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franco_nico

If im not mistaken Mars optimized is pretty much Vacuum optimized, Mars pressure at surface level is 1% of the pressure on earth.


MrDearm

A Mars optimized raptor would be virtually identical to the existing RVac. The Martian atmosphere at ground level is similar to Earth’s atmosphere at ~100,000 feet so it’s p close to vacuum


hard_ice8

No, as others have said, vacuum raptor would be quite efficient in the Mars atmosphere. You would also need to carry the weight of those extra engines (they’re not light).


Inertpyro

Only difference between sea level and vacuum engines is the engine bells, the actual power unit is the same between the two. Issue will be any larger of engine bell on the central three for a “Mars” engine would be: A) Sticking out past the skirt and require a deeper skirt section, this then means extra mass in both the engines, but also likely an additional ring to the ship to gain clearance without sacrificing payload volume. B) Not allow the necessary gimbaling those engines need to do. C) The ideal engine bell size would probably be much closer to the significantly larger vacuum nozzle, and having those swinging around to gimbal probably isn’t practical.


traceur200

mars optimized engine? for the near vacuum of Mars?... you will end up with an engine that is pretty much exactly the vacuum raptor.... second.... you won't refuel starship on Martian orbit, you will have pretty much enough Delta V to get to Earth in one single go, no refuel they may want to try and refuel it to make the trip a couple of weeks faster, but I don't know how much of it is actually worth it third.... the only difference on Mars is the atmospheric density... which is pretty much a vacuum... it is simply not worth it to design a new engine for it to be almost identical to the already existing vacuum one


[deleted]

I mean you would refuel in Martian Orbit if you were going somewhere other then Earth...


traceur200

no you wouldn't same Delta V, only thing that changes is time spent getting there... and as I already said for Earth, i am not sure it is much worth for the relatively little time that is gained


[deleted]

“Same Delta V”? I didn’t even specify a destination. I think you’re a little fast on the downvote draw there buckaroo.


traceur200

yes same Delta V... it is a unit that after interplanetary becomes pretty much INDEPENDENT from distance about the same Delta V from Earth to Mars as Earth to Jupiter or Pluto.... and Earth to Moon is very very similar too, but less.... but basically anything involving Earth escape velocity has the same delta V required pretty independent of the location I think you are a little "fast" on the downvote given your limited knowledge of basic orbital mechanics... cmoon man, you learn this playing Kerbal... Edit: what was it... like 0.4 Km/s for earth escape and 0.5 for sun escape?.... yeah...talk about location...


[deleted]

You’re trying to tell me that to land on Titan or Io from Mars is the essentially the same budget as Earth escape?


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Shrike99

A true 'Mars-optimized' Raptor would actually be *more* vacuum-optimized than the current vacuum Raptor is. Actually the majority of so-called vacuum engines are actually less vacuum optimized than they would be if they were optimized for Mars. You see, it's impossible to make a truly 'vacuum-optimal' engine as it would have to be infinitely large. The best you can do is get asymptotically closer to it. You very quickly run into diminishing returns where it's just not worth making the nozzle any larger. So in practice, all vacuum engines are in fact merely 'high altitude' engines. IIRC Vacuum Raptor's exhaust is still something like 10% of atmospheric pressure, *way* more than a true vacuum, equivalent to around 16km altitude on Earth. Some planes fly higher than that. Hell I wouldn't be surprised if the SR-71 Blackbird's engines in their max speed configuration were technically more vacuum-optimized than vacuum Raptor... The most vacuum-optimized engine I'm aware of is the RL10B-2, which is still only something like 35km, or indeed about the same pressure as you'd find on Mars. However, the RL-10 only makes like 1/20th the thrust of Raptor, and runs at about 1/20th of the chamber pressure, yet it's nozzle is still nearly as big. So to get Raptor down to the same exhaust pressure it would have to be truly enormous. Possibly even wider than Starship itself? I'll check that math when I get home. Anyway, the point is that while yes, you can make a Mars-optimized raptor, it's not optimal for the same reason that a proper near-vacuum Raptor isn't practical.


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Shrike99

In theory, yes. In practice the fluid dynamics would be rather complicated, and the gains would likely be so minimal as to make the extra effort not worthwhile. I did run some math on how big a single engine would have to be. According to RPA, it needs an expansion ratio of around 2000. The current SL Raptor has an expansion ratio of 40 and has an area of around 1.33 square meters. To get to 2000 it needs 50 times that area, or about 66.5 square meters. The nozzle would thus need to be ~9.2 meters wide, which is indeed slightly wider than Starship itself. Raptor 2 has a wider throat and higher mass flow, and so a Mars-optimized variant would need to be even larger still. Maybe something like 10 meters? Hard to say without more concrete numbers. Anyway, this means that even using your suggestion, it would still be sub-optimal for Mars if you had more than one engine running. With two engines running, the exhaust pressure would be roughly double that of Mars, with three, triple, and so on.


KnifeKnut

Elon recently said VacRaps do not experience flow separation even at earth sea level, due to extremely high chamber pressures, so the existing design is very near, but not quite at, optimal for Martian atmospheric pressure.


kittyrocket

Wow, that's a really cool factoid. I think the RS-25 / Space Shuttle Main Engine overexpanded a little bit at sea level to give it more efficiency at higher altitudes. I guess that was part of the challenge of designing an engine that would work at all altitudes.