Most enviromentally friendly Propellant choices

[Incarnation of Chaos]

The most enviromentally friendly propellant would be LH2 and LOX produced with electrolysis of water; you could also intentionally exploit the action of high-energy gamma rays inside a critical reactor to cause hydrolysis of water. However running a Nuclear reactor intentionally critical (In the sense of close to meltdown; i know all reactors are "Critical" in another sense of the word) to produce fuel is just bad ideas, so you're better off just using a conventional reactor or solar power to produce LH2/LOX. Which also could serve as a chemical battery for your renewables if you're doing this on a massive scale ironically, and since going 100% renewable also normally requires excess production to cover fall/winter months (around 2-3 times base load).....this actually seems like a great idea....except having massive stores of cryogenic oxidizer on hand is also bad ideas....

You then have your Hydrolox rocket loft a NTR upper stage which is just using LH2 or NH4 into LEO, and then go from there.

But i think the focus on "Enviromentally friendly Propellent" really misses the big elephant lurking in the corner; the dirtiest part of any industry in Aviation isn't burning hydrocarbons....

Aluminum is incredibly energy-intensive to produce from Bauxite, and accounts for more pollution than i think most people are aware. Production of Steel REQUIRES emitting CO2, as you must blow pure oxygen into your molten iron to first remove the Carbon that's already in it (It becomes CO2 after bonding to the Carbon). Then add the correct amount of carbon and whatever other additives/alloying metals your customer desires, Concrete also requires emitting CO2 both in production (Which could be eliminated, but not in a economically viable way) and while setting.

Composites also aren't eco-friendly, and disposing of them isn't really something anyone has figured out. The production of the glues and solvents to work with them also isn't something you'd call eco-friendly....

Now does that mean i'm saying we shouldn't come up with solutions? Absolutely not; we should always be thinking of different ways to do things! Even if they don't bear fruit; they might be the missing piece for someone down the line. But i really get tired of the discussion about propellant; cleaning up the production chain before the rocket is on the pad would reduce CO2 emissions far more than even if every single future rocket launch was Hydrolox and reusable. Getting more electric cars on the road and powering them via renewable sources would offset a massive increase in launches.

I'm just going to point you to Everyday Astronaut's video; he did a decent job of explaining it. And he also pointed out, but didn't breakdown the impact of production.

[wafflemoder]

Realistically, any form of space elevator, space tower/space fountain, launch loop, or orbital ring will be more environmentally friendly in the long run as no propellants are needed, and all components are multi use. But, its nonsensical to say that any of these methods are "propellants". 

The original question was about the most eco-friendly propellant, not the most eco-friendly surface to orbit transportation system, or even the most eco-friendly rocket design. These are all complex and intertwined questions for sure, and the question is framed in such a way that some pollution will be unavoidable from the production of the launch vehicle and the propellants. It is even fair to assume that a "nuclear space faring civilization" is one that uses nuclear propulsion (either in part or exclusively), which may make any chemical propellants considered irrelevant if that is the case.

Based on all of this, and trying to pool everything together, here are 4 answers previously suggested that could be valid answers.

• LH2 would be the cleanest nuclear propellant, if only the impact of the launch itself is considered.
• H2O (water) would be the cleanest nuclear propellant if the impact of propellant production is considered, with minimal effort to produce. It remains a possibility for LH2 to be cleaner than water if there are significant advances in how its produced, but currently this is not the case.
• Hydrolox would be the cleanest chemical propellant, ignoring the impact of propellant production.
• Either methalox or hydrolox would be the cleanest chemical propellant if the impact of propellant production is included. As methane is more naturally abundant than hydrogen, and hydrogen typically being produced from methane or water, methalox may be cleaner overall. But this also depends on how these technologies would develop.

[wumpus]

Pretty much everything emitted beyond a planet's orbit will be swept out by the solar wind into interstellar space, probably decaying into iron or something over the billions of years before anything encounters it (or more likely just decaying into energy and winking out of existence after 10-100s of billions of years with no encounters).  Production of said material matters (and again I'll nominate argon), but spewing nuclear material is a non-issue.  Note that unlike leaving chemical residue between planets, nasty nuclear issues clean themselves up in days, rougher stuff in years (just in case it gets near a planet or trapped by an asteroid), and any measurable radioactivity will be gone after millions of years.

Between orbit and escape velocity I'd like to nominate ion drives driven by argon.  Obviously, xenon is just as inert as argon, but argon is so wildly common (more common than carbon dioxide in the atmosphere) that scaling up production to any level isn't a big deal.  Power is obviously required, but that's typically solar (especially in Earth orbit.  Mars and beyond probably requires nuclear power, but that's pretty green on its own).

Generating fuel/power on Earth is always the tricky part, and expect much of the exhaust to remain on Earth.  Hydrolox is famed for providing nothing but water, but produces plenty of carbon in production.  Kerolox and methalox produce carbon dioxide during flight, but also don't appear to release more during production (although recent reports imply that this is completely wrong with respect to methane.  Methane production appears to be far worse than we thought).  I'm no fan at all of hypergolics, and don't think they really should be used for first stages at all, but I understand that they make a lot of sense for ICBM applications, and ICBMs get the funding that space programs piggypack on (and not vice-versa).

[kerbiloid]

Actually the safest fuel is air.
I.e. hot nitrogen.

***

Maybe the rocket principle is just too limited to be used by the really future tech.
It's obviously not an option for regular interstellar flights, even crewless. It's hardly an option for mass passenger LEO flights.

Maybe the LEO shuttle should be a magnet interacting with the magnetosphere.

Also maybe it's easier to develop some kind of energy transmission to keep all fuel and power source on ground, than try to place it inside every vessel.

***

Back to the rockets.

It's not normal when a regular shuttle withstands 4 g overloads on launch and aerobraking.
It's relatively appropriate for brave spacemen, but it's silly for regular passengers (~1.5 g in regular planes).

Also it's not normal to protect the regular spaceship with expendable heat protection.
It's not normal to fly in plasma cloud, and so on.

A regular passenger shuttle should ascent and descent at 1.5 g or less.
It shouldn't fly in plasma cloud or hit the ground by shockwave.

So, it should slowly get to ~30..40 km at plane speed, when air is enough thin to stop dragging, then accelerate to the orbit.
On return it should decelerate before entering the atmosphere, then slowly get down from 30..40 km at plane speed.

***

So, it should make horizontal 8000 m/s at 1.5 g.
Say, it's hovering by engines, so from 1.5 g in total it makes sqrt(1.5² - 1²) ~= 1.1 g horizontally. So, the "burn" time ~800 s.

This means it would spend ~800 * 9.81 ~ 8000 m/s for hovering.
Actually it will spend ~sqrt(2) * 8 = 12 km/s because the faster it moves horizontally, the less thrust it needs to spend vertically.

To get to the 40 km altitude before the acceleration, at subsonic speed, it has to spend 40 000 m / 300 m/s * 9.81 m/s2 ~ 1.5 km/s.
Basically it can do this by throwing down the air by thermal nozzle.

Same delta-V it needs to return.

***

So, a typical comfy LEO shuttle flight requires ~50 km/s of delta-V, almost totally above the atmosphere.

The shuttle needs no heat protection or shroud, it needs just a tin hull to hold 1 atm.
(Maybe it should have it for flight abort).

The shuttle is single-staged, the fuel tank is plane-like.
So, the fuel is ~25% of total mass.
Mass ratio ~4/3.

ISP*g required ~50 / ln(4/3) ~= 173 km/s = 200 km/s.

***

Obviously, no chemicals can produce this ISP, so it can be only a gas heated by a reactor (onboard or remote).

So, let's just fill the tank with air (on ground or while ascending to 40 km), and use this air as chemically inert monopropellant.

We can treat this air as pure nitrogen (or separate it and store only the nitrogen).

So, the propellant is hot nitrogen.

***

Molar mass = 28 g/mol (in molecular form)

Thermal speed ~200 km/s corresponds to the temperature (2*10⁵)² * 28*10^-3 / (3 * 8.3144) ~=45 MK.

Obviously, atoms are fully ionized at such temperature, so actually the molar mass is 14 g/mol.
But this doesn't matter.

What matters, no material can survive at that temperature, so the ionized nitrogen jet can be only magnetically accelerated.
So, they must heat the nitrogen up to ~10 kK to ionize and accelerate it in a magnetic nozzle up to 200 km/s.

***

Total kinetic energy to receive during the acceleration or deceleration ~= 20 000² / 2 = 200 MJ/kg.
Let it be 300 MJ/kg (potential energy and so on).

Acceleration duration ~= 800 s.
Average net power = 300*10⁶ / 800 ~=400 kW/kg.

***

Say, it's a beam of energy received from the orbital powerplant.
Say, 90% of energy gets used, 10% heats the receiver.

So, heat power is ~40 kW/kg.

Say, the receiver temperature is ~1000 K.

Heat radiation power density = 5.67*10^-8 *1000⁴ ~= 60 kW/m².

So, the required area of beam receiver is ~40 / 60 ~ 0.7 m²/kg.

Say, the ship is lens-shaped, diameter : height ~ 6:1
Say, it's average density is ~250 kg/m³.

Then its mass is ~ 250 * pi * diameter² / 4 * diameter / 6 ~= 33 diameter³, kg

The receiver area = pi * diameter2 / 4 ~= 0.8 diameter² .

So, it needs
0.8 diameter² >= 33 diameter³ * 0.7

diameter <= 0.8/(0.7 * 33) = 0.04 m.

Actually this means that car-sized shuttles can be powered by a beam, but bigger ones need an onboard powerplant.

As the onboard powerplant is obviously thermonuclear, the LEO shuttles can be of two types: small drones and individual copters, and huge platforms.

***

Say, you have a huge platform with 200 km/s of exhaust speed, 40 km/s of delta-V above the air without additional tanks.

This means you can use it also for interplanetary flight at ~20 km/s hyperbolic speed.
So, 40..120 days to Mars. With additional tanks - faster.

So, this LEO shuttle is at once an interplanetary shuttle for inner planets, and you don't need another.

Hence you get a universal standard shuttle using nitrogen, the ultimate environmentally friendly ecofuel.

***

They are shuttling between Earth and LEO, LEO and LMO, LMO and Mars, and in all other places.

Same shuttles are shuttling inside the Jup and Sat sat systems.

They shuttle everywhere where they can get nitrogen.

Around Titan they are shuttling especially with pleasure.
And around the Pluto and Triton.

***

And as you can separate its isotopes, ¹⁵N gives a relatively aneutronic fusion fuel, much more cheap and accessible than boron.

Also you can separate the N isotopes right onboard, so the ship can infinitely refuel in situ from air (or ice).

So, you use deuterium in monumentally huge onground powerplants, and nitrogen in the transplanetary shuttles. Screw the helium, who needs it.

***

So, the ultimate best ship is the nitroship.

Edited April 14, 2020 by kerbiloid

[Spacescifi]

15 hours ago, kerbiloid said:

Actually the safest fuel is air.
I.e. hot nitrogen.

***

Maybe the rocket principle is just too limited to be used by the really future tech.
It's obviously not an option for regular interstellar flights, even crewless. It's hardly an option for mass passenger LEO flights.

Maybe the LEO shuttle should be a magnet interacting with the magnetosphere.

Also maybe it's easier to develop some kind of energy transmission to keep all fuel and power source on ground, than try to place it inside every vessel.

***

1.  Magnetic shenanigans are possibe, but not simply magnetic. I have read that magnetizing air with only a ma gneticfield is really hard, but it can be more easily done with lasers fired in a circular way. Magnetohydrodynamics and plasma might help too.

https://physicsworld.com/a/optical-centrifuge-magnetizes-molecular-gas/

 

2. They tried this already. Look up Myrabo's lightcraft. It worked great going up, but past enough hundreds of meters and the laser begins to lose focus due to distance, and the craft topples over, which is bad (since that will mean it will fall out the sky).

To make it work would require zepplins tetheted to the ground to fire lasers at all the up until the air ran out.

Distance is a problem that can be engineered away.

The funding is not there though.

Edited April 14, 2020 by Spacescifi

[farmerben]

Ethanol mixed with hydrogen peroxide is an environmentally friendly choice.

While it's not the highest ISP for a main engine, it is totally viable for maneuvering thrusters.   For a long mission to the moons of the gas giants, you would want a lot of thruster capability, but who knows exactly how much.  Reserve propellant can be converted to life support supplies... oxygen and vodka.  

The hydrogen peroxide decomposes/ignites against certain hot metal catalysts.