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How much more efficient would it be to use a launcher big enough to go directly to the surface of the Moon like Apollo did, compared with using two launches with a lesser rocket, say one with the lander and the second with the crewed command module? By "economy" here I don't mean dollars and cents, but more principally in terms of delta-V and such fundamental measures. If the Apollo program had continued, would it have been beneficial to use Saturn V to build a Gateway in Lunar halo orbit? Was this suggested in the 1960s-70s?

SLS Block I is too small to launch Orion as the command module with a proper Lunar lander attached to it. That's a motivation for the Lunar Gateway concept. How can the relative (physical) costs per Lunar landing of 1) going directly, 2) docking in LEO and 3) building and docking with the Lunar halo Gateway, be estimated and compared?

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  • $\begingroup$ Do you mean direct as in how Luna 1 and 2 were sent to the moon with a single burn and no parking orbit? Also-- whats the lesser rocket you referred to in your first sentence? $\endgroup$ – Magic Octopus Urn Jul 21 '19 at 14:45
  • $\begingroup$ Once in LEO, delta-v requirements to reach the Moon are basically fixed and not dependent on what vehicle you are using to reach the Moon or if you've docked with another spacecraft to get the required fuel/extra modules. Therefore, any differences between single-launch and multi-launch (with orbital rendezvous) must occur before the spacecraft departs from LEO to the Moon. This means you're essentially comparing Earth-to-orbit efficiency of two types of rockets; the big one and the smaller ones. Without knowing exactly what rockets you're talking about, an exact answer can't be given $\endgroup$ – Dragongeek Nov 13 '19 at 13:20
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Without considering money or engineering complexity as the measure, this is hard to measure. In theory earth orbit rendezvous is not much more costly in terms of delta-v. This is because there is a slight unavoidable cost of the ability to do the transfer burn while still inside the atmosphere. However the Apollo launches got into orbit before transferring anyway even though in theory they could have.

Other than that there is not much in theory that has to be different. Flying two rockets: bolted together/flown side-by-side/flown 1 orbit-behind-each-other-and-docked are all pretty much the same thing (except from an engineering and money point of view...).

In practice:

  • The gateway won't orbit just off the surface (for other reasons) so there is an extra landing cost as you cant do a no-altitude change landing.

  • The rendezvous are never completely free, even if there is no meaningful lower bound on their cost.

Without a more specific picture of those scenarios with numbers, its hard to put a meaningful number these questions other than: they are theoretically pretty similar.

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How can the relative (physical) costs per Lunar landing of 1) going directly, 2) docking in LEO and 3) building and docking with the Lunar halo Gateway, be estimated and compared?

Let's assume that all cases include at least a brief stay in LEO, as was done with the Apollo missions (about 2 and a half hours).

In case 2, we add the cost of rendezvous and docking in LEO. This is probably well under 100 m/s of delta-V cost to manage the rendezvous. Some additional time must be allowed for the approach, so there will be some consumables use and propellant boil-off. Soyuz missions to ISS have demonstrated 6-hour rendezvous from second launch, and the docking phase proper may take quite a while for two large spacecraft.

For the case 3 lunar Gateway mission, according to this Ars Technica article, LEO-to-Gateway-to-Lunar-Surface costs 6.85 km/s compared to 6.1 km/s for LEO-to-Lunar-Surface direct flights.

This is a significant increase in required delta-v, and a number of big names have spoken out against the Gateway concept for this reason.

In exchange, the Gateway could provide some long-term amenities that wouldn't be practical in an Apollo style LOR mission. Astronauts will appreciate the chance to rinse off the moon dust before the long trip home. Reusable big-ticket equipment (e.g. rovers) could be serviced at Gateway instead of returning all the way to Earth. If lunar missions are going to become a regular event, Gateway or something like it is probably worthwhile, but it may not be worth the cost if we're only going to make one or two return visits.

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  • $\begingroup$ The orbit of the Gateway is not ideal for lunar missions. It is ideal for a direct to earth comm link and low Delta v maintenance requirements. $\endgroup$ – mothman Nov 14 '19 at 4:48

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