Doug Messier is skeptical about the overhype from Virgin about their vehicle. So am I. Not that I care, of course. Of course, I think that the safety of hybrids is overhyped as well, and the tradeoff with operational costs isn’t that great. But maybe it takes overhype to be successful in business.
5 thoughts on “How Green Is My Spaceship?”
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Well, I’m pretty familiar with the environmental and safety issues of hybrids, since I wrote most of the original claims on those topics. The first point is that both issues depend upon the actual propellants used. With a pure hydrocarbon fuel grain, such as the HTPB used at AMROC and most subsequent hybrid houses, and LOX, the exhaust issues are pretty much identical to those of a LOX-kerosene liquid-fuel engine. It is preferable only in the relatively minor consideration of fuel spills, which just don’t happen with the solid grain. This does mean that you don’t have to have fuel spill countermeasures or training, and you can eliminate that part of an EA or EIS.
However, at the time I was discussing these issues, the comparable vehicles in the classes we were dealing with (Scout, SSI’s Conestoga, etc.) were all solids; the other application that was being looked at seriously, replacing the strap-ons on Atlas, also involved a hybrid-solid comparison. And there, there’s no question about it, hybrids are far cleaner so long as you don’t load aluminum into the grain, which we didn’t.
As far as safety is concerned, there are genuine advantages for hybrids over liquids or solids. But again, it depends on the propellants. Hybrids aren’t magic that can turn a self-pressurizing substance such as N2O into a benign, non-self-pressurizing substance. However, a fully-fueled LOX-hybrid vehicle just won’t explode, either on the pad or in flight. You can have an overpressure rupture in the LOX tank, which would be a problem, but that’s the only hazard. Its quantity-distance limit is set only by the overpressure hazard, and it is far smaller than the equivalent. There are significant operational advantages from this fact, particularly in pre-flight operations. Additionally, hybrid strap-ons have some unique advantages over liquid or solid strap-ons: they can use main-tank oxidizer, and thus flight termination can be done simply by cutting the oxidizer feed. You don’t have to have the strap-ons wired up to split the case. The Q/D issues of the solid strap-ons complicates the pre-flight operations of launchers that use them, which is why their operators had studied hybrids.
Hybrids are not magic, and they aren’t a general solution for space flight. However, they are a useful niche tool and should be viewed that way. It’s important to understand where the advantages and drawbacks are real, and where they are not.
Rand,
That is the core of successful marketing, making people believe your widget is better then anyone else’s widget. That is what brand positioning is all about. Look at how well Microsoft used it in building its dominance in the computer market over Apple’s much better machine.
Tom
Sure it’s hype. Like Thomas sais, that’s marketing. We shouldn’t buy into the hype too much but it’s great that there are starting to be vehicles to hype. Vehicles built with private money anyway (we’ve had more than enough NASA hype over the years).
With a solid grain, you don’t get fuel spills – but you do get lots of non-biodegradable HTPB sawdust that is impossible to contain and lasts forever, and you do get lots of extra waste from casting that you don’t get pouring kerosene. Kerosene also burns cleaner than tire rubber in almost every measure – less CO2 for given thrust, less incomplete combustion products, fewer suspended impurities, less two-phase particulate flow. The combustion products of a NOx/rubber rocket are not nice stuff. Not highly toxic, but not nice. Some of them are much more powerful GHGs than CO2, and some others are major destroyers of ozone, and components in smog and acid rain. You don’t get nitrogen compounds in any meaningful quantity burning LOX either with a hybrid or a liquid – CO is pretty much the worst stuff coming out the back in that case, and that can be reduced with a longer chamber length. And lets not even get into the relative energy and emissions between LOX production and NOx production – it’s ugly.
NOx is not quite a nightmare in terms of safety, but LOX is far better whether you’re using a hybrid or liquid engine from both efficiency and safety standpoints. Proponents like to claim that NOx has been used in dentists offices for a hundred years – true enough, but when you are dealing with a quenchable monopropellant theres is big difference between a needle valve trickling a few mg/s and a 12″ hole belching the stuff out. If you have a liquid engine on a strapon you can just cut off the fuel – actually, you can cut off the fuel OR the oxidizer and still have a condition that is reasonably safe – it is essentially built-in double redundancy, although trailing a cloud of atomized kerosene is not ideal. And it is also easier to re-start a liquid engine versus a hybrid, so you aren’t stuck with a split-second decision between ditching and continuing a marginal burn – you can shut down the motor, coast for a minute and review data, and then make an informed decision. You also won’t get into a runaway feedback detonation loop with a liquid, which is a highly unlikely situation with a hybrid but possible if you are running lean and your chamber pressure spikes, and which is always a possibility with a solid. You can’t plug the nozzle with a chunk of fuel in a liquid engine like you can with a hybrid or solid.
Finally, in a hybrid you replace the chamber after every run. That is a major component. With a liquid engine, there is no major overhaul each time. You can very cheaply test the thing to failure, or to an arbitrary number of starts, so you can get a very detailed profile of how the engine behaves. You could qualify a hybrid chamber by running it several times at great expense, but each time is a major operation with lots of steps that can introduce hidden variables. The cost/benefit analysis for qualification comes out on the side of not as safe, in other words.
Hybrids are marketed as the best of both worlds, but the flip side is they are also the worst of both worlds. They have all the same opportunities for plumbing leaks and tank and manifold overpressures, along with all the unpredictable – and potentially runaway – issues that go with an exposed grain, along with the recurring operations and qualifications costs of frequent major system overhaul. I have heard Burt Rutan say publicly that this is the last vehicle he will do with hybrids. Considering the investment he has made in them, there must be some good reasons for that.
There’s a youtube video of a project at Purdue called “ALICE” using ultrasonically nanosized aluminum suspended in ice as fuel. Alumina as exhaust…. Curious about impulse, etc.