The planning for the Heim Prime project only started in earnest just over two months ago but the team has achieved an incredible amount in that short time, particularly since its move to the actual site a few weeks ago.

You may recall from reading Chapter 114, Chapter 116 and some later references, that original concept was for a torus or donut shaped half kilometre wide habitation ring with eight tunnels (later modified to six) leading from the torus ring to a hub at the core of the asteroid. The hub being connected by shafts to surface bases at both of the intended rotational axis points and the ring outer perimeter being connected to the 'equator' of the surface by a (then unspecified) number of emergency escape lifeboat tunnels.

The broad concept still holds but a significant number of design changes have since been incorporated into the, hopefully final, basic layout. The changes revolve around a mixture of security, practical and aesthetic issues.

The matter of security was touched on in earlier chapters Chapter 124 - Defence and Chapter 127 Heim Prime - Planning update. In response to these issues the design team has completely redesigned the two surface bases, which are to be constructed at the sunward and anti-sunward points on the intended spin-axis. These bases will each include all the facilities depicted in the Alpha base diagram [Chapter 47], together with a few more, but they will now be largely sub-surface and more widely spread out than those on the earlier settlements. The concept is for the warehouses, hangars and ship building sheds, which are currently located on the surface on the other Heim worlds, to be located below various camouflaged landing pads with aircraft-carrier-style lifts to bring craft and goods to and from the surface, thereby eliminating visible structures. Additionally, critical infrastructure such as chemical and water conversion plants, storage tanks and nuclear power generation systems will be concealed underground and be more widely dispersed.

These underground facilities will include such things as the command-and-control centres, the research labs and hospitals, maintenance workshops, accommodation and catering blocks and, of course, the Interchange halls with the shafts leading to the interior. As an additional defence precaution, the base facilities at each of the axis-point bases will be duplicated at the other in all critical areas, enabling them to service the interior independently of each other. Some of the less-critical facilities will necessarily need to be constructed on the surface but will be located some distance away from the main surface bases. These include the observatory on the 'dark side' base and the solar panel arrays on the sunward side.

In terms of the practical issues leading to changes, these were mainly to do with the strategies for tunnelling and for the distribution of mined material. They relate to the order of work and the location of the emergency access tunnels. In all previous asteroid conversions, the tunnelling order has been to first bore the main shaft to the core, secondly to excavate the hub 'hall', thirdly to dig out the 'lift shaft' tunnels horizontally through to the proposed habitat domes, fourthly to excavate the dome areas, fifthly to excavate the small road/rail connecting ring and finally to bore the emergency egress tunnels out to the surface. The problem with that order was that stages three to six could not begin until one and two were complete. Not too much of a problem on those (relatively) smaller projects but a significant delaying factor on a project of this size.

Other design changes have been incorporated due a mixture of aesthetic and functional considerations. There had been some discussion and concern about the relatively small diameter of the main torus tunnel - it being just a half kilometre wide - smaller than even Alpha's domes. Even though the length of the ring tunnel itself will be almost twenty-five kilometres some were of the opinion that its width might make it seem slightly claustrophobic. Other were concerned about the possible difficulties in creating 'climates' in such relatively low-roofed areas. The consensus was that this was something that had to be addressed at this point in the design, as it would be more difficult to make any changes after tunnelling had begun.

During our deliberations there was much concern amongst the engineers about the time, effort and material requirements for any significant widening. At that point I had a thought, and asked the engineers, "Is there any structural reason why the tunnel has to have the same dimensions throughout its length. What I'm getting at is, would it be possible to make just some parts wider?". That caused a lot of discussion and a bit of excitement, as I believe we all saw a possible solution emerging. They discussed the idea at some length and agreed that, subject to a few changes in the tunnelling plan and structural design, there were no radical objections to making some parts wider.

The upshot of that were design changes which left more than half of the habitat tunnel just five hundred metres in diameter but, at the location of the six 'towns' where the lift shafts intersected the tunnel, it would be expanded to two kilometres wide by two kilometres long with a higher roof, thus creating six dome-like 'towns' around the habitat ring. It was then suggested that most of the population and light industry would be concentrated in these six widened areas with the upper floor of other twelve to thirteen kilometres of narrower sections being used for a combination of climatically varied 'natural' systems and agriculture.

A second proposed change was to dispense with middle level in these narrower sections making level two the uppermost level with just the bottom, services-level underneath. The effect of this would be twofold. One to simplify construction and two to increase the 'ceiling' height in those sections from just under 300 metres to almost 400 metres, thereby making them more climatically adaptable. This design would see the ring-road and the monorail emerge 'above ground' in the narrower tunnels and return 'underground' beneath the upper level in the wider, longer and higher 'town' stretches of tunnel.

These changes will result in the need for modifications to tunnelling and excavation equipment, structural redesign, extra materials and the removal of significantly more tunnelling waste. However, the consensus and conclusion was that now is the time to make them and 'get it right'. The engineering team were also of the opinion that the changes would not delay the project too much, if at all, because they have been decided before equipment and component design has been started. A brief meeting of the Grand Circle was convened, and the changes were all approved.

Over the past two days we have put these modifications into our digital modelling and with the new 3D concept view I can really begin to visualise the final result. Of course, all we are seeing at this point is a picture of a hollow shell with basic flooring locations. The task of filling that shell with all it takes to make it into a self-sustaining world is, to use a somewhat over-used exaggeration, mind-blowing. To my mind, the tunnelling and lining is a massive task that will take many years, but I suppose that gives the experts the time needed to come up with plans and designs for the minutiae of turning it into a functioning, self-sustaining mini-world. However, the actual time it will take to fit it all out is another matter altogether. My enthusiasm is building up and I'm all 'gung-ho' for us to get started and try to shorten the time-frame a bit. I just want to be around to see it completed.

Update March 2025

Since writing this chapter, I have obtained details of the final design. [see Ch 167]