You're quite right that 150kg is unlikely to be the final maximum payload for Electron: SpaceX's Falcon 1 was almost immediately uprated with the addition of a Merlin-1C engine, and was nominally planned to be offered in an extended variant, Falcon 1e, that could take 1010kg to LEO.
The most likely upgrade path probably consists uprating the Rutherford engine and extending the tankage structure in length to accomodate for propellant.
Now: how feasible is it to develop, or otherwise convert such a small payload orbital-rocket design to that of an offensive weapon?
I'll try to roughly address your points case-by-case.
1. Development costs & timeline
The question presumes that this technology would be falling into the hands of a company or country. It's worth noting even RocketLab have required significant injections of money for operation:
- In late 2013, RocketLab secured venture capital Series A funding from Silicon Valley company Khosla Ventures.
- They were heavily backed in 2014 by U.S. defence conglomerate Lockheed Martin.
- The New Zealand government has subsidised their development through innovation awards, totalling some eight digits of funding.
- In May 2017, they raised $75 million as part of a private Series D funding round.
Make no mistake: rocketry is an expensive business. Your actor will need either deep pockets or very good friends to make it through development hell. Even ignoring finances, it takes a lot of people and a lot of time to get off the ground:
- RocketLab currently employ 200 engineers, developers, and other staff.
- Development started on Electron in 2014. You'll need a very stable political and physical environment to make sure development is seen through.
2. Operations needed
If you're a company, you're going to need approval of your country first to authorise and sign off your launches:
The Government has today signed a contract authorising Rocket Lab’s space activities from New Zealand. [...] It is an interim measure to allow Rocket Lab to commence launching rockets before the Bill establishing a regulatory regime comes into force.
I hope you've got considerable sources of liquid oxygen and your preferred choice of fuel, as well as power available!
You'll need steady access to range assets both on ground at your launch site, and around the globe to ensure tracking and communication with your vehicle. A ground tracking error resulted in the flight termination of the first Electron flight in 2017.
You'll probably need access to an unlocked GPS unit, which requires special authorisation and exports licensing from the U.S. government.
Which leads me to my next point: RocketLab were incredibly lucky to make it to orbit on their second flight. It took SpaceX four attempts to do so. The failure rate on maiden rocket launches is very high, so you will need the money and resources to continue manufacturing, because a single shot success cannot be guaranteed.
3. Weaponization & miniaturization
What weapon fits into 150kg? The obvious answer to maximise a destruction to weight ratio would be minituarized nuclear warhead. The United States have some! (Good luck "acquiring" them though)
If you were able to scale down one ~164kg W76 warhead, designed for the Trident I & II ballistic missiles, you'd have a very nice sub-100 kiloton nuclear weapon available at your disposal.
This might not be enough however...
But, somehow, you managed to make it into orbit on your first try with a prohibited device onboard — well done!
It's safe to say that most, if not all orbital vehicle designs currently, are not well suited for reentry. If you've ever watched a SpaceX launch and landing, you'll be aware that during reentry:
- The steerable grid fins on the vehicle heavily abate and catch fire as the soupy atmosphere pushes through them.
- The vehicle is covered in its own soot as it reenters the atmosphere, and must endure high-g decelerations as it hits the stratosphere.
- Unless specifically designed for reuse, the vehicle is practically throwaway after 1-2 flights — reuse isn't easy on a rocket.
What really drives home the point here is that Falcon 9's first stage is only reentering from a relatively mediocre speed of ~1-2km/s. And as kinetic energy increases at the square of velocity, going from a suborbital reentry to orbital reentry will strain even the best designed architecture: you're looking at at a 31x increase in kinetic energy.
So, your going to need to have your weapon encapsulated in a blunt body with sufficient heat shield. Small blunt bodies have been designed before: the Stardust capsule was targeted to land back on Earth in a 76 by 44 kilometre ellipse in the Utah Test & Training Range.
You'll need to be a bit more accurate than that though, a 100 kiloton nuclear weapon will only create a thermal radiation blast with a ~3.9km radius.
5. Speeds involved
Say your bad actor gets this far. As we're in orbit, the minimum reentry speed you're going to get without significant propulsive reductions is that of orbital velocity: ~7.6km/s. You might be able to lower this slightly by ensuring you hit your target on first pass and aim for a suborbital trajectory, but there isn't much difference here unless your target is very close by, and you wouldn't use an orbital rocket for that anyway — far more conventional forms of attack are more suited to this subdomain.
You would need to make that 150kg of payload capability almost all fuel to get any meaningful reductions out of it. You'll also be slowing yourself down, making it easier for the enemy to target you with anti-ballistic missile technology. Any rational actor with the capability to develop this technology would likely prefer to reenter at high velocity, and shed the speeds using an ablative heatshield.
At such a speed, you'll be able to target anywhere on Earth in just under 90 minutes. A very useful capability for sure — but my personal expectation is that they'll be either sabotaged, forcibly shutdown, or reasoned with beforehand :).
Yep, this is technically possible — but it has been for a while. RocketLab are not the first to pioneer any of this small orbital technology, beyond some carbon composites manufacturing or a novel engine design. There simply just hasn't been a market for it before now.
The reason you don't see bad actors popping up everywhere with their own small orbital offensive weapons is because it's still damn hard to do.