Hacker News Reader

Comment by pauljurczak | Hacker News Reader

> Exotrail’s nanosatellite thruster runs on 50 watts of power and is equivalent in volume to 2 liters of soda.

Don't you confuse it with 2 liters of milk!

Perhaps an American will confirm but I believe that 2L bottles of soda are one of the few common items sold in metric quantities in the US. Smaller soda bottles are measured in fl. oz. and milk in gallons.

Correct.

A writeup for a cubesat I once tangentially worked on measured the propellant in "shots" (IIRC multiples of 44mL), which I always thought was hilarious.

Americans will do anything to not use the metric system, lmao.

It’s a real pity that Americans don’t talk about how awesome our base-2 volume measurement system is. All the benefits of metric and it’s computer friendly. If our length, area, and mass systems had been suitably adjusted at the same time, that’d’ve been the bees knees!

Only a a subset of our volume measurements are base-2. There are 16 tablespoons in a cup, 2 cups to a pint, 2 pints to a quart, and 4 quarts to a gallon, plus a few lesser used units in between (for example, a "gill" is apparently a half-cup) which is all well and good. But then there are 3 teaspoons to a tablespoon, and an assortment of units larger than a gallon: 63 gallons in a hogshead; a barrel, or half-hogshead, with 31.5 gallons; alternatively 42 gallons in an oil barrel.

Personally I think the "best" US unit is Fahrenheit, though only when used for human purposes. Going from 0-100 takes you from very cold to very hot, roughly the range of reasonable temperatures for a human to live in. A bit outside that range is also survivable with a bit more careful preparation. A place which experiences temperatures from 0-100 across a year isn't outrageous. Compare to Celsius where 0-100 ranges from only somewhat cold to far beyond what humans can reasonable exist in. Fahrenheit temperatures are just a bit more friendly to work with in my biased opinion.

This is exactly my take on the metric system. I'm all-in, except for Fahrenheit for human-related things.

Along with the benefits you mentioned, the Celsius scale, being a scale, does not have most of the benefits that the rest of the metric system has. You don't take advantage of powers of ten (we don't talk about kilodegrees Celsius), and you don't do conversion to other units. For those things, you need Kelvin anyways.

Fahrenheit makes it really awkward to think about whether it's wet, snowy or slippery ice outside. Celcius is great for that, emphasis on the phase transitions of water.

I think that falls under the same temperature intuition that people will naturally get about whatever system they're using. Just like you have to memorize that 70F or 20C are roughly room temperature, you have to memorize that 32F or 0C are the freezing point of water. I'll admit that this is a tiny point in favor of Celsius, but is mostly won out because thinking in terms of 0 to 100 is more convenient than thinking in terms of roughly -18 to 38, and a "tens" of Fahrenheit (like "it'll be in the 70s today"), is considerably more informative.

My understanding of the 3U cubesats that these are intended to be used on was that they weren't much bigger than a 2 litre bottle. Am I way off in my understanding, or do these just use most of the available volume?

It's simple, really. A 1U cubesat is defined as a 10 cm by 10 cm by 10 cm device, i.e. 1 litre in volume.

A volume of 2 litres therefore implies a 2U unit, in other words 20x10x10 cm (the actual dimension of the 20 cm part can be chosen arbitrarily).

Ah didn't realise the dimensions were quite that small! That does mean that this takes up 2/3rds of the cubesat, which sounds like it would be impractical for many purposes.

This one flew on a 6U (~35x20x10) and that's really the minimum size for this kind of engines. And even on a 6U, the power limitations make us think twice before putting an engine, it really has to be needed.

What does this even mean? DeltaV would have been much more understandable.

This comment isn't trying to convey the capability, it's talking about the physical size. Perhaps you want to know about the DeltaV, and perhaps at some point that should have been mentioned, but at this point, that's not what they're talking about.

You cannot really talk about deltaV capability for a thruster. You can talk about specific impulse, power usage, power dissipation, total impulse. But deltaV implies you know the size of the satellite, which varies for all their customers.

Reply on news.ycombinator.com