JacobCooper wrote:I'm not sure if it's possible to call it technology. Still, I advocate for sustainability in aerospace engineering, and it's great to see that many space agencies take action to become eco-friendly.
The only truly non-polluting option for launch is H2/O2 which also happens to be the most efficient chemical option as measured by specific impulse. Unfortunately, it is much more difficult to work with H2 vs other fuel options and the low density of it imposes substantial mass penalties due to required added structure to contain the fuel.
The only major systems I can think of using this for first stage lift are Space Shuttle and Delta 4. Both were ruinously expensive and shuttle required horribly polluting solids on top of the main engines to get off the ground.
The good news is that we're seeing a big move away from hypergolics for anything other than maneuvering even by the Russians and Chinese who have stayed with them much longer than the US. We're even starting to see some designs like F9 and Starship that use cold gas thrusters or methalox for maneuvering rather than hypergolics. For those who don't know, hypergolic fuels are horribly toxic.
Going back to H2/O2, it is very well suited for second stages where the added volume of H2 vs other propellants imposes a much lower structural penalty allowing the high Isp to work for you. That said, there is a lot of added complexity for logistics adding a third propellant to handle at the launch site. In addition to this, engines are very expensive to develop so we're seeing more and more instances where the second stage uses the same engine as the first but with a different bell/nozzle optimized for performance in vacuum. It minimizes development costs and maximizes production efficiency. SpaceX Merlin and Raptor engines as well as Rocket Lab Rutherford engines are examples of this.