How tyrannical is the rocket equation for a human RT to Mars?

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Discussion Overview

The discussion revolves around the challenges and considerations of using the rocket equation for human missions to Mars, particularly focusing on the implications of Delta-V requirements, mass lifting from Earth, and the feasibility of orbital refueling strategies. The conversation touches on engineering problems related to propellant transfer in low Earth orbit (LEO) and the safety concerns associated with cryogenic fuels.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • Some participants express concern that the Delta-V requirements for a Mars mission, especially with a spacecraft carrying supplies for several years, may result in overwhelming mass that needs to be launched from Earth.
  • Others propose that orbital refueling could mitigate mass issues by allowing a rocket to launch with empty tanks and then be refueled in LEO, potentially enabling faster trips to Mars and reducing radiation exposure for the crew.
  • There are discussions about the engineering challenges of transferring propellant in orbit, with some participants questioning the safety of such operations and others arguing that fuel transfer is a routine procedure for the ISS.
  • Concerns are raised about the risks associated with cryogenic propellants, with some participants noting that while cryogenic fuel transfer is more complicated, it is not unprecedented.
  • Participants debate the explosion risks associated with different types of fuels, with some arguing that the risks of cryogenic fuels differ from those of hypergolic fuels used on the ISS.
  • There is a technical discussion regarding the effects of explosions in a vacuum, with participants speculating on the scaling of shock and thermal effects.

Areas of Agreement / Disagreement

The discussion features multiple competing views regarding the feasibility and safety of orbital refueling and the handling of cryogenic propellants. There is no consensus on the level of risk associated with these operations or the best approach to address the challenges posed by the rocket equation for Mars missions.

Contextual Notes

Participants highlight limitations in their understanding of the specific engineering challenges related to cryogenic fuel stability and the differences in handling fuels at various pressures. The discussion also reflects uncertainties about the implications of using different types of fuels in terms of safety and operational procedures.

swampwiz
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I was reading this:

https://www.cnbc.com/2020/12/01/elo...-spacex-will-land-humans-on-mars-by-2026.html

It just seems that the Delta-V requirements - especially with a spacecraft that is stocked with supplies for a few years or for the Hohmann transfer helio-orbits - will make the amount of mass that needs to be lifted off from Earth overwhelming. That said, I suppose that stacking a launch vehicle in LEO would be similar to that for the ISS (albeit with the huge mass of the propellants) - but my experience working on the SSET has taught me that keeping the propellants at the right conditions would be a huge problem without the launch gantry. Would NASA take the risk and use SSSR Roman candles?
 
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The answer - at least for SpaceX - is orbital refueling. Launch a rocket that can deliver ~100 tonnes of dry mass and ~100 tonnes of payload to low Earth orbit with its tanks empty, then launch several refueling missions to fill the tank. Once it is filled with ~1000 tonnes of propellant again it can go to Mars.
Going from LEO to Mars with a fully fueled single-stage rocket can make the trip quite fast, which lowers radiation exposure of the crew.
 
mfb said:
The answer - at least for SpaceX - is orbital refueling. Launch a rocket that can deliver ~100 tonnes of dry mass and ~100 tonnes of payload to low Earth orbit with its tanks empty, then launch several refueling missions to fill the tank. Once it is filled with ~1000 tonnes of propellant again it can go to Mars.
Going from LEO to Mars with a fully fueled single-stage rocket can make the trip quite fast, which lowers radiation exposure of the crew.
So the idea would be to move the propellant liquid from the tank coming from Earth and put into the tank sitting in LEO, as opposed to stacking the tank coming from Earth? OH BOY, do I see an engineering problem here! It would definitely have to be a bit away from the ISS in case it explodes.
 
swampwiz said:
So the idea would be to move the propellant liquid from the tank coming from Earth and put into the tank sitting in LEO, as opposed to stacking the tank coming from Earth? OH BOY, do I see an engineering problem here! It would definitely have to be a bit away from the ISS in case it explodes.

If you think refueling is that dangerous you should stay away from the ISS. It is the only spacecraft that is regularly refueled (with hyperbolic fuels no less)...
 
swampwiz said:
So the idea would be to move the propellant liquid from the tank coming from Earth and put into the tank sitting in LEO
Yes.

Fuel transfer in orbit is nothing new, it's routinely done for the ISS. Cryogenic fuel transfer is more complicated, and of course it hasn't been done on the scale needed for Starship, but it's not that revolutionary. I don't see where the explosion risk would be.
 
Explosion effect in vacuum -- would that scale as inverse square or inverse cube? I suspect that the shock effects would go as inverse cube and the thermal (and shrapnel) effects as inverse square.
 
mfb said:
Yes.

Fuel transfer in orbit is nothing new, it's routinely done for the ISS. Cryogenic fuel transfer is more complicated, and of course it hasn't been done on the scale needed for Starship, but it's not that revolutionary. I don't see where the explosion risk would be.
Well, the cryogenic propellant was what I was getting at here.
 
swampwiz said:
Well, the cryogenic propellant was what I was getting at here.

The ISS's pressure-fed hypergolic systems involve three fluids at extremely high pressures, and two of them are highly reactive compounds that spontaneously combust on contact with each other. LOX and CH4 won't do anything without an external ignition source, and the main tanks are likely to be no more than a few bar during the transfer. What about being cold would make them more dangerous to the ISS?
 
glappkaeft said:
If you think refueling is that dangerous you should stay away from the ISS. It is the only spacecraft that is regularly refueled (with hyperbolic fuels no less)...
I don't think the fuel is hyperbolic at all --- it's real. :smile:

1607221538641.png
 
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  • #10
cjameshuff said:
The ISS's pressure-fed hypergolic systems involve three fluids at extremely high pressures, and two of them are highly reactive compounds that spontaneously combust on contact with each other. LOX and CH4 won't do anything without an external ignition source, and the main tanks are likely to be no more than a few bar during the transfer. What about being cold would make them more dangerous to the ISS?
Hmm, I guess hadn't thought that keeping LOX & LH2 stable in the ISS would be different (i.e., the opposite problem) than at the pad at the Cape.
 
  • #11
Liquid hydrogen is difficult because it needs to stay so extremely cold (20 K). Oxygen (90 K) and methane (110 K) are easier.

Note: I took the 1 bar values, but they don't increase that much for common tank pressures.
 
  • #12
phinds said:
I don't think the fuel is hyperbolic at all --- it's real. :smile:

View attachment 273772

That's not grammar. It's diction.
 
  • #13
Vanadium 50 said:
That's not grammar. It's diction.
I know, but I don't have a badge for diction.
 
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