Jon Goff has an interesting post (if you’re into rocket propulsion) on a technology that’s been lying fallow for decades. But this post isn’t about that concept per se, but a more general one:
I stumbled on this while trying to track down some old Aerojet papers about a sort of forced flow separation control technique that they researched back in the late 50s. I had noticed that most of the papers that cited the research talked about how Aerojet’s had concluded that the approach didn’t yield any net benefit, however the way they discussed it made me somewhat suspicious of their conclusion. You can sometimes get a sort of telephone effect with academic citations–where someone will read someone else’s review of some obscure and hard to locate article, and instead of reading it themselves, they’ll just summarize the summary, and before long who knows what the original article said. To make a long story short, I had good reason to be suspicious that there was something of that sort going on with this paper (especially since the two abstracts I was able to find online for their research seemed to directly contradict all the claims I’ve seen in citations of their work elsewhere).
Assuming that it was the case here, this happens more than you might think, and more than it should. This kind of thing, in fact, is the source of a lot of false mythology about space technology (e.g., highest vehicle performance is achieved with LOX/hydrogen, air breathers are the key to low launch cost, etc.). Many “rules of thumb” and conventional wisdom are based on a limited analysis, and used by people unfamiliar with their origin, or the underlying assumptions. I’ve written about an example of this before from my own early career:
Back when I worked at the Aerospace Corporation, a couple decades ago, I was fresh out of school, and sitting in a meeting with more senior people, discussing a conceptual design for a new military geostationary satellite. The subject was how to provide orientation. The two traditional choices were spin stabilization (many of the Hughes communications satellites used this technique) and active reaction control, which was more accurate, but limited the lifetime, due to depletion of propellant.
I (or someone, but I think it was me) suggested using gravity gradient stabilization (that is, taking advantage of the fact that a non-spherical satellite will naturally orient itself in the local vertical position, due to differential tidal forces between the line of the orbit and the small distances of the appendages from that line). The response of one of the supposedly experienced engineers was, “There’s no gravity gradient at geosynchronous altitude.”
I was a little surprised. “Oh, you mean there’s not enough to do the job?” (I was thinking that perhaps he’d already considered it, and run the numbers.)
“No, there is no gravity gradient effect that high–it only applies in LEO.”
Note that he wasn’t making a quantitative argument, he was making a qualitative one. Low orbits had gravity gradient, high ones did not.
…What happened? Sometimes even engineers don’t always apply good scientific principles. In this case, I suspect that he was an airplane guy who’d migrated into the space business (as often was the case in the beginning decades in the space industry), and had never really learned the fundamentals of orbital mechanics, or the underlying principles. Instead, he’d probably taken a space systems design course, and been given a lot of engineering rules of thumb, one of which was, no doubt, that gravity gradient can be used in LEO, but not in GEO.
And that’s not a bad rule of thumb, as long as you understand where it comes from. Gravity gradient is indeed much less at twenty thousand miles altitude than at two hundred miles, and for most satellites could be considered, for practical purposes, to be non-existent. But we weren’t talking about most satellites–we were looking at a new concept, much larger than anything previously deployed in GEO, with long booms and appendages that might, in fact be used for G-G stabilization. But because he didn’t understand the physics, he mistook a rule of thumb for natural law, even though the law of gravitation says that the earth’s gravity extends out to infinity, though it drops off as the square of the distance.
Often someone will perform an analysis, and people in a hurry will simply look at the bottom line, while ignoring the assumptions that went into it, which, if altered, might completely change the conclusion. Worse yet, sometimes the author hides the assumptions, making it even more pernicious (this, to me, is one of the primary reasons that we make so little progress in advancing a useful space policy–there are too many hidden assumptions on the parts of debaters, and everyone assumes that they’re shared, when they’re often not).
This is why it’s important to properly document a trade analysis–so that when the assumptions change, it’s easy to go back and determine whether or not the trade conclusion has, or whether or not it has to be redone. This is also why it’s important to perform sensitivity analyses in the course of the trade–to make it easy later to determine, perhaps at a glance, whether an assumption change is critical or not.
I don’t know whether or not the augmented thrust technology that Jon unearthed will find its way into future vehicles, but I’ll bet that the original authors of the study didn’t consider all of the potential applications for it when they published it–they were probably working on an engine for a specific vehicle concept. XCOR has been doing a lot of this kind of archaelogy of the early space age, and (at least it’s my understanding) have found it a rich ore of untried but promising concepts. When one considers how much money was spent on the development of space technology in the early days (and how chaotic and largely undirected the vehicle development process has been over the last few decades), it would be surprising to me if there aren’t a lot of old tricks in there that can find applicability in the twenty-first century. But one has to read carefully, and hope that the papers were documented properly. And when documenting our own results now, we should think of those who may be reading them in the future.