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ASEI
2006-Jan-23, 08:15 PM
From what I know about hybrid rocket engines, they appear to be a lot better in terms of performance and safety than solid engines. They usually have less brittle solid fuel than a solid engine (like rubber or some other plastic type material) and so you don't have to worry about fuel cracks or absurdly pristine fabrication conditions as much. You can throttle them by controlling the oxidizer flow rate. They acheive slightly higher specific impulse than solid rockets do.

So why aren't they being investigated to replace solid boosters and 1st stages on launch vehicles? Is there any push out there to engineer a full scale hybrid rocket booster?

Nicolas
2006-Jan-23, 10:19 PM
Storage of SRB's is much easier (no cryogenics), so is segmented processing I assume. I assume cost is also an aspect, as is reusability.

Bob B.
2006-Jan-23, 10:54 PM
I've heard it is difficult to achieve even combustion in large hybrid engines, thus they are not practical for big boosters. Though they work okay in a small applications like SpaceShipOne.

joema
2006-Jan-24, 03:14 PM
...So why aren't they being investigated to replace solid boosters and 1st stages on launch vehicles?...
Because hybrids don't have sufficiently compelling advantages. E.g, the SpaceShipOne hybrid rocket has a specific impulse of about 250 sec. This is lower than the shuttle SRBs.

Re amount of solid fuel, cracks, etc, this is still an issue for large hybrids. Consider a hybrid shuttle SRB. Even about 1/2 the solid fuel is still a lot and fabrication inconsistiencies would still cause problems. The same inspections and meticulous fabrication would still be required.

With hybrids you retain the checking/fabrication issues of solids, plus the liquid issues (pumps, leaks, etc).

The main advantage of hybrids is thrust termination and some degree of dynamic throttleability. However I don't know how throttlable they really are. All you can control is amount of oxidizer, and adding oxidizer usually causes hotter combustion, so there are limits. I seriously doubt a hybrid could be deeply throttled like the Lunar Module descent engine (10% to 100% thrust).

Pure solids can also be "throttled", just not dynamically. E.g, the shuttle SRBs are throttled back along with the SSMEs to reduce thrust before Max Q. Then the SRBs are "throttled up" after Max Q along with the SSMEs for increased thrust. This is done by altering the solid propellent interior surface area during manufacturing. It obviously can only be done for one static profile, but that's all rockets generally need.

Thrust termination is also possible for pure solids, and the shuttle SRBs were originally intended to have that. It would have been achieved with pyrotechnic devices that precisely blow the top off the SRB. NASA ultimately decided against this, I think because it was unclear the ET structure would survive the shock.

Huevos Grandes
2006-Jan-24, 04:02 PM
So why aren't they being investigated to replace solid boosters and 1st stages on launch vehicles?
$$$


Is there any push out there to engineer a full scale hybrid rocket booster?
The United States is using Apollo-era capsule design with a modified shuttle booster. The thinking goes: why spend money on new technology, when the old one works satisfactorily enough ?

Bob B.
2006-Jan-24, 04:41 PM
Because hybrids don't have sufficiently compelling advantages. E.g, the SpaceShipOne hybrid rocket has a specific impulse of about 250 sec. This is lower than the shuttle SRBs.
SpaceShipOne used nitrous oxide for the liquid oxidizer and some type of rubber for the solid fuel (I've never been able to find out exactly what type of rubber was used). This is not a particularly efficient combination, though it worked well enough for the application.

Building a hybrid with a markedly better performance would probably require the use of liquid oxygen for the oxidizer. But in that case you've introduced all the problems of handling large quantities of cryogenic liquid, thus you've lost one of the advantages of using a hybrid in the first place. You might as well just go all liquid rather than messing around with a solid/LOX motor.

I seriously doubt other oxidizers -- N2O4, RFNA, H2O2, N2O etc. -- can provide a performance any better than an all-solid motor.

joema
2006-Jan-24, 06:02 PM
...I seriously doubt other oxidizers -- N2O4, RFNA, H2O2, N2O etc. -- can provide a performance any better than an all-solid motor.
How about a hybrid of liquid flourine and solid lithium? I wouldn't want to be anywhere close by during either manufacture or use, but in theory the Isp would be pretty high.

joema
2006-Jan-24, 06:53 PM
In case anybody's wondering, hybrid boosters would not have prevented the Challenger disaster.

In theory you could have detected the burn through, shut down the boosters, then jettisoned them. However the vehicle had insufficient energy for an RTLS abort. The earliest you could detect the problem was about T+59 sec, at which point the orbiter/ET thrust-to-weight ratio minus the boosters was WAY less than 1:1. IOW it would essentially fall out of the sky. Pre-Challenger there was no bailout option.

Post-Challenger (bailout available), it's slightly conceivable bailout would have been possible. However the energy state was so low it's questionable. You've got to shut down the boosters, jettison them, shut down the SSMEs, position attitude for ET separation, jettison the ET, get oriented for gliding flight, etc. All that takes some time. At T+59 sec, the vehicle was only at about 33,000 ft (10058 meters) and moving at about 1000 mph (450 m/sec). The trajectory might peak at about 60000 ft (18300 m), roughly 20 seconds after shutdown. You've got to begin bailout no lower than 20,000 ft (6100 m). In lower altitude gliding flight, orbiter descent rate is at least 10,000 ft/min (3048 meters/min). Even assuming no structural problems, timing would have been very tight.

Bob B.
2006-Jan-24, 07:04 PM
How about a hybrid of liquid flourine and solid lithium?
I thought about fluorine but didn't mention it because I don't think anyone is really seriously considering using it, particularly for a booster engine ignited on the ground. I believe the Russians built an upper stage using fluorine, but that's the only example I know of beyond test firings.

I'm not familiar with the fluorine-lithium reaction. What is the exhaust product? The examples I've looked at have all involved fluorine, or a fluorine containing compound, and a fuel containing hydrogen, such as hydrazine. The product is hydrofluoric acid (HF), which is highly toxic and corrosive.

From what I've read, fluorine may be a good oxidizer for deep space work but will probably never find use in an Earth launch vehicle.

joema
2006-Jan-24, 07:44 PM
Don't know what the flourine/lithium byproducts are, but I'm sure they're bad. However I think flourine & lithium are the highest specific impulse bipropellants -- higher even the hydrogen & oxygen. The only chemical propellents with higher Isp is a tripropellent of flourine, lithium and hydrogen.

Unfortunately flourine and lithium are two of the most tricky, reactive substances I can think of.

Nowadays nobody in their right mind would use them for a surface launch.

Bob B.
2006-Jan-24, 08:16 PM
I think flourine & lithium are the highest specific impulse bipropellants -- higher even the hydrogen & oxygen.
Interesting; I didn't know that about fluorine & lithium. I may have to do some performance calculations on that someday.

Fluorine/hydrogen also gives a higher specific impulse than oxygen/hydrogen -- about 5% more by my calculations. Fluorine has a slightly higher molecular weight than oxygen (19 vs. 16) but burns much hotter, thus giving higher exhaust gas velocity.


The only chemical propellents with higher Isp is a tripropellent of flourine, lithium and hydrogen.
I'm aware of tri-propellant engines but haven't studied the technology very closely. Isn't the hydrogen injected into the thrust chamber after combustion of the other two propellants to lower the overall molecular weight of the exhaust gas?

joema
2006-Jan-24, 09:41 PM
...Isn't the hydrogen injected into the thrust chamber after combustion of the other two propellants to lower the overall molecular weight of the exhaust gas?
Sorry, I don't remember any more details. I think the Ignition! book by John Clark has more info on these specific propellants.

Hugo Drax
2006-Jan-24, 11:02 PM
I managed to dig up an interesting newsgroup article/thread on tri-propellants here (http://yarchive.net/space/rocket/fuels/fluorine.html)

Interesting snippets:

- Rocketdyne managing to achieve specific impulses of 542 in an engine with a 40:1 expansion ratio fuelled with a combination of fluorine, hydrogen and liquid lithium

- fluorine & lithium are both extremely nasty, and keeping lithium in a safer solid form (liquid lithium is hypergolic with air and highly corrosive) prior to combustion is difficult because of its low melting point.

- most fluorine salts (inc. lithium fluoride) are toxic in bulk, and hydrogen fluoride is even worse.

As joema said, nobody in their right mind would consider using this stuff for a surface launch.

joema
2006-Jan-24, 11:25 PM
...As joema said, nobody in their right mind would consider using this stuff for a surface launch.
Can you imagine a collision between a tanker truck carrying cryogenic liquid flourine (-363F) and one carrying liquid lithium(+356F)? The Soviet N-1 moon rocket crash would likely be less damaging.

Bob B.
2006-Jan-25, 12:04 AM
I managed to dig up an interesting newsgroup article/thread on tri-propellants here (http://yarchive.net/space/rocket/fuels/fluorine.html)
Thanks for digging that up, it was pretty interesting.

publiusr
2006-Jan-25, 10:16 PM
SDLV or bust.

lord bytor
2006-Jan-27, 04:32 AM
Can you imagine a collision between a tanker truck carrying cryogenic liquid flourine (-363F) and one carrying liquid lithium(+356F)? The Soviet N-1 moon rocket crash would likely be less damaging.

Wouldn't a cesium/flourine rocket be better? Cesium is more reactive than lithium and is a liquid near room temperature.

I'd still hate to be the poor sob who had to fuel it though.

Jerry
2006-Jan-30, 04:11 AM
There are many types of solid rocket propellant, most of which have been formulated long after the basic type used in the shuttle (actually developed in the early 1960's). Today's solids have higher specific impulses, more energy per unit volume, and tailorable burn characteristics.

There are solids that can be stopped and started (by quenching, or varying the pressure,) and solids that burn at a resonant pressure, meaning that by varying the nozzle diameter, the thrust could be throttled. Enriched hybrids, binary solids, environmentally friendly solids, and high energy solids with very complex fuels.
The space shuttle family are the only man-rated solids, so these are what are usually used for comparative purposes, but they are certainly not the best for all applications.

Bob B.
2006-Jan-30, 05:06 AM
Jerry,

Do you have any information of the formulations of these solid propellants?