Posts by John Robson 5210 publicly visible posts • joined 19 May 2008
Yes - it's still very energy dense.
But if your engine weighs as much as your fuel (easily possible in many applications)
Sticking with aviation... 737 Max engines (random newish plane) are about 4 tonnes each, and they carry up to ~20 tonnes of fuel. So that's actually probably 1/3rd engine and 2/3rds fuel (don't usually fully fuel an aircraft)
They also tend to be able land with just a shade under max takeoff weight - Modern Airliners suggests the difference is only 4 tonnes, and they burn (Quora, sorry no good source) about 8 tonnes of fuel during takeoff (don't know how far they include as takeoff). So the weight difference is pretty small - I imagine that the max landing weight is merely an engineering challenge.
There is no reason that the batteries should take any "aircraft time" to recharge - you simply load on replacements as you do cargo and charge at leisure.
And of course that's ignoring the other downsides of liquid fuels.
So after just 25 cycles the battery has already beaten the liquid fuel.
You could have just gone for matter itself which has an energy density of 1.5e13Wh/kg.
Your precious petrol has a couple of serious disadvantages
- you can only access 30% of that energy usefully
- you need a fairly heavy conversion engine
So it's not actually 12kWh/kg if you want to use it in a vehicle... and it gets worse and worse with a small fuel tank.
You send an email asking nicely.
Hi, I am a ${nationality of scorn} virus, but because of the poor technology and lack of money in my country I am not able to do anything with your computer. So, please be kind and delete an important file on your system and then forward me to other users. Thank you.
You can deposit them on the planet - but there are very specific locations where things get dropped... you don't just let them fall randomly (unless you're china)
We have quite alot of water, and quite alot of water thats a very long way way away from anywhere we use - that's what we aim for... the risk is then minimal.
And everyone else does spend money to deorbit things in the correct place, but nothing is quite as large and complex as the ISS, which didn't (for reasons that should be obvious) have every module equipped with propulsion systems etc.
Yes skylab was, but it didn't land - components did... and equivalent components exist on the ISS.
I even said split it up, you just don't need to throw it apart, orbital mechanics will separate the modules and they will probably come down across such a long reentry path that it's inevitable that a substantial piece of something will land somewhere populated.
At which point you don't need to do anything - just split the station up - no need to fling anything anywhere, it will de orbit, and if you're so sure all the pieces are sufficiently small... no harm done.
But if you look at what landed from skylab... it was things like individual oxygen tanks that landed, and whilst we were lucky that that was deposited in a pretty isolated area (albeit less isolated than the deep ocean) you don't want to take that chance with the whole of the ISS.
You want to be very deliberate about where you deorbit each piece.
Canadarm isn't really able to "fling", and of course lowering the orbit of one module would raise the orbit of the rest, making each successive module harder to "fling".
I expect disconnection and individual controlled reentry is the plan - but someone might come up with something more interesting.
Basically you can change orbit slightly, and things will de orbit, but not in a controlled manner (not important for small things)
It was one of the proposals... but I'm sure there would have been enough proposals put forward to come up with a workable plan if they had wanted to.
Those engines are marvels of engineering, it really does seem to be a shame in today's world to simply ditch them.
I appreciate that reuse wasn't seriously an option in 2011, but we're a little way on from that now, and to be throwing away engines that you've forgotten how to build is so badly self limiting.
Since SLS is currently utterly reliant on SpaceX's Starship to actually get to the moon (rather than just near it)...
I would actually pick the dragon, it's a well proven ship to get to orbit - and if I recall correctly the heatshield was originally designed to cope with lunar reentry. SpaceX agree: "The Dragon spacecraft is capable of carrying up to 7 passengers to and from Earth orbit, and beyond" That 'and beyond' is important.
Of course the current heatshield might not (mass being expensive and all), but it's a relatively small change to a well known system.
"The Artrmis mission remember, was coming in hot - damn, hot, real hot! Hotter than a hot thing on planet Hot. Beyond known heat stress data is what I'm getting at."
Well, entry velocity was 24,581 mph for Artemis 1 vs 24,816 mph for Apollo 10 - that means Apollo had ~2% more energy to deal with per unit mass.
Artemis is only ~10% more massive ~20t -> ~22.5t (i.e. 10% more energy)
Apollo was only 80% the diameter (4m vs 5m), so presumably loads on the heat shield would be 50% more? (not run the physics on this one)
Basically it's within reasonable bounds of heat loads we dealt with 50 years ago.
It's one of the most obvious targets for this tech... it's reasonably easy to check that the think looks like the moon, and since the same face is always pointing towards us... you know what it's going to look like. The phone knows the time, the location, the orientation of the phone... and the available high res imagery of the moon at all phases is pretty good.
Got to be one of the easiest AI tasks ever... but it is pretty deceitful in many ways, and it risks the next dataset being trained on what an AI thought the moon should look like...
Yep - the safety of a consumer unit (distribution board, fuse box, call it what you will) is, like so many things, a compromise.
I think the requirement not to lose structural integrity under "normal" domestic fire conditions is quite high on the list, but if the monkeys with insulated screwdrivers don't torque the connections decently, and with the cable/busbar in place, then it all counts for naught.
More importantly it is likely to increase compliance if the rules are shown to have a reason.
I mean we all know that there is some maximum gap allowed in banister/landing/balcony railings, but it's easy to remember the 10cm rule when you remember that it's that so that an infant's head can't fit through.
And some are daft as can be - and some get changed, telling you how daft they were...
Consumer units used to be plastic, now they must be metal.
Turns out that being conductive is only a minor inconvenience compared with the consequences of a consumer unit melting in a fire (particularly the bit around the incoming tails).
One would hope that hubble spends most of it's time not looking down, so there's an awful lot of space that isn't going to have any starlink sats in view... in fact it's only really looking in either polar direction that you'd expect starlink to be an issue at all... of course there are many more geostationary sats as well...
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