What are some examples of stepping-stone applications that would be suitable for iteratively advancing mass driver technology?

Question

Since their inception, rockets have improved over many design iterations. Each iteration was funded because it unlocked some kind of new value, such gaining an advantage in a geopolitical conflict, launching a satellite into orbit, or landing astronauts on the moon. We didn't go straight from Chinese fireworks to the Apollo program, we got there through a series of "stepping stone" applications of rocket technology.

If there were a similar set of stepping-stone applications for mass driver technology, then perhaps mass drivers could gradually become as sophisticated and mainstream as modern rockets.

What are some examples of stepping-stone applications that might serve to spur along the development of mass driver technology?

Note: Stepping-stone applications with these requirements are preferred:

1. Suitable for almost any kind of payload, as opposed to only payloads that can withstand extremely high gee-forces,
2. Cost-effective,
3. Rapidly reusable.

Applications that are less well known, than, for example, EMALS or railgun artillery are also preferred.

Note: Stepping-stone applications that were attempted in the past, but did not work out for various interesting reasons, are very much "in-scope" for this question as we should attempt to learn from past experiences.

Answer 1

Answer for December 2023, since this will probably change with time.

Much like rockets, the most obvious driver will be use in weapons. Events of 2023 have re-affirmed the advantages of a weapon system that can be fed from any available source of electricity and reloaded with cheaply mass producible rounds, rather than more capable smart/guided systems that have limited inventory and long lead time on new build.

Which means unfortunately the most likely jump in electrical launch technology will involve a major war. In particular if it features a non nuclear adversary trying to overwhelm a sophisticated static air defense by using a fixed gun system.

The more hopeful pathways are high speed rail and related systems running in tunnels. Superconducting power transmission will also be relevant, both to get power the (hopefully) remote launch site and for the launch system itself.

Air launch will be relevant to payload design, since a Pegasus type launch vehicle could also be fired from a mountain top catapult system using plausible current technology at a similar altitude/speed profile, should there be a economic justification for doing so. The demise of Pegasus and Stratolaunch is not a good sign for ground assisted launch of conventional rockets.

The activities of spin launch, successful or not will be informative both from directly developed technology and from third party studies into potential payloads for that system with very high G loads.

Relevant to any gun type system is if plasma windows or similar technology can in fact be both scaled up and fired through since being able to do so is critical to achieving the launch tempos that justify the infrastructure costs of 100km mountain top tunnels.

Further in the future, the Moon is a potential site for purerly ground based train, sub orbital 'fling and catch' and fully orbital systems involving various flavors of electrical launch. The vacuum, lack of existing infrastructure and much lower velocity needed mean useful devices could be built using existing technology, and potentially from mostly local materials.