Some of you might know of the OTRAG rocket, an interesting launch vehicle design from the 1970s, intended to use large numbers of relatively simple, mass-produced Common Rocket Propulsion Units (or CRPUs), fueled by a kerosene/nitric acid mix, clustered together and staged to launch payloads into orbit. A 64 CRPU craft was intended, from the data released, intended to put a one metric ton payload into Earth orbit.
For various reasons, including political and economic ones, the venture didn’t succeed, only launching some test rockets from Zaire and Libya before going under in the mid-1980s. Although it hasn’t completely died, as the modern Interorbital Systems is developing a similar rocket with a similar payload, the “Neptune-36,” with technical advice from the OTRAG consortium itself.
But, that brings me to my question, and one I’ve not been able to find any information on: how long would it have taken to assemble, fuel, and ready for launch one of those 1-ton cargo OTRAG rockets, assuming the CRPUs were already assembled?
I’m aware that this is speculative in the extreme, but if there’s anywhere I could turn to for enough expertise to make even a rough estimate, it’d be here. Besides that, I’ve already emailed Interorbital Systems, but haven’t heard back—that’s probably the closest I’ll get to a primary source from my home (or maybe my state).
So…can anyone here venture to make even a ballpark guess on how long it would take to prep and launch one of these little giants?
Without knowing technical details about the system and the process for integrating it, any answer would be purely speculative. The plan for OTRAG–and presumably IOS–is to actually store the modules fully loaded with storable oxidizer and propelant “on the shelf” and integrate as needed. A lot of the effort in integration and testing is in verification testing and having a bunch of modules would seem to compound that but perhaps the individual modules (which are pressure fed and therefore don’t have turbopumps or other complex propellant feed and regeneration systems) are simple enough to offset that and the vehicle has sufficient stability that coupled loads analysis margins are assured.
Practically speaking, I see numerous problems with both this scheme (using pressure fed parallel staged propulsion modules) and the construction of the vehicle in general, but it may be that IOS has addressed these. I follow small satellite launch systems for both professional and personal interest and IOS has put out little in terms of technical details to evaluate the viability of their engineering solutions, so I can’t make a credible estimate of when they will fly or how successful they may be in achieving their goals.
It is possible to passivate containers storing white fuming nitric acid (WFNA) by the addition of an inhibitor, most frequently hydrogen fluoride, whic produces an oxidation layer protecting the surface from reactions. I don’t know if IOS plans to do this, but that was the plan for OTRAG, and the simple systems would be pulled off the shelf and integrated in plug & play fashion to build up scalable rockets of the required capability. Since there is no TVC system–the rocket is steered by differential throttling of the outboard modules–it was mechanically very simple.
However, both OTRAG and IOS would seem to have very large inert mass fractions due to having so much tankage, which is hugely inefficient, and IOS is using WFNA and turpentine as propellants. I have never heard of anyone using turpentine as a propellant for a orbital space launch rocket, or indeed, anything intended for launch above a few thousand feet, and given the high ratio of carbon and strong bonds of monoterpenes it just doesn’t seem like a particularly good fuel despite the long term storability as compared with more volatile petroleum distillates.
I don’t have any detail information about the IOS Neptune system so every observation I have made is purely speculative, and the little amount of information I have may be out of date or not reflect the full capability of the system, but these issues combined with the low performance pressure fed engines just make me question the viability of the concept, but I have to assume that the Millirons have done the necessary analysis and testing to validate the concept for investors.
The “Big Dumb Rocket” concept is that if you can build a rocket from the absolute cheapest off-the-shelf components and fuels, the cost saving more than makes up for the poor payload fraction. You need more payload, you just build the whole thing bigger (hence the modular approach). And especially, too-crude-to-fail reliability minimizes the ground infrastructure needed. They’d use black powder if it was only cheap enough.
The Big Dumb Rocket (BDR) approach is basically levying off of scaling efficiencies; by going big, you can tolerate lower performance, and by staying simple, you end up getting more reliability. Depending upon details of the configuration a BDR may also get better mass ratio and effective payload capacity by using a larger squat form rather than a long slender vehicle. The OTRAG and Neptune aren’t really BDRs; they’re designed as scalable systems from the smallest to largest possible payloads specifically to minimize non-recurring engineering and manufacturing costs, and don’t really benefit from scaling efficiencies because they’re just small rockets stuck all together rather than getting geometric and volumetric efficiencies of being larger and rounder.
However, there should be several caveats to the idea that you can just make a launch vehicle cheaper by making it larger and more simple. For one, you need some minimum degree of performance to make it to orbit; a vehicle that can barely lift itself fully loaded and spends too much time just fighting gravity is not going to be functional even though it is simple. A vehicle that can’t fill its payload capacity enough to justify launch cost is not economically viable even if the cost per kilogram of hypothetical payload is low. And even though everyone who talks about making space launch cheaper wants to talk about trading reduced reliability for lower cost, at the end of the day nobody wants to launch with a vehicle showing high risks.
One thing that is often missing from otherwise erudite discussions about reducing launch costs is that most of the actual cost of a launch isn’t the structure and propulsion system (typically ~10% of total cost) or propellants (<1% for common propellants), or even the engineering flight analysis (coupled loads analysis, flight stability analysis, et cetera), but rather all of the costs and effort associated with integration, inspection, qualification/certification/acceptance testing, and software maintenance and upgrades. A conventional rocket launch vehicle requires hundreds of people to assemble, test, verify, and document every critical function of the vehicle and all of the administrative overhead, and the failure of a single subsystem or critical component can result in complete loss of vehicle (LOV). This is even true with supposedly “redundant” systems, because such systems only protect against random failures (latent defects, statistically unlikely weaknesses, uninspectable damage) not design limit or system failures. So the only real way to significantly reduce costs is to design a system–not just the launch vehicle but everything associated with system integration–such that it requires a minimum of human labor to perform all of the critical tests and verifications.
This turns out to be difficult, especially for launch vehicle designs that are in constant flux and too costly to do for vehicles which only fly occasionally, so this has only been attempted in a couple of cases and has never really proven out or been taken to the useful extent, but this is what is done with many consumer electronics and products to achieve high reliability with low human labor cost. It’s a major up front cost, but for high rate throughput it is essentially required, regardless of efficiencies of the vehicle, and in fact this kind of automated checkout is actually more crucial for small payload launch vehicles with low profitability margins.
