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

In summary: 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...
  • #1
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.
 
  • #3
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.
 
  • #4
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)...
 
  • #5
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.
 
  • #6
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.
 
  • #7
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.
 
  • #8
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?
 
  • #9
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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1. How does the rocket equation affect the feasibility of sending humans to Mars?

The rocket equation plays a crucial role in determining the amount of fuel and resources needed to launch a spacecraft from Earth and successfully reach Mars. It dictates that the amount of propellant required increases exponentially as the mass of the spacecraft increases, making it a major factor in the overall cost and difficulty of a human mission to Mars.

2. What is the rocket equation and how does it work?

The rocket equation, also known as the Tsiolkovsky rocket equation, is a mathematical formula that calculates the velocity of a rocket based on the amount of propellant and the mass of the rocket. It takes into account the concept of specific impulse, which measures the efficiency of a rocket engine, and the initial and final masses of the rocket to determine the change in velocity (delta-v) that can be achieved.

3. How does the rocket equation impact the design of a spacecraft for a Mars mission?

The rocket equation heavily influences the design of a spacecraft for a Mars mission. Engineers must carefully consider the mass of the spacecraft, the efficiency of the propulsion system, and the amount of propellant needed to reach Mars and potentially return to Earth. This often leads to the use of lightweight materials and advanced propulsion systems to minimize the impact of the rocket equation on the overall mission.

4. Can the rocket equation be overcome for a human mission to Mars?

While the rocket equation cannot be completely overcome, there are ways to mitigate its effects. One approach is to use in-situ resource utilization, where resources found on Mars, such as water ice, can be converted into propellant. This reduces the amount of propellant that needs to be launched from Earth, thus decreasing the impact of the rocket equation.

5. How do advancements in technology affect the rocket equation for a human mission to Mars?

Advancements in technology, such as more efficient propulsion systems and lightweight materials, have the potential to decrease the impact of the rocket equation on a human mission to Mars. However, these advancements must be balanced with the cost and feasibility of implementing them. Ultimately, the rocket equation will continue to be a crucial consideration in any human mission to Mars, but technological advancements may make it more manageable.

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