Why does nobody use Liquid-Propellant rockets?

Discussion in 'Propulsion' started by awseiger, Jul 15, 2012.

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  1. Jul 15, 2012 #1

    awseiger

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    Why does nobody use Liquid Propellant rocket motors? If someone were to develop one that was "pop in/out" like our AP and Hybrid motors are, could it be certified by the NAR and used?

    It just seems odd. I feel like constructing a little mini liquid rocket wouldn't be terribly hard. Of course, you'd have to supply the fuel, which is probably not the safest thing to have hanging about in your basement. These things can be averted, however. A tank of oxygen and a bottle of kerosene are sitting in my garage...

    Is there anything that explicitly denies Liquid propellant motors from being certified?
     
  2. Jul 15, 2012 #2

    cwbullet

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    Liquid rocket fuels are dangerous, potentially explosive, and potentially expensive to use. You have to keep them under high levels or pressure and super cooled. At a minimum, they are highly flammable (see the Hindenberg).

    They are just not a viable option.
     
    Last edited: Jul 15, 2012
  3. Jul 15, 2012 #3

    fyrwrxz

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    There was a pressurized gasoline powered rocket late 80's-early 90's but it was expensive and there really is no way to control all the variables so it was an EX. Most of the alloys and machining required is beyond the reach of the all but the most hard core fliers and then most of those have some serious shops to back them up. Your thrust to weight ratios for Ap-based motors are very efficient in comparison. Hybrids basically came about as an answer to the threat of us losing the new (then) reloadable systems that lead to the court challenge ruling AP as an explosive. Thankfully that was overturned and a huge victory for us rocketeers. Hybrids are very cool, but there is still all the GSE to lug out to the field to fly and you have to have electronics on board for deployment. Having said that, check out Reaction Rocket Society for some awesome liquid fueled birds and you can see why it can be addictive.
     
  4. Jul 15, 2012 #4

    COrocket

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    You really need to know what you are doing engineering wise in order to make a liquid rocket work. It's not a "hook up a oxygen and kerosene bottle and see what happens" type of deal. Liquids are considerably more complex in design and building than solids/hybrids, and even those take a lot of expertise to make. With complexity comes price, and they also don't scale down too well. Also you would need a significant ground support system to fuel the rocket. They just aren't practical for hobby use.
     
  5. Jul 15, 2012 #5

    JStitz

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  6. Jul 16, 2012 #6

    awseiger

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    But... If someone developed one, it could be certified, am I correct?
     
  7. Jul 16, 2012 #7

    cwbullet

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    I have not found a rule against it, yet.
     
  8. Jul 16, 2012 #8

    troj

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    I know Tripoli made an exception for the Ratteorks tribrid with its alcohol. Dave Griffiths had to do lots of proving of its reliability and safety before it was allowed to be certified.

    A true liquid motor will be an uphill battle, at best - liquids have tremendous complexity, and when they go wrong, they tend to go VERY wrong

    Add to that the fact that they typically use cryogenic liquids, and it gets even more dicey
     
  9. Jul 16, 2012 #9

    troj

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    Forgot to add...there's an organization with the knowledge and facilities to do liquids safely -- RRS
     
  10. Jul 16, 2012 #10

    cwbullet

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    That is why I would leave this to NASA and SpaceX.
     
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  11. Jul 16, 2012 #11

    oddmanrockets

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    some friends of mine had that gasoline perioxide rocket about 10 or so yrs ago and after lots of engineering and multiple failed launches they finally gave up also anything that the fuel got on (ex.my borrowed parachutes ) it turned it poop brown . it sucked
     
  12. Jul 16, 2012 #12

    MarkII

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    There have been a few discussions about this on TRF over the past couple of years. This one was typical:

    http://www.rocketryforum.com/showthread.php?28092-liquid-nitrogen-alchohol

    I think that COrocket's post (post #21) in that thread was particularly informative. Here is a direct link to it:

    http://www.rocketryforum.com/showthread.php?28092-liquid-nitrogen-alchohol&p=250506#post250506

    There were a number of other good posts made as well, so it is worthwhile to read through the whole thread. Bottom line: this is way, way beyond anything that one would pursue as a hobby. Elon Musk needed investors and had to hire a whole slew of professionals and obtained more than a little help from NASA in order to do it.
     
    Last edited: Jul 16, 2012
  13. Jul 16, 2012 #13

    Rex R

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    well Quest has one rocket that uses DHMO for a propellent mass :).
    rex
     
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  14. Jul 16, 2012 #14

    MarkII

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    Would there be any obstacle to getting a liquid-fueled engine certified? Oh my yes, starting with the fact that the existing safety codes wouldn't even cover it. Propellant storage and handling? Fueling procedures and safeguards? Do you think that any club would have a heavily-reinforced blockhouse at their launch field? And just imagine the field dimensions and set-back distances that would be required!
     
  15. Jul 16, 2012 #15

    MarkII

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    It's a reaction mass but I think it would be a stretch to call it rocket fuel.
     
  16. Jul 16, 2012 #16

    cwbullet

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    Cost: typically you have to prove they are safe and reliable to be certified. Liquid motors require a higher level of support that most club would not be able to provide. I welcome someone to try, but I think it would be difficult to obtain the level of safety and reliability with a liquid motor. Imagine the fires from a CATO.
     
  17. Jul 16, 2012 #17

    Nerull

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    Reading some of the RRS reports, they had to heat 85% hydrogen proxide to 137F during fueling. Hydrogen peroxide undergoes rapid decomposition into oxygen and a lot of heat (A very dangerous combination) at 140F. :y:

    Hydrogen peroxide of that concentration will also immediately burst into flames if it comes into contact with your skin or clothing.
     
  18. Jul 16, 2012 #18

    cwbullet

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    This is why H2O2 can be used in IED type devices by McGiver types.
     
  19. Jul 16, 2012 #19

    awseiger

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    Hmm... Everything said here makes sense. There goes my plans of Liquid rockets, even sometime in the future. I was thinking of something kind of like the hybrids now, with the tanks/nozzle all stuck together in a single casing.

    I'll stick to the Dihydrogen Monoxide/N2 when I want to go liquid :D

    And another thought... Why is it that the "real rockets" (cause ours are real rockets) use Liquid motors, when a solid motor is far simpler to implement? I know that liquids have a lower thrust/longer burn time, but the added complexity for the gain of longer burn time seems silly. There's now way the thrust/weight ratio for a liquid can be anywhere near that of a solid.
     
    Last edited: Jul 16, 2012
  20. Jul 16, 2012 #20

    gpoehlein

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    A big part of it is that liquid fueled rockets can be throttled. Except under certain circumstances, once you light a solid fueled rocket, it goes full bore until it runs out of fuel. So if you want to shut down and restart a rocket (course corrections, de-orbiting burns, etc.) you want a liquid fueled rocket.

    Of course, there were these liquid fueled model rockets back in the early seventies:

    http://www.ninfinger.org/rockets/catalogs/estes73/73est42.html
     
  21. Jul 16, 2012 #21

    JStitz

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    Don't forget we have a lot better recovery rate with solid motors and electronics.
    Most of these RRS rockets are meant for a single flight.(and aren't named by tradition)

    Plain and simple Solid Motors give you more satisfaction for your $$. It also helps to live near the Mohave Desert to cha cha with these liquid guys.
     
  22. Jul 16, 2012 #22

    JDcluster

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    To justify the time, money, and development efforts: you would be looking at much larger engines/ motors for liquids.
    The complexity alone is: 10x that of solids.
    If liquids are your thing, I suggest you hook up with the RRS not the NAR.....


    JD

     
  23. Jul 16, 2012 #23

    Nerull

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    It's all about efficiency. Liquid motors tend to have the highest ISPs, which means you get more impulse per unit weight of fuel. The shuttle used SRBs to lob it clear of most of the atmosphere, as they produce massive amounts of thrust - but would be much heavier than the liquid fuel used for most of the flight.

    When you need to lift a whole lot of weight off the ground and get it moving quickly, thrust to weight is the most important factor, and solids excel at that. That's why a lot of launchers use solid rocket boosters. But once you get above the atmosphere, an efficient engine is better than one that produces a lot of thrust. An efficient engine allows you to carry less fuel by weight than a less efficient engine, making the entire rocket lighter and cheaper. In spaceflight, a pound saved is worth more than it's weight in gold.
     
  24. Jul 16, 2012 #24

    COrocket

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    Liquids are more practical in "real rockets". As someone said earlier the ability to throttle/shutdown/restart is very advantageous. Also, the solid motors used on the space shuttle, for example, are far more complex than the ones in our hobby. Most notably they have a gimbaling system.

    The thrust/weight of a rocket motor isn't really the most important performance parameter in commercial rockets. Obviously if you have longer burn times your thrust/weight will be lower. You would crush your payload/astronauts if your T/W was too high. Guidance systems allow rockets to accelerate much slower than hobby rockets, and thats not a bad thing. There is a lot of optimizing the burn profiles of each engine to reach orbit or whatever the goal is in the most efficient way.

    Things like specific impulse http://en.wikipedia.org/wiki/Specific_impulse can be higher in LH2/LOX liquid engines than solids, and propellant mass fraction http://en.wikipedia.org/wiki/Propellant_mass_fraction is also interesting to read about, considering that the liquid rocket propellant tanks are constructed differently than the solid rocket motor casings, which also play into how efficient the rocket as a system performs.
     
  25. Jul 16, 2012 #25

    spacecadet

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    The tanks can derive their structural rigidity simply from the pressurised liquid, so they don't need much in the way of mechanical support. Blue Streak's skin was not much thicker than a Coke can.
    At 1:20 in this film
    http://www.nationalarchives.gov.uk/films/1951to1964/filmpage_rocket.htm
    you can see the chap shoving it with one hand.
    I believe Atlas was similar.
     
    Last edited: Jul 16, 2012
  26. Jul 16, 2012 #26

    Nerull

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    With modern materials, there isn't really enough advantage to this to make up for the handling and transport issues a balloon tank has. The Atlas V has changed to a rigid self-supporting structure.
     
  27. Jul 16, 2012 #27

    RadManCF

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    IIRC, the current crop Of ICBMs, SLBMs etc. all use solids for simplicity, and safety. They get stuck out in the boonies for extended periods of time, and are supported by very small crews, not a good setting for liquids. I would suggest reading up on the BOMARC missile, particularly the reasons the liquid fueled version was phased out in favor of the solid fueled version. It boils down to a: the missile is ready for launch immediately, as it does not need to be fueled, and b: the potential for godawful fires in greatly reduced. However, since Cape Canaveral and Vandenburg operate under a different set of pressures, the benefits of solids are outweighed by the benefits of liquids.
     
  28. Jul 16, 2012 #28

    GregGleason

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    Well, taking a look at what NASA has done with reliability, one could take a look at the LM/LEM engines, especially the ascent engine. They HAD to make them as reliable as possible, because if they didn't work ...

    The fuel and/or oxidizer were very toxic. IIRC instead of fuel pumps, pressurized helium was used. The pressurized system had a burst disk as part of the design, which is a metal diaphragm that is designed to fracture to vent at a preset pressure so that the major tank doesn't rupture. This actually happened on Apollo 13 on the return leg, but providentially after their final motor burn of the LM's descent motor.

    I will agree that the ISP of liquids is typically better, but the ISP/dollar sure isn't: The development costs as a commercial venture would be prohibitive. Do I wish someone could do it? Sure! But for the foreseeable future I would expect this to remain in the domain of the experimental and research enthusiasts, and never be a part of hobby rocketry.

    Greg
     
  29. Jul 16, 2012 #29

    luke strawwalker

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    Liquid engines are very complex... they might SEEM outwardly simple, but they are not. The experts like Goddard, Von Braun, Oberth, Korolev, Glushko, Isayev, etc. experimented with liquid rocket engines for YEARS before getting ones that would operate safely without melting down or exploding outright. Establishing safe, smooth, steady combustion at startup is absolutely critical and the difference between a motor that explodes and one that actually thrusts. Of course things can go wrong after startup that can quickly melt an engine down, or cause it to explode, but startup is usually the first MAJOR hurdle...

    Liquid propellants are incredibly volatile. "A tank of oxygen" won't cut it... that's GASEOUS oxygen (GOX or GO2) and for a rocket engine you need LIQUID oxygen, which is cryogenic and INCREDIBLY volatile in it's liquid form (basically anything liquid oxygen comes into contact with that's flammable will cause it to burst into flames... go to YouTube and look up "liquid oxygen fires" and you'll see what I mean... even a hot diamond dropped into liquid oxygen will burn away to nothing... even the oil from your fingerprints, if left uncleaned in an LOX line, can combust or explode under the right conditions...) Room temperature storable propellants (such as rocket grade hydrogen peroxide, red fuming nitric acid, nitrogen tetroxide, hydrazine, etc.) are either extremely volatile or highly toxic, or both. Some storable propellants aren't so bad, like Nitrous Oxide, which is what makes the liquid part of a hybrid actually possible.

    You have to force the propellants into the combustion chamber against the pressure inside the chamber, with sufficient pressure to atomize or mix the propellants in the combustion chamber, and feed it at predictable rates, IN THE PROPER PROPORTIONS. An engine running oxidizer-rich will very likely melt down or explode due to the excess heating and corrosive effects of the extremely hot excess oxidizer. The metering of the propellants and the valving is no small undertaking in itself. Metering propellants through an orifice can give markedly different delivery rates based on the pressure differential across the orifice opening (delivery doubles as pressure squares). To pressurize the propellants, you either need something that's self-pressurizing (like nitrous oxide) by boiling off as a low cryogen as head pressure in the tank reduces from propellant expulsion from the tank, or you need a pressurant gas (for say kerosene or other non-cryogenic propellants that do not self-pressurize by boiloff at standard temperatures.) This would require the design and inclusion of a pressurization system to inject the propellant(s), with their associated lines, valves, regulators, etc. All this would be necessary for even a "simple" pressure-fed rocket engine-- a pump fed engine at this scale would be an extremely advanced design and not in the realm of a hobbyist's capabilities.

    Besides, it violates all the safety codes, and IF you could build such a motor that would operate safely and reliably, it would cost literally hundreds or thousands of dollars and STILL be incredibly complex for other hobbyists to use, even compared to the already complex operation of "relatively simple" hybrid motors, which are WAY less complicated than a liquid rocket motor would EVER be...

    Later! OL JR :)
     
  30. Jul 16, 2012 #30

    MCS

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    Because liquids are really hard.. no more..

    solids are hard
    hybrids are harder
    liquids are hardest

    Each of those steps involves an order or magnitude increase in complexity and cost. And let's be honest with ourselves.. most Tripoli-types don't have the commitment and checkbook to chase a liquid.

    If you think you have "the right stuff"... go for it!! (seriously; hell of a journey)

    Having said that, here is my favorite liquid project. Robert Watzlavick 3D printed a working biprop regen liquid motor. He's has several dozen 1 minute tests on the motor. 3D printing could be a game changer.

    http://www.youtube.com/user/RocketMooonlighter?feature=mhum


    -->MCS


    .
     

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