SpaceEngineer & Strelok
Strelok
Okay, let’s break this into layers. If you were to build a propulsion system that slashes delta‑v while keeping weight in check, what would be the first constraint you’d lock onto?
SpaceEngineer
The first thing to lock onto is the propellant mass fraction – basically the mass ratio of propellant to the total vehicle mass. If you can’t keep that in check, the delta‑v you’re chasing will always come at the price of a heavier craft.
Strelok
Good, but that’s just the headline. What’s the actual propellant? Liquid hydrogen? LOX? Or a solid grain? Each gives a different Isp, and that changes the mass ratio curve. Also, don’t forget structural mass – a light frame isn’t enough if the engine’s heavier than the fuel pack. Give me the numbers, and we’ll see if the design really holds together.
SpaceEngineer
I’d start with a liquid bipropellant combo – LOX/H2 gives the best specific impulse, about 450 s for a modern cryogenic engine, but it’s a lot of weight in the tanks. A LOX/LCH4 mix drops the Isp to roughly 360 s but the propellant density is higher, so you can fit more mass in the same volume. For a solid grain you’re looking at 140–160 s, but the structural mass of the grain and casing adds another 15–20 % of the propellant mass. If you target a structural mass fraction of 10 % of the total, a liquid system with a lightweight composite pressure vessel can push that down to 8 %. So with LOX/H2, you’d have a propellant mass fraction of about 90 % and a structural mass fraction of 8 % – that’s the sweet spot for high delta‑v without a runaway weight penalty.
Strelok
Nice numbers, but remember the tanks will be a weak point – cryo tanks need heavy insulation, which creeps up that 8 % structural figure. If the insulation takes another 2 %, you’re looking at 10 % again. Maybe tweak the methane mix for a lighter envelope and still keep Isp high enough. Keep a margin on the mass budget, or the whole thing will collapse on the first load test.
SpaceEngineer
You’re right, insulation is a silent weight killer. With methane the tanks can be thinner, but you still need a decent thermal shield. I’d look at a high‑temperature composite lined with a lightweight vapor‑gap layer, which cuts the insulation mass by maybe 30 %. That brings the structural fraction back to about 8 %, and you still get a 360 s Isp. Adding a 5 % margin in the budget should give us a cushion for the first load test.
Strelok
Sounds solid, but don’t forget the engine mass. Even with a lighter tank, a high‑performance methane engine will still add a few hundred kilos, and that pushes the overall mass fraction up. Keep an eye on the thrust‑to‑weight ratio for the first stage – if you’re still hunting 450 s, you’ll need an extra 10 % structural buffer or the vehicle will buckle under its own weight. Check the margins on the pressure vessel and the insulation thickness; a 5 % over‑design on those usually saves you from a first‑flight failure.
SpaceEngineer
Engine mass is the real bottleneck; a high‑performance methane engine sits at around 200 kg, so the vehicle’s dry mass climbs fast. If we keep the pressure‑vessel and insulation to a 5 % over‑design, the total structural fraction stays near 9 %. That leaves just enough margin to hit a 4:1 thrust‑to‑weight on the first stage without the frame giving way. Keep tightening the propellant fill level and the tank shape – every gram saved there pushes the delta‑v up.
Strelok
Nice, but “just enough margin” is a risky phrase. If the engine stalls on a mis‑tuned thrust curve or the tank leaks, that 4:1 ratio drops fast. Consider a staged approach—get the first burn to a clean 3.5:1 and then add a margin in the propellant load. Also, keep a contingency for thermal cycling; a single micrometeorite puncture could shift the whole balance. Tighten the shape is good, but don’t let the weight savings compromise the structural integrity under load.
SpaceEngineer
You’re right, a 3.5:1 T/W baseline is safer. I’ll add an extra 8 % propellant load to cover any engine hiccup and give the tanks a thicker, but still lightweight, liner to survive a micrometeorite. The pressure‑vessel will be a double‑wall composite with a sacrificial buffer; that keeps the structural margin high while we still hit the delta‑v goal.
Strelok
That extra 8 % is a nice safety net, but it also drags the dry mass up. Make sure the buffer layer doesn’t become a permanent weight penalty; you could use a single‑layer sacrificial film that tears off on impact, rather than a full second wall. Keep the T/W calculation tight and double‑check the thermal gradients—any asymmetry will create a bending moment that your double‑wall won’t cancel. And remember, the best weapon against micrometeorites is a predictable, symmetrical load, not a haphazard over‑design.
SpaceEngineer
A single sacrificial film is the way to go; it lets the main composite keep its mass down and only loses a few grams if struck. I’ll model the load curve to stay within a 2 % symmetry tolerance so the bending moments stay negligible. That keeps the pressure‑vessel light but still able to handle the thermal cycling. And the extra 8 % propellant is kept in reserve, not as a permanent weight—withdraw it after the first burn if all goes as planned.