely unrelated in my infallible filing system. They appear about half way down the web page.

Interesting question. I would say that a major initiative behind the migration from hydraulic to electrical actuators is the automotive industry. As long as we drive automobiles we will continue to have accidents that destroy hydraulic components such as brakes, steering, etc. and dump hydraulic fluids into the environment. As a result, there is pressure to eliminate these polluting actuation systems. Electrical actuators will probably win this contest, and a huge amount of development money is being invested in these devices and their safe use. Therefore, I suggest that a symbiotic relationship exists between the automotive and the space industry and that relationship is the driving force (excuse the pun) behind the development of safe electrical actuation systems.

I was mentioning about the thrust vector control (TVC) actuators of launch vehicles. These are used for engine gimbal control for the vehicle steering. Now a days high power electromechanical actuators having maximum force capability of 11 KN (11,000 Kgf) is used in launch vehicles which were conventionaly handled by hydraulic actuators. These actuators have roller screw/ballscrew to convert the rotory to linear motion. Can anybody explain about any additional issues if any in handling high voltages associated with high power electromechanical actuators in satellite launch vehicles?

I believe one may agree that the duty cycle for the actuator in flight is important and there can be little doubt that hydraulic actuators can hold a stall load for longer duration than does a electrical actuator if such a load/duty cycle were to demand. Nevertheless it will be still possible to have a suitably designed power electronic device(IGBT/MOSFET) in conjunction with its heat sink for the electrical actuator drive system.

I'm no Rocket scientist but it seems to me that oil would boil in outer space and at -247 f What did not boil would freeze.

as for hear on earth I worked on GM cars that use eclectic steering motors for assist only not drive by wire and they failed all the time but are relatively cheap and when they fail its worse than a Mack Truck to steer. We also are using 36v 64v and 300 v systems for Hybrids