Today I learned that John Carmack's volunteer aerospace startup Armadillo Aerospace have been thinking for some time about using an OTRAG style modular booster to get things into orbit, just like us (in fact, they've even been corresponding with Lutz Kayser about it). Their website's FAQ section contains a few sections on this idea that are probably worth us paying attention to.
The first thing has an idea that we may want to consider, with regard to minimising drag during launches:
With your current modular design, won't drag on a high number of modules be a deal-breaker? What about adding a fairing?
Our current plan of record is to fly the modular clusters at subsonic speeds through the thickest parts of the atmosphere. This means that typical concerns about transonic conditions and high drag are not as relevant. We will pay a price in efficiency, since delaying higher speeds until much higher altitudes expends more energy, but we feel we will more than make up for that in the simplicity and flexibility of the modular design. As such, we feel a fairing may not be necessary, only adding unnecessary weight. But higher altitude testing will prove this out.
The second thing addresses reliability concerns with the OTRAG design, which I seem to recall being a worry for some people back at /r/tothemoon on Reddit:
Won't the modular approach be more prone to mishaps since there will be more individual systems within which things can go wrong?
Very simple modules should have much higher reliability than conventional, complex rockets. Even without that:
If each module only had 99% reliability, an eight module system would have an 7.7% chance of having a failure of some kind (1.0 – 0.99^ 8 ). However, the configurations we will likely use would require two modules in opposite "banks" to both fail to bring the vehicle down, which would only have a (1.0 – 0.99^4) * (1.0 – 0.99^4) = 0.00155 chance of happening. We expect the real number to be much smaller than that, because we expect we will have better than 99% reliability per module. Each bank should have its own guidance electronics as well, so that will also be redundant.