Rocket packs have drawbacks.

Jet packs carry chemical fuel but uses atmospheric oxidizer. About 10 minutes before the fuel is used up.

Rocket packs carry both fuel and oxidizer. About 30 SECONDS before the fuel is gone.

Hard to increase duration, short of nuclear energy.

Users typically fly with rocket packs at an altitude too low to use a parachute (below 30 meters) but too high to survive the fall (above 9 meters)

@nyrath
seems like, for in-atmosphere use, some kind of ducted fan gear would be a *lot* more practical for personal flight

(wouldn’t really help with the issue of operating in that no-survival envelope, though...)

@tkinias @nyrath The no-survival envelope is not necessarily a big deal. I mean, let's assume you are NOT going for a military application. So it's recreational. You can fly over water, which makes a fall pretty survivable.

@isaackuo
Oh yeah, I was thinking about practical applications rather than pure recreation—though I guess we can argue about how much the term ‘practical’ applies to military...
@nyrath

@tkinias @nyrath Cynicism aside, we can analyze how practical a system is for military uses. And any sort of jet/rocket pack looks ... really really impractical. The noise alone says, "Shoot me! Here!"

@isaackuo
yeah, unless we’re going full-on Traveller gravbelts, I don’t see something like this being terribly useful (even in the relatively practical fan version)
@nyrath

@tkinias @isaackuo

A physicist I knew gave me equations to calculate the power usage of a scifi antigravity device that obeyed the laws of physics

https://www.projectrho.com/public_html/rocket/antigravity.php#parahover

@nyrath
I love his approach to these things.

(I got some of my students using his work on alien sky and plant colors in our worldbuilding class this semester...)
@isaackuo

@nyrath
If I ever do anything with Travelleresque space opera that requires antigrav, I will be using Luke’s antigrav math.
@isaackuo

@tkinias @nyrath Something might have been lost in the translation of quoting this. Luke and I have done lots of calculations on all sorts of things through the years, and I don't think he wouldn't be careless about power vs energy.

Anyway, in case you didn't follow the calculations, the energy requirements for hovering in place - with these equations - is zero. Gravitational potential energy is neither lost nor gained during hover.

@isaackuo @tkinias

If you like, you can give me your comments on his discrepancies and I will add them .

@nyrath @tkinias I'll write up an addendum commentary and run it by Luke Campbell first.

@nyrath @tkinias Here's my addendum commentary (I ran it by Luke first):

ISAAC KUO:

To clarify, the equations calculate energy requirements, rather than power requirements. To get the power, you divide the energy by the time in which to make the change.

[part 1 of 2 or 3, because Mastodon sucks]

@nyrath @tkinias

But for the original question, the point is moot. In order to hover in place, the energy requirement according to these equations is zero, and zero divided by any time interval is also zero - thus the power requirement is zero. (Luke Campbell explains this principle with the example of "a chair".)

[part 2 of 3, because Mastodon sucks]

@isaackuo
Presumably we have some amount of power, though, to keep the device ‘on’—if the antigrav turns off you fall like a rock, so that suggests *something* is consuming power to keep the lights on (as it were).

Perhaps this is the antigrav equivalent of keeping your superconducting magnets cool?
@nyrath

@tkinias @isaackuo @nyrath

I really want to see, sometime, antigrav packs with big radiator fins blazing cherry-red from dissipating the energy returned in the course of slowing your plummet from rock-like speeds.

@cerebrate @tkinias @isaackuo

In a few Larry Niven stories with something like that. He called it a "gravity drag"

@cerebrate @tkinias @isaackuo

Larry Niven mentions the gravity drag in his short story "Flatlander".

"...The beautiful thing about a gravity drag is that it uses very little power. It converts a ship’s momentum relative to the nearest powerful mass into heat, and all you have to do is get rid of the heat..."

I'm not sure what that means, exactly. Sort of like aerobraking without the air. Needs a heat radiator. How does it know which mass is nearest?