Hybrids also don't mix the propellants as well as liquid engines
Yesterday I got into a debate (more like an argument, of course) with a Youtube hoaxer about mixture ratios in rocket engines. He thought the "rich black smoke" from the Saturn first stage (where?) somehow proved NASA wasn't serious about making an efficient rocket engine.
I pointed out that
every bipropellant liquid rocket engine I know runs rich to lower the temperature in the combustion chamber and to improve I
sp by lowering the average molecular weight of the exhaust. Then I realized I didn't know the mixture ratios of hybrid rockets, or even how you might control it.
Looking through
Rocket Propulsion Elements by Sutton & Biblarz I found that the mixture ratio of a hybrid varies with the oxidizer flow rate, richer at low flow and leaner at high flows. That makes sense, and I guess hybrids are harder to throttle than I thought.
This happens a lot, and it's the one reason I still argue with hoaxers. In the process of rebutting their nonsense I often learn something even though they never do.
problematic vibrations at large scales
When I first got into high power rocketry a few years ago, APCP was in short supply because of the disasters at their production plants, so hybrid rockets were the rage. I noticed they had a very characteristic "throaty" sound, apparently because of significant combustion oscillations. I can think of several causes. The oxidizer tank is just above the combustion chamber with the N
2O fed by both vapor pressure and acceleration. There could be a lag in getting the necessary heat of vaporization into the tank to maintain pressure. Increasing combustion chamber pressure could slow the oxidizer flow. And changes in acceleration could change the flow rate too (i.e., pogo).
Unless the engine's pressure fed, in which case you need a much heavier oxidizer tank.
Right. Fine for (relatively) small hobby rockets, not so fine for space launch systems.
with an interesting disposal issue for failed or expired fuel
What's the shelf life?
Minuteman and
Peacekeeper (sic) missiles are/were on standby for many years.
Can't it just be burned in small amounts in an incinerator? Assuming it hasn't already been cast into a huge grain...
The fuel grains have to be installed while the vehicle is being assembled, which leads to incidents like this.
I've heard of at least one fatal accident with a solid rocket motor at a NASA facility, in the early 1960s I think. But given how many SRBs have been handled since without incident, apparently the proper safety precautions pay off. I can think of a few: protective covers on the nozzles, installing igniter pyros as late as possible, electromechanical safe & arm systems, grounding everything, and the obvious "no smoking" rule. What else?
Their poor performance also means an all-solid system has to use a large number of stages. 2 or 3 stages is about ideal for liquid rockets, all-solid launch systems are rarely less than 4 stages and I've never heard of one with just 2 stages.
Yes.
Scout was 4 stages. The
Pegasus that largely replaced it is only 3, but the carrier aircraft could be considered a zeroth stage. There are several ground-launched versions of the Pegasus (the
Minotaur) that add another solid stage.
The fact that they can't be shut down non-destructively is really a major issue...the most recent Falcon 9 launch would have been a loss-of-vehicle accident if it had been using a solid first stage, instead it shut down before liftoff on the first attempt and was launched a few days later.
To be fair, solids generally do start more reliably. Both can certainly fail during flight, though as we also saw with the recent Falcon 9 liquid failure mechanisms aren't necessarily as catastrophic as those of solids so it's easier to design in some redundancy.
All-solid launch systems are limited in capability and expensive...the cost per kg to orbit of the Pegasus air-launched all-solid system was about twice that of the already-expensive Shuttle, making it the most expensive launch system on a kg to orbit basis, and not even being particularly affordable on a cost per launch basis (where tiny all-solid systems try to compete with liquids).
How much of Pegasus' high cost is due to being solid, and how much simply to its small size? People don't use it for low cost per kg, they use it to give their small satellite its own orbit instead of having to hitch a ride with a primary payload going somewhere close.
Solids are used in US and ESA systems as boosters, but Russian and Chinese systems seem to favor liquid fueled boosters. These give better performance and don't seem to be overly expensive
The Ariane 4 also had liquid boosters as an option.
How are those Russian and Chinese boosters fueled? The Ariane used hypergols, so you don't have to load a whole bunch of cryogenic tanks shortly before launch. It seems likely that both Russia and China have less stringent environmental and safety regulations than either Europe or the US and that could make it cheaper for them to use hypergols.
