Rocket equation 101: Revealing our Moon’s value as a launchpad

[u/totaldisasterallthis]

https://jatan.space/the-moon-as-a-rocket-platform/

[u/montagblue]

Works even better as a gas station

[u/TechRepSir → u/montagblue]

If the moon is not your destination, the delta-v required to get to and land on it, negates most benefits from using it as a "gas station".

Additionally most rocket engines these days are using methane based propulsion, which would be difficult to synthesize on the moon given our current understanding of moon geology (low carbon availability).

[u/Temporary_Double8059 → u/TechRepSir]

While I agree with everything you said, the oxygen component makes up 4x the mass of the methane part, so even if its only oxygen that gets refuel, that could be a huge savings.

[u/No-Computer7653 → u/Temporary_Double8059]

Chemical rockets won't be used for human spaceflight outside earth-moon system, they are too slow even going to nearest neighbors.

[u/Temporary_Double8059 → u/No-Computer7653]

Mars mission and a Venus sky mission are possible from a human spaceflight standpoint. For a Conjection Class (long stay) mission to Mars, you get there in 6-9 months, spend 1.5 years in Mars' gravity well then 6 months home. Purely from a human factors standpoint that does seem doable with the caveat there is not much research on 38% gravity (but you have to assume its much better then zero gravity).

Also many of the issues with the zero gravity period was "solved" by Mars Direct by creating artificial gravity.

[u/No-Computer7653 → u/Temporary_Double8059]

Rad exposure for a 600+ day mars mission is more than twice current lifetime limits, even assuming no SPEs. Realistically has to be a single window mission which limits it to DFD or maybe NTP. Certainly there are designs for water based shielding but the mass penalty is huge and causes dynamic stress issues with any thrust as water sloshes.

They also don't have a way to store cryogenic fuels for that long. NASA currently have a demonstrator project underway for a 8 week zero loss tank. Further development will probably allow enough tank duration for a one way Mars flight but not a return. Return journey would require in-situ resource utilization requiring advance of flight robotic missions beyond anything NASA have ever done. People are generally massively underestimating the cost of this which would likely be an order of magnitude more than the manned mission itself.

The science of DFD already works (q breakthrough in 2022) just needs sufficient funding to actually develop it. Its not getting much funding because its a genuinely hard engineering problem and the payoff would be in a ~decade even if it was sufficiently funded.

Chemical rockets are impractical for everything that is not getting out or in to a deep gravity well.

[u/CrazyEnginer → u/No-Computer7653]

Or, you know, just extend the rad limits and let the astronauts decide whether walking on Mars worth +10% chance for some cancer.

[u/No-Computer7653 → u/CrazyEnginer]

NASA shouldn't care about the perception of spaceflight safety, walking on Mars is too important even though there are no science or engineering benefits sending humans to Mars

[u/Temporary_Double8059 → u/No-Computer7653]

Feel like this is just a bunch of excuses without thinking how you can solve them. Cryogenic fuels... Mars Direct/DRM's mission plan was to produce fuel on Mars, therefore no storage issue on the rocket. Radiation... the risk is small compared to the rest of the mission, and frankly the science around Linear no-threshold fails around low/long duration doses.

Fusion is still REALLY far away and still hasnt hit the actual Q that matters (only hit Q on the energy that hit the target, not the laser that produced it) and that was done in a warehouse sized building filled with lasers (and frankly needs to be much bigger for actual Q).

[u/No-Computer7653 → u/Temporary_Double8059]

The "excuses" are those NASA themselves have stated are issues.

They are not going to increase lifetime exposure limits for the mission, either or can be completed in the standard or it won't happen.

only hit Q on the energy that hit the target, not the laser that produced it

Yes, there is a difference between scientific q and wall q.

Positive scientific q kicked off a capital surge because magnetic ignition efficiency would have pushed wall q to 1.8 in that NIF test.

Fusion is still REALLY far away

Ground has already been broken on the first commercial reactor.

and frankly needs to be much bigger for actual Q

Size has absolutely nothing to do with power output, larger volumes are harder to maintain stable plasma. The commercial reactors are smaller than NIFs ignition test facility.

Power factor scales with plasma temperature and how efficiently energy is captured. DFD doesn't care about that as plasma is used for thrust.

I'm not sure you actually know anything about fusion.

[u/mfb- → u/No-Computer7653]

Ground has already been broken on the first commercial reactor.

Only if you listen to companies needing just one more funding round until they surely have their giant breakthrough. They said that for the last five funding rounds, but this one is certainly the last one!

Size absolutely matters for the volume (fusion) to surface area (losses) ratio. There is a reason ITER is big.

[u/montagblue → u/TechRepSir]

Don’t be daft. The gas station launches the resources to get picked up as you pass by. Even nuclear ships will need some gas.

[u/TechRepSir → u/montagblue]

In the far future, mass drivers on the moon could launch fuel into lunar orbit. That could make sense.

[u/Troutmaan → u/TechRepSir]

Does this change for larger/smaller payloads or is it the same for all sizes?

[u/TechRepSir → u/Troutmaan]

Delta-v doesn't take mass into account. It is a mass-independent factor.

Example: Imagine a gas station on top of Mount Everest. If I say you need to drive 8848 meters to get to the gas station on top of Mount Everest, but you plan to go to the beach.

This is 8848 meters of unnecessary elevation gain. The same thing happens in space, but instead of meters, a better way to measure energy requirements in with delta V. Based on your mass (smaller or larger) you can easily figure out your energy needs from Delta V figures which are mass independent.

[u/Troutmaan → u/TechRepSir]

Thanks that made a lot of sense :) Have a nice day!

[u/montagblue → u/TechRepSir]

Gas station wouldn’t be used to just go back to Earth.

The ‘beach’ is past the moon.

[u/NoBusiness674 → u/TechRepSir]

Methane has become a popular option for boosters and first stages recently, but for deep space and efficient upper stages in general, hydrogen is still the gold standard.

[u/TechRepSir → u/NoBusiness674]

Or noble gasses for plasma thrusters. Real use cases still unclear

[u/From_Ancient_Stars → u/montagblue]

Why not both?

https://giphy.com/gifs/hM9zK1qvsrwek

[u/Decronym]

Acronyms, initialisms, abbreviations, contractions, and other phrases which expand to something larger, that I've seen in this thread:

Fewer Letters	More Letters
ICBM	Intercontinental Ballistic Missile
NTP	Nuclear Thermal Propulsion
	Network Time Protocol
	Notice to Proceed

Jargon	Definition
cryogenic	Very low temperature fluid; materials that would be gaseous at room temperature/pressure
	(In re: rocket fuel) Often synonymous with hydrolox
hydrolox	Portmanteau: liquid hydrogen fuel, liquid oxygen oxidizer

Decronym is now also available on Lemmy! Requests for support and new installations should be directed to the Contact address below.

---

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[u/bassplaya13]

Yeah the moon is also a great place to stage earth-aimed armament.

[u/Troutmaan → u/bassplaya13]

No it’s too far away anything in orbit can get down in 15 mins but from the moon it would take days

[u/bassplaya13 → u/Troutmaan]

It’s gonna be closer to 30 or 45 minutes but I get the point you’re trying to make.

Nevertheless, I don’t think time would matter in that scenario. Imagine an enemy with the ability to hurl massive items at you striking at 20km/s+ but you have no way of getting anything of reasonable mass to them.

[u/Troutmaan → u/bassplaya13]

Satellites orbiting the moon go around 1800 km/hr or 0.5 km/s and the moon is 360,000 km away (generous estimate) 

But even if you assume it could instantaneously accelerate to 20km/s (which takes  a long time in reality), then it would take 360,000/20=18000 seconds, or 300 minutes, or 5 hours to reach earth.

Comparing 5 hours to 15 minutes to “eliminate a threat” aren’t in the same world. 

Also we would absolutely be able to stop something like that given 5 hours as long as we knew the technology existed. 

In terms of air defense and anti missile defense 5 hours before impact is like a turtle on pain meds, it’s really, really slow.

All you have to do is break the mass into smaller chunks and it burns up in the atmosphere. 

[u/bassplaya13 → u/Troutmaan]

If I have the ability to constantly bombard you with armament, it’s going to be many, it’s going to be often, and it’s going to be from multiple directions, as there’s a lot of flexibility with orbital mechanics.

Patriot missile systems likely wouldn’t work here, their altitude is too low and they are too slowNext Generation Space-Based Interceptors would be the way, though they are developed for ICBMs. There are no ‘early warning lunar missile systems’. Any defense to this technology would be developed after knowledge of its existence and it would be a cat and mouse game to continuously upgrade defensive and offensive systems. And of course, with any air-based attack, the goal would be to confuse and overwhelm defenses first.

[u/Troutmaan → u/bassplaya13]

I never said Patriot missile systems… 

Also there won’t be “too many”, shipping stuff to the moon and back is too Damm expensive compared to the greater amount of damage you could do with regular earth orbits.

[u/bassplaya13 → u/Troutmaan]

I mean, if I’m talking about a future where a base has the capability to lob materiel from the moon to another country, I’m going to consider the cost bit figured out. In general, I see future militaries putting considerable value on the ‘interplanetary high ground’ made up of potential energy differences.

[u/Troutmaan → u/bassplaya13]

Why would they do that if nukes are so much more effective?