With current propellant/motor Isp and assuming a full from-scratch design (i.e. not optimizing for low cost using existing engines and avionics), what's the lower bound on how small an orbital launch vehicle could be if payload capacity is not a concern?
In other words, suppose some company/country/rich person wanted to put a very particular grain of sand into low earth orbit without using an existing vehicle and didn't care about the program scaling to bigger payloads, how big would the rocket need to be?
@azonenberg Isn't fluid dynamics, at small scale, really iffy?
@miermont As an initial random example, according to some very rough numbers and assuing you start at high altitudes with negligible air drag, a 30kg rocket with 1kg dry mass (i.e. 29kg of propellant, 1kg of airframe/payload) with a 2800m/s exhaust velocity (typical for APCP solid fuel) would give a delta-V of 9.5 km/s, which should be enough to go from a standstill to LEO.
So if you managed to make an upper stage that tiny and overpowered, and stick it on top of a first stage that got you above most of the atmosphere (but not needing to impart a ton of horizontal velocity), you could plausibly end up in orbit.
@miermont I mean, I expect it'll still be a "large" rocket (like 10m tall or so) because it has to be able to lift the motor itself, plus all of the fuel to accelerate the motor and not-yet-consumed fuel enough that you end up with the upper stage in LEO.
I'm just wondering how big you'd need to go, and what would make sense propellant wise. For a really small launcher you'd probably be well served with something like APCP because no need for heavy tanks and plumbing. Lower Isp than cryogenic liquid propellants but would probably also be lower dry mass.
Several amateur folks have sent model rockets above the Karman line, but none got anywhere close to orbital velocity. How far are we from a group of motivated hobbyists putting a GoPro and radio transmitter in LEO just because?