LOX LH2 rocket engine

[jhoblik]

Is it necessary to have Oxygen and Hydrogen in liquid form to properly work inside rocket engine? Or is it just way how to deliver enough fuel to rocket engine? Is it possible to have lower thrust rocket engine and hydrogen and oxygen in gas form?

 

[najab]

Liquid propellants are several hundred times more dense that the gaseous forms. This means that you get several hundred times more propellant in the tank when it is liquid - to store the same amount of gas would take tanks several hundred times bigger.

 

[gofer]

Two words - energy density. Here are the densities for the 2 phases of Hydrogen:<br /><br />rhog = 0.089 87 kg/m3 - gas<br />rhol = 70.99 kg/m3 - liquid<br /><br />energy is proportional to the mass - thus the liquid phase of the Hydrogen packs about 800 *times* more punch per pound. The 2 significant metrics of rocket propulsion, the Thrust and the ISP, are dependent on this. The extra engrgy is put into the fuel during the liquifying process of H2 on the ground. Btw, ordinary car gasoline packs even more energy, but is too heavy per joule. <br /><br />Although, I beleive there have been proposals for water steam powered (low thrust manuevering) rockets.<br />

 

[jhoblik]

Question was not about storing Oxygen or Hydrogen, but about using in rocket engine and if it is going to change ISP.<br />Thank you for any answer.

 

[jhoblik]

I understand about density. But are you sure that same amount of fuel 1LB in liquid form give more thrust than 1LB in gas form.<br />Thank you for any help.

 

[strandedonearth]

Storage issues aside, liquid propellants should give more thrust than gaseous propellants simply due to the huge increase in volume in the phase change from liquid to gas (less volume entering the combustion chamber for liquids over gasses). The same phase change is what makes steam such a powerful working fluid in turbines, even though there is no combustion in the steam system (combustion to heat the water to steam, yes, but it's the phase change from water to steam that produces the pressure).<br /><br />In other words, it's my semi-educated understanding that burning gaseous propellants would not produce as much pressure in the combustion chamber as burning the same liquid propellants, simply due to the extra expansion from the phase change. Not to mention it's easier to feed more molecules of liquid propellants into the chamber than gaseous propellants.<br /><br />If I'm wrong, then someone please tell me, I barely made it though thermodynamics in university.

 

[avmich]

Yes, it is possible to burn gaseous hydrogen (GH2) with gaseous oxygen (GOX) in the chamber. The chemical energy freed with creation of water heats the reaction products, and that energy later gets converted to the kinetic energy of the flow.<br /><br />Note that chemical energy which is freed in reaction 2H2 + O2 = 2H2O doesn't depend on the phase state of the propellants. If you add some energy to propellants before they enter the chamber and start reacting - by heating them - that added energy remains with them, adding to the resulting temperature. So, if you burn GH2 with GOX, the resulting temperature will be higher, and the thrust (per unit of reacting mass of propellants) will be higher.<br /><br />To be more precise, the increase of the temperature in the chamber shifts the equilibrium point in the burning reaction. That means, the composition of resulting gases changes with initial temperature of propellants - if you feed some plasma of H and O into chamber, they won't react much, in a sense that the resulting H2O will just as easily dissolve back. But for "normal" temperatures of liquid H2 (LH2) and O2 (LOX) versus GH2 and GOX that's a rather small adjustment.<br /><br />The volume of components does complicates things in term of construction, but doesn't reduce the thrust by itself. As an example, the Centaur's engine, of RL-10 family, feeds GH2 into chamber, after that GH2 was used to rotate the turbine which drives pumps.<br /><br />Be aware that the biggest pressure point in the whole system is immediately after pumps. Pressure gradually lowers from that point - first in the cooling jacket, then in injectors, and in the chamber it is even lower - and that is the reason why burned gases don't go back into injectors and fuel pipes.

 

[barrykirk]

There are two factors to take into account when comparing gaseous verus liquid fuel and oxidizer.<br /><br />1) The storage density of the liquids versus the gases. The tanks are much smaller for the liquids.<br /><br />2) Just as important it is much easier and energy efficient to pump liquids than gases.<br /><br />The energy required to pump a liquid or gas for that matter is dependent on the volume and the pressure. With a liquid the volume is much lower so the pumping energy is much lower. This means that the pumps can be vastly smaller and are vastly more efficient.<br /><br />Even with pressure fed rockets, a working gas is used to push a liquid into the combustion chamber, the gas itself never goes directly into the combustion chamber.

 

[avmich]

I agree that to put a GH2 into tanks will require vastly bigger and hence heavier tanks, so nobody does that. However, for some engines you still send GH2 into chamber, not LH2 - as it is the case with RL-10. Note that pumps still push the LH2, thus saving power.<br /><br />At the same time liquid H2 has its own drawbacks, which makes reasonable to test chambers using GH2 all the way, without first liquifying it. To make LH2 is much harder than to make LOX, as LH2 has the temperature only 20 K versus 80 K for LOX. In addition to that LH2 requires some better materials to be handled with, since it tends to infiltrate metals.

 

[darkenfast]

I hope this isn't construed as "hijacking" but since there are some educated people here, I have a question regarding LOX/LH engines. Are there any fundamental reasons why a big (F1 or larger) engine can't be built? I'm just speculating about the future. While I'm a big supporter of the policy of using the SRB-derived boosters for now, someday we are going to have to face up to the fact that they pollute. We have a good infrastructure at KSC for handling Hydrogen, and I would like to see it become the main propellant. The SDHLV follow-on might look a bit like a big Delta IV, with three engines. Any thoughts in that direction?

 

[spacelifejunkie]

Is there any way to store hydrogen or a hydrocarbon in solid form? The density would be even that much higher than a liquid and cold I'll bet. In pure form solid hydrogen is probably impossible to maintain in bulk values but is there any new research out there working on similar ideas?<br /><br />SLJ

 

[Swampcat]

<font color="yellow">"Is there any way to store hydrogen or a hydrocarbon in solid form? The density would be even that much higher than a liquid and cold I'll bet. In pure form solid hydrogen is probably impossible to maintain in bulk values but is there any new research out there working on similar ideas?"</font><br /><br />I can think of a few solid (at room temperature) hydrocarbons right off the top of my head...coal, asphalt, paraffin, sugar. Some of these have been used as fuel in hybrid rocket motors. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[spacelifejunkie]

I understand that coal, sugar and the like are possible hybrid rocket fuels but you lose a lot of ISP. SpaceDev's NO2/Solid Rubber is a perfect example of a great hydrocarbon, hybrid rocket. I am speaking of a solid hydrocarbon that is kept cold, melted and used as a liquid rocket fuel. Water expands when frozen but this is unusual for most substances. How much advantage could you gain from say, solid kerosene versus liquid? What about other traditional liquid fuels? Any exotic, experimental stuff being researched out there?<br /><br />SLJ

 

[mlorrey]

Kerosene is pretty solid at temps below -150 F or so. It will melt, though, pretty quickly once you start burning, unless maybe you are going to extrude it like a glue stick, keep the stick cooled with LOX, then spray the LOX on it hypergolically at the end. You might be able to design it to operate as a form of pulse detonation propulsion, that would give you a much higher Isp (1000+)

 

[Swampcat]

<font color="yellow">"I am speaking of a solid hydrocarbon that is kept cold..."</font><br /><br />Understood. Now.<br /><br />Just pointing out that, although you mentioned cold, you did not specify the temperature you were looking for in a solid hydrocarbon <img src="/images/icons/wink.gif" />. <div class="Discussion_UserSignature"> <font size="3" color="#ff9900"><p><font size="1" color="#993300"><strong><em>------------------------------------------------------------------- </em></strong></font></p><p><font size="1" color="#993300"><strong><em>"I hold it that a little rebellion now and then is a good thing, and as necessary in the political world as storms in the physical. Unsuccessful rebellions, indeed, generally establish the encroachments on the rights of the people which have produced them. An observation of this truth should render honest republican governors so mild in their punishment of rebellions as not to discourage them too much. It is a medicine necessary for the sound health of government."</em></strong></font></p><p><font size="1" color="#993300"><strong>Thomas Jefferson</strong></font></p></font> </div>

 

[henryhallam]

<font color="yellow"><br /> Are there any fundamental reasons why a big (F1 or larger) engine can't be built? <br /></font><br /><br />No, but I think a couple of studies have shown that <i>for first stages</i>, if you consider the whole system including tanks etc, then it is more "efficient" / a better idea to use LOX/Kerosene than LOX/LH2 despite the lower specific impulse. I guess LOX/Kerosene is more polluting than LOX/LH2 but it's still a heck of a lot better than SRBs.

 

[propforce]

<blockquote><font class="small">In reply to:</font><hr /><p>"Are there any fundamental reasons why a big (F1 or larger) engine can't be built?" <br /><br />No. <p><hr /></p></p></blockquote><br /><br />Allow me to elaborate further.<br /><br />No, there's no fundamental reasons why a big (F1 or larger) engine can't be built. <img src="/images/icons/smile.gif" /> <br /><br />Sizing up LH2/LO2 engine is much easier than say, LO2/Kerosene engines. The later propellant combination tends to involve combustion instabilities in the chamber as it gets bigger and bigger. That is a tough problem even today, as much of an art as it is science. That is another reason why you don't see big chambers on the Russian engines, they tend to use multiple chambers to make up the total thrust, e.g., RD-170/ RD-180 instead of solving the combustion instability problem.<br /><br />The latest LH2/LO2 engine, RS-68, is already more powerful than the SSME. Either one can be scaled up to bigger and more powerful engine if money is available. <br /><br />The <i>gaseous</i> propellant engine, however; tends to be smaller thruster because of this density/size/efficiency issue many posted above. But yes, there are some GH2/GO2 thrusters available.<br /><br /><blockquote><font class="small">In reply to:</font><hr /><p>No, but I think a couple of studies have shown that for first stages, if you consider the whole system including tanks etc, then it is more "efficient" / a better idea to use LOX/Kerosene than LOX/LH2 despite the lower specific impulse. I guess LOX/Kerosene is more polluting than LOX/LH2 but it's still a heck of a lot better than SRBs.<p><hr /></p></p></blockquote><br /><br />I don't think that is universally the case. One must look at the <i>total system</i> as well as economics (if commercial) and constraints when design a launch vehicle.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[rybanis]

<blockquote><font class="small">In reply to:</font><hr /><p>The latest LH2/LO2 engine, RS-68, is already more powerful than the SSME. Either one can be scaled up to bigger and more powerful engine if money is available<p><hr /></p></p></blockquote><br /><br />Argh, money!<br /><br /><br />Also, isn't the RS-68 heavier than a SSME? I think I remember SG saying that the SSME still has some advantages over the RS-68. <div class="Discussion_UserSignature"> </div>

 

[propforce]

<blockquote><font class="small">In reply to:</font><hr /><p>Also, isn't the RS-68 heavier than a SSME? I think I remember SG saying that the SSME still has some advantages over the RS-68. <p><hr /></p></p></blockquote><br /><br />Of course SG is correct !! <img src="/images/icons/tongue.gif" /> But the RS-68 has a higher thrust too.<br /><br />The SSME is a <i>turbo-charged</i> engine having 4 turbopumps and uses a higher performance cycle, whereas the RS-68 was designed for "economical use", being an disposable engine as such. The chamber pressure, Pc, of SSME is almost double of that the RS-68, so theoretically one can build a 'supersized' SSME to the same size of RS-68 but with 200% of thrust !! <div class="Discussion_UserSignature"> </div>

 

[rybanis]

With twice the blast radius, too!<br /><br /><br />I just reciently saw a host of early SSME test videos. Fun stuff if you're a fan of explosions. Egads though, I just cringe when I think of what it could do to the back end of a shuttle... <div class="Discussion_UserSignature"> </div>