An interesting discussion on the subject, involving Jim Benson, at Space Transport News.

My opinion? I think that the danger of liquids is overrated, but may the best concept win. The ultimate answer is competition, on both safety and cost.

I think that this like most rocketry debates has been done to death. Hybrids are safer in that they don't usually explode (unless your oxidizer or fuel is itself unstable), and it's very easy to throttle such an engine (only one propellant stream needs be adjusted) and maybe even to restart it. But they are extremely long (parafin might get around that), relatively low ISP, and have burn through problems. I'm sure other people can come up with further pros and cons with little prompting.

At this point, I agree with you, Rand. We just have to see what designs work out.

Not only can you throttle and restart hybrid rocket motors, but more importantly, you can instantly shut off the oxidizer, hence quenching the combustion -- something that is not possible with solid motors.

Other oxidizers can be toxic, highly unstable, and extremely dangerous -- high test hydrogen peroxide for example. Benson Space Company Chief Test Pilot Hoot Gibson refers to pilots "dissolved" by hydrogen peroxide in the early days of rocket powered flight. Contaminated N2O is not unstable and cannot explode.

You might want to check in at:
www.FreeTripToSpace.com

Onward and upward!

What about N2O/Propane? That doesn't sound too dangerous.

Jim,
I probably shouldn't get involved in this, and I know most of the people who've called you on this have done so anonymously, but I think you're doing yourself and all of us a disservice by some of your accusations you made over there on hobbyspace. And as someone who respects your work, I felt obligated to say something.

I don't mind hearing claims about hybrids being safer--I'm not sure I fully agree with them, but claims like that seem perfectly fine. I think you're overselling some things a bit, but you've more than earned a right to boost your preferred solution. What decent businessman wouldn't talk up his company?

It's just when you've been intentionally insulting other companies, making accusations like the following that I lose some of my respect for you: "Companies claiming to use horizontal takeoffs to reach 100 KM have not done their basic math (or are just plain dishonest) and as a result, their public relations performance claims are at odds with the most simple laws of physics."

100km is just not that hard, whether you approach it from a VTHL angle like you prefer, an air launched HTHL approach like Burt prefers, a ground launched HTHL like Rocketplane or XCOR prefer, or a VTVL approach like MSS/BlueOrigin/TGV/Armadillo prefers, all of them are definitely feasible. Claiming that you think you have a better approach is perfectly kosher--I would expect you to support the approach you're trying to convince investors to fund. Claiming that you're skeptical that ground-launched HTHL will work very well for suborbital or orbital flight for one reason or another is also fine. Skepticism is always warranted for any technical project in this field. It's just when you cross that line into accusing other companies of dishonesty and fraud (particularly when you're wrong about the physics), that it doesn't help anyone. Least of all yourself.

This industry is big enough for several players, and I wish you luck in your ventures. I have nothing against your approach, and am eager to see how things go for you. Heck, you even personally offered me a job at my first Space Access conference back in 2003.

I'm just suggesting that slagging your competitors isn't just inappropriate, it also makes you look bad too.

~Jon

Whoa! Dressed down on the internet. I imagine Jim sitting at home, twirling his mustache between his thumb and forefinger, and plotting Jon's imminent liquidation.

Josh,
I'm not to worried about Jim plotting my demise. :-) Jim's a good guy, and a heck of a lot classier than that. I hope he takes it more as a constructive piece of advice than a dressing down.

~Jon

Hybrids are defintely safe to transport and handle on the ground.
It seems once they start burning though, it's still an issue.

Hybrid or solid the pressure case is holding pressure, any
grain failure will lead to burnthrough which is never good.
Also grain failure can block the throat, which has some
real trouble if it over presses the case.

The only safe rocket is on the ground and unfueled - the same as an airplane, or even a car. That said, an acceptable level of safety can be achieved using any technology base - even antimatter. The only difference is the trade off between cost, time, and safety.

I'm with Rand on "may the best concept win." I know I'm preaching to the choir here, but that's one thing I love about commercial space development as opposed to government space development. If this were a big government program, the "people in charge" would have to "make a decision" about which way to go--never mind that that decision could be wrong, could be influenced by considerations far outside engineering, safety, and potential profitability, and could even be right with current knowledge but wrong because of things we don't yet know. Instead, in the market, everyone can stump for their ideas, attract the investors they can, and many ideas will actually get tried. (I hope.) If one thing emerges over time as the best idea (commercial airships were cool, but planes have pretty much won out), that's great, and if we end up with multiple competing technologies for years to come--that's great too.