Kerosene & LOX Limits

[spacelifejunkie]

Everyone, including myself are on the edge of their seats waiting for the Falcon 1 to fly. The posts on this site are a testament to that. My humble opinion is that it is a matter of time before someone posts an "I told you so!" on the success of Falcon 1. Musk and SpaceX have the vision of colonizing the solar system, namely Mars. What are the performance limits of his kerosene/LOX rocket? The isp is pretty good but does not match the space shuttle's performance. How big a lifter can SpaceX possibly create? A trip to Mars will require a mega heavy lifter. How close to the theoretical/practical limit was the Apollo rocket for kerosene and LOX? Also, Musk has a knack for lowering the cost of rockets but what would a SpaceX super heavy lifter cost?<br /><br />Your thoughts?<br /><br />SLJ

 

[vogon13]

As Von Braun envisioned with the Nova class rocket, you just keep making the kerosene/LOX stages bigger, till they lift the load you want.<br /><br />At some point though, you might hit upon Orion Nuclear Impulse Propulsion as being the better answer.<br /><br /> <div class="Discussion_UserSignature"> <p><font color="#ff0000"><strong>TPTB went to Dallas and all I got was Plucked !!</strong></font></p><p><font color="#339966"><strong>So many people, so few recipes !!</strong></font></p><p><font color="#0000ff"><strong>Let's clean up this stinkhole !!</strong></font> </p> </div>

 

[comga]

Focusing on Isp is a trap. Offerings to the temple of Isp require hydrogen, which has very low density, leading to enormous first stages with thick insulation, some of which falls off sometimes (Shuttle) or burns on stage ignition (Delta IV by design). Have you seen how high the Delta IV goes before it begins it gravity turn? Its due to keeping the enormous structure light at the expense of strength.<br /><br />There was a NASA study not that many years ago that concluded that the optimum fuel for first stages is LOx/Kerosene. Even the SDHLLV will use hydrogen as a true second stage, not lighting the SSMEs until at altitude. Now as an upper stage fuel, like the Centaur, H/LOx works great, even if it doesn't work cheaply.

 

[scottb50]

LOX/H2 works just fine as a first stage, and yes the physical size of a tank may have to be bigger, but it more than makes up for taking all that Carbon along for the ride. <br /><br />With proper tanks and an outer covering icing should not be a problem either. The main problem with LOX/H2 engines is the optimization of nozzles. The SSME is a compromise, but if it didn't have to operate over such a large range it could be better tuned for low altitude thrust. <div class="Discussion_UserSignature"> </div>

 

[darkenfast]

Comga, I understood that the SSME's on the Heavy would be ignited at launch, not at altitude. Do you know this for a fact or does anyone else have that info?

 

[propforce]

Yes for the cargo version (SDHLLV), the SSMEs will ignite on the ground during launch based on NASA's latest release (ESAS) "1.5 stage" concept. http://images.spaceref.com/news/2005/nas.esas.19.l.jpg<br /><br />The CLV, however; will use the SSME as a 2nd stage engine therefore will be air-lit. http://images.spaceref.com/news/2005/nas.esas.18.l.jpg <div class="Discussion_UserSignature"> </div>

 

[propforce]

<font color="yellow">Offerings to the temple of Isp require hydrogen, which has very low density, leading to enormous first stages with thick insulation, some of which falls off sometimes (Shuttle) or burns on stage ignition (Delta IV by design). Have you seen how high the Delta IV goes before it begins it gravity turn? Its due to keeping the enormous structure light at the expense of strength. </font><br /><br />Where are you getting these stuff???? <img src="/images/icons/laugh.gif" /><br /><br />Did you notice on the Shuttle, the ET foam insulation fall off is from the <i> top</i> of the ET, which is where <i>liquid oxygen tank</i> is located??? The foam falling off has to do with the processing chemistry of making the foam insulation, not because of the choice of hydrogen propellant. <br /><br />Likewise on the Delta IV, the burning of external insulation at lift off has to do with a combination of enclosed exhaust duct with escaping hydrogen gases, and the choice of insulation used. <br /><br />The Delta IV flies its trajectory differently also mainly because it has a lousy thrust 2nd stage engine, the RL10B-2, thus has to fly a high lofting trajectory to make up its weakness on that engine. The hydrogen/ oxygen 1st stage has so much energy, it alone delivers the vehicle to 100 nmi (IIRC, will check it tomorrow), so technically the first stage is SSTO <img src="/images/icons/smile.gif" />.<br /><br />While it is true that for the same delta-vee performance, a LOx/Kerosene first stage will result in a smaller propellant tank, but it will also be REQUIRING A MUCH HIGHER THRUST ENGINE because the density of Kerosene is much heavier than hydrogen. Otherwise, you'll seriously suffer the vehicle lift-off thrust-to-weight.<br /><br /><br /><br /><font color="yellow"> There was a NASA study not that many years ago that concluded that the optimum fuel for first stages is LOx/Kerosene. Even the SDHLLV will use hydrogen as a true second stage, not lighting the SSMEs until at</font> <div class="Discussion_UserSignature"> </div>

 

[chriscdc]

I thought they went with the SRBs due to budget constraints when they developed the shuttle and planned on moving onto a liquid fueled rocket later on. Then they didn't have the money to develop the system.<br />Can someone clarify this?

 

[comga]

OK OK Igniting the SSMEs at altitude must be an outdated or just incorrrectly read stuff from NASA. I got it wrong and will go back to the latest and greatest from NASA. And the foam shedding isn't the worst thing, if you are not in its path. (The last foam shedding from the PAL ramp probably had little to do with the underlying temperature, although I thought it was from the aft end, which is outside the large hydrogen tank.) We can argue some other time about the Delta IV lofting and whether the absurd burning of the aft end insulation is a perfectly logical engineering choice, comical, or both.<br /><br />However, one fact remains. Hydrogen is a very low density fuel. The resulting structures are much larger than those for a comparable (in terms of delta-V) first stage. Therefore it is not obviously superior to kerosene and LOX for a first stage. The largest liquid engines have been kerosene and LOX, although the SRBs are still larger. <br /><br />In short, there are no more limits to Kerosene and LOX rockets than any other, and perhaps fewer.<br /><br />

 

[comga]

The Delta IV is no more an SSTO than SpaceShip 1. As all the SS1's critics point out, its not altitude that counts but velocity. The Delta IV spends an enormous amount of time and energy going up, which doesn't help at all. The energy it takes to raise a 300km by 20km orbit to 300km by 100km is a small fraction of the energy it would take to go straight up from 20km to 100km. The earlier the turn towards horizontal is made, the better. Lofting is not going to make up for a weak second stage. <br /><br />The NASA study that determined the superiority of kerosene & LOX was done only a few years ago, decades after the Shuttle was designed. I will look for the reference. And it was only a study. Studies have a way of being generated and either being publicized or ignored by how their conclusions fit with other realities, including political and comercial. <br /><br />I can't argue with your statement about the needing a much higher thrust engine with kerosene & LOX. However , much larger kerosene/LOX engines have been built, and engine size, like Isp, is just another thing to be traded in optimizing a rocket. That trade still comes out well for kerosene & LOX rockets.

 

[propforce]

<font color="yellow">The Delta IV is no more an SSTO than SpaceShip 1.</font><br /><br />No doubt. But you can't deny SS1's claim of having reaching 'space'. Likewise, the DIV first stage having so much energy has to be credited to having hydrogen as fuel, despite the relatively lousy performance of RS-68.<br /><br />I agree that lofting is <i>not</i> going to make up for a weak second stage, but this was the decision of the 'brilliant management' made <rolling eyes big time here> just as the dumb decision of putting LOX tank on top of Hydrogen tank <big sigh>. Unfortunately, engineers are not in charge of their own destiny <sobbing>.<br /><br />A study was made a few years back and determined that DIV could substantially increase its payload performance simply by changing out the 2nd stage engine from 25K lbf to a 60K lbf engine. But the economic reality (no money) soon killed that idea. The RS-68 is undergoing some IRAD upgrade design and will improve much of its lacking performance. Still, the engine cycle selected (GG cycle) pretty much limited it's performance ceiling.<br /><br /><br /><font color="yellow">The NASA study that determined the superiority of kerosene & LOX was done only a few years ago, decades after the Shuttle was designed. I will look for the reference. And it was only a study. Studies have a way of being generated and either being publicized or ignored by how their conclusions fit with other realities, including political and comercial.</font><br /><br />You must be referring to the Space Launch Initiative (SLI) study done a few years ago where NASA came out and stated that they want a LOX/RP first stage. <br /><br />There's a lot of merit for having kerosene (RP) as fuel for a launch vehicle, particularlly the first stage. I am not 'anti-kerosene' by any means. In fact, I've briefed our company VP on the pros & cons of RP vs. Hydrogen. The bottom line - you only want a hydrogen vehicle because you <div class="Discussion_UserSignature"> </div>

 

[nacnud]

<font color="yellow">...just as the dumb decision of putting LOX tank on top of Hydrogen tank...<font color="white"><br /><br />Why is that a daft idea? I would have thought that putting the denser liquid at the top of the rocket reduces the amount of effort needed to keep the rocket staight when flying (thrust vector), or does the extra structure needed to support the heavy LOX out weigh this?</font></font>

 

[spacelifejunkie]

"So the message here is that, if the U.S. want to build bigger launch vehicles using LOX/RP as first stage, there's no U.S. manufactured LOX/RP engines available."<br /><br />Since there are none available, could a small startup like SpaceX make this happen? Building the Falcon rockets so far has been a challenge but what about an Apollo or larger rocket for the moon and Mars? Can they pull that off from scratch? Not without major funding. How much? One of SpaceX's advantage is its lean style of management. They would have to hire a lot of new people to build that kind of rocket. Can they maintain their vision considering a massive growth in the company and answering to stock holders wishes for profits simultaneously? A lot of variables.<br /><br />On the science...Can SpaceX still build an Apollo class rocket (they affectionately call it the "BFR") with complete reusability like the rest of their rockets? Two stages or three? Boosters plus two stages? How many boosters? Even an upgraded Merlin at 100,000 lbs. thrust would necessitate using about 75 of them to match the Apollo's total thrust. When does the practicality break down so that SpaceX can no longer maintain its cost advantage? <br /><br />SLJ

 

[nacnud]

Well Musk has stated he wants to create an F1 class engine.

 

[henryhallam]

<font color="red"><br />...just as the dumb decision of putting LOX tank on top of Hydrogen tank... <br /></font><br /><font color="yellow"><br />Why is that a daft idea? I would have thought that putting the denser liquid at the top of the rocket reduces the amount of effort needed to keep the rocket staight when flying (thrust vector), or does the extra structure needed to support the heavy LOX out weigh this?<br /></font><br /><br />Perhaps something to do with having to pipe the warm LOX through the cold LH2 tank, leading to increased LH2 boiloff and/or heavy insulation?

 

[nacnud]

Increased boil off...<br /><br />I've no idea but if this was a problem then why the repress lines on the ET?

 

[tap_sa]

Because ET LOX line doesn't go through the hydrogen tank but runs on the side of it? AFAIK Delta IV has the same config. Having the LOX feed on the outside of the rocket poses some aerodynamic annoyance but it seems to be much better than trying to fight hydrogen boil/LOX freeze problems with more insulation.<br /><br />And you are right about the stability issue, the denser stuff usually goes in front.

 

[propforce]

<font color="yellow"> Why is that a daft idea? I would have thought that putting the denser liquid at the top of the rocket reduces the amount of effort needed to keep the rocket staight when flying (thrust vector), or does the extra structure needed to support the heavy LOX out weigh this? </font><br /><br />It does keep the vehicle Cg forward, but it also add lots weight for the vehicle. Since LOX is 8X heavier than LH2, By putting LOX tank forward, the LH2 tank wall has to be mch thicker than it had to be. Since hydrogen tank has lots of volume, the extra increased in weight is substantial.<br /><br />Also now the LOX has a long way travel down to the engine, a separate anti-geyser line must be added in addition to LOX feedline. POGO suppression device has to be added as well because of this feedline length. All these added weight to the rocket.<br /><br />It is not true on other rockets put denser liquid forward. On LOX/RP vehicles, RP is put forward of LOX. The Cg issue can be controlled by engine gimbaling where as dry weight takes away payload performance.<br /> <div class="Discussion_UserSignature"> </div>

 

[propforce]

<font color="yellow">Since there are none available, could a small startup like SpaceX make this happen?</font><br /><br />He would need to buy existing engines or commission an engine company to develop one for him. As I've said, high power LOX/RP engine with Pc much higher than that of F-1, has not only funding difficulties but technical issues that the U.S. engine manufacturers have yet able to over come.<br /><br /> <div class="Discussion_UserSignature"> </div>

 

[tap_sa]

<font color="yellow">"On LOX/RP vehicles, RP is put forward of LOX."</font><br /><br />On which vehicles? Saturn V, Soyuz, Atlas all have LOX on top of RP-1.<br /><br />What does anti-geyser line do?

 

[barrykirk]

That doesn't surprise me that LOX/RP1 might be optimal for a first stage. Do you have a link to that study?<br /><br />

 

[propforce]

<font color="yellow">On which vehicles? Saturn V, Soyuz, Atlas all have LOX on top of RP-1. </font><br /><br />You're right. I stand corrected! I knew the Delta II has the LOX at the bottom of RP tank last night but should've check all other vehicles first <img src="/images/icons/blush.gif" /><br /><br />Thanks for the correction <img src="/images/icons/smile.gif" /><br /><br /><br /><br /><font color="yellow">What does anti-geyser line do? </font><br /><br />When cryogenic propellant travel down the feedline, the 'warmth' of feedline may cause some propellant to vaporize and become "bubbles". These "bubbles", if with enough quantity, could rise up causing the collaps of flow, and creat "water hammer' effect damaging engine and/or feed line. As minimum, it will cause engine to injest bubbles which could suffer operationally. An anti-geyser line allow some recirculation of propellant by its thermal gradient, and sometime aided with ingestion of gaseous helium, thereby minimize the two-phase flow of propellant from occurring.<br /><br />POGO effect is a vibration phenomena coupling between fluid (propellant), engine and vehicle structures such that, if not addressed, could cause structure failure of launch vehicle - not a good thing !! <div class="Discussion_UserSignature"> </div>

 

[drwayne]

I am not the brightest person in the world mechanically, but I am still amazed that the plumbing issues that arise in these systems are dealt with as successfully as they are.<br /><br />Wayne <div class="Discussion_UserSignature"> <p>"1) Give no quarter; 2) Take no prisoners; 3) Sink everything."  Admiral Jackie Fisher</p> </div>

 

[scottb50]

That is not a problem, it could be an advantage. The warmer the Hydrogen, and the higher the pressure, the better. The colder and denser the Oxygen the better, also. <br /><br /> <div class="Discussion_UserSignature"> </div>

 

[tap_sa]

<font color="yellow">" An anti-geyser line allow some recirculation of propellant by its thermal gradient"</font><br /><br />Very interesting info, thanks! Couple questions: Is the antigeyser line used only during chill-down process to cool the feedline or does it operate continously while engine burns? If latter then is there some mechanism that separates the unwanted bubbles and sends them back to tank via the antigeyser line? I assume the ag-line separates from the main feed line before (first) pump, does the ag-line discharge to the top of the tank (ullage) and if so what prevents the circulation going the wrong way ie. pump sucking even more gas through the ag-line?<br /><br />I really want to know how these thing work so any tidbits are greatly appreciated <img src="/images/icons/wink.gif" /> This ag-line thing is especially interesting because couldn't find anything about it from Sutton's book so it seems these are the lesser known items in the art of rocket propulsion.