Rocket Science - why do many rockets stage from kerosine to liquid hydrogen?

Factual Questions
1

Ok - serious rocket science question. Saturn V, and other rockets to my understanding, start with a first stage of kerosene, and switch to liquid hydrogen in the second stage. LO is always the oxidizer. From the wikipedia article on “Tripopellant rockets” which I’ve quotes from below, kerosene has higher energy density, and has more “thrust-to-mass”, but hydrogen has more “specific impulse”, which means it has more “impulse” (change in momentum) per unit of propellant. The higher the specific impulse, the less propellant is needed to gain a given amount of momentum. So kerosene has more thrust? But Hydrogen causes a greater change in momentum? I would think they would be about the same thing. If your basic force of thrust doesn’t cause a change in momentum -
then how is it different?

So I’m fuzzy on a few things.
How can one fuel have “more thrust”, and the other “increase momentum” more. I thought perhaps it was because Liquid oxygen and liquid hydrogen burn “faster” and send fewer molecules out at a faster rate than kerosene. I do not
know. The other point made in this article is that containing liquid hydrogen
requires a very serious container since it is under a huge amount of pressure.
So, if we made the entire rocket using only liquid hydrogen, the container might
weigh too much - requiring a bigger container, more weight… ad infinitum. But then this makes me wonder, what would happen if we made the rocket stages to use nothing but kerosene - it would take up too much room?
How’s that if it has more “thrust”. Any insights appreciated.

– quote from article
The other kind of tripropellant rocket is one that uses one oxidizer but two fuels, switching between the two in mid-flight. In this way the motor can combine the high thrust-to-mass of a dense fuel like kerosene early in flight with the high specific impulse of a lighter fuel like liquid hydrogen (LH2) later in flight. The result is a single engine providing some of the benefits of staging.

Although liquid hydrogen delivers the largest specific impulse of the plausible rocket fuels, it also requires huge structures to hold it due to its low density. These structures can weigh a lot, offsetting the light weight of the fuel itself to some degree, and also result in higher drag while in the atmosphere. While kerosene has lower specific impulse, its higher density results in smaller structures, which implies less loss to atmospheric drag. In addition, kerosene based engines generally provide higher thrust, which is important for takeoff, reducing gravity drag. So in general terms there is a “sweet spot” in altitude where one type of fuel becomes more practical than the other.

2

http://history.nasa.gov/SP-350/ch-3-2.html

3

Well that’s consistent but it contradicts the wikipedia article which says that kerosene has more thrust per mass. This can’t be right. If there is “less energetic kerosene” in the first stage it does not have higher energy density. Is there something wrong with the paragraph I quoted?

4

Kerosene gives you more force, but for a shorter time. Force that is necessary to overcome gravity and air resistance. A “comparable” Liquid hydrogen rocket would accelerate slower, and thus spend more time in high gravity.

Hydrogen has a higher specific impulse (change in momentum per mass), so it’s preferable when you have no air resistance. Also it requires a heavier support system, which is more important for the initial stage.

I think both the wikipedia article and the Nasa page are mixing units in a manner that makes things hard to understand. I’ve tried to unravel the mess, but instead of presenting a discussion of those units that might be wrong I went with the simplified explanation, which has the benefit of being right, for a given definition of “right”.

Force per mass, I think, depends on the design of the rocket. I suspect you could design a liquid hydrogen rocket with the same force per mass as a kerosene rocket, but at the cost of a heavier rocket with more wind resistance, which is why I put in “comparable”.

Kerosene is more dense, so you can feed it at a higher rate through a smaller engine, than you can with hydrogen, which is useful in the stage of flight when your force isn’t only adding to the crafts momentum, but is also fighting gravity and air resistance.

I think if you add in the increase in potential energy, and factor in the air resistance of a practical rocket design, you’ll find kerosene has higher “net power”. So when the NASA page speaks of the “higher power offered by liquid hydrogen” it confuses someone like me.

Caveat: IANARS, just a high school physics teacher

5

As discussed in the recent thread on Saturn rockets, the LH2 and LO2 are not under pressure in the rocket’s fuel tanks - or at least not under much pressure. That’s why the stuff is so damn cold: it’s the only way it will remain liquid at low pressure.

In pressure vessel design, for a constant pressure, as the diameter of a cylindrical tank increases, the thickness of the wall must increase. So the compressor tank out in my garage is two feet in diameter, has 1/4"(?)-thick steel walls, and is rated for 150 psi; if I want a tank with twice the diameter and the same pressure rating, the walls will need to be 1/2" thick. The space shuttle’s LH2 tank is nearly 28 feet in diameter. And hydrogen can be stored at pressures as high as 10,000 psi without liquifying. To maintain a substantial pressure for a tank that large would require ridiculously thick walls of high-strength steel.

As it happens, the shuttle’s LH2 tank operates at a pressure of only 33 psi; The Saturn V and other LH2-fueled rockets, no doubt, operate with similar tank pressures.

6

I think I’m beginning to get a better handle on it.
Maybe if I asked it this way -

What problem would I have if I had both stages use kerosene?
What problem would I have if I had both stages use liquid hydrogen?

Thanks,
Dc.

7

Let me try it this way.

ISP is a measure of how efficient a rocket engine is and how fast it will end up going (omitting lots of details).

If you have plenty of time and are just floating out in space, higher ISP is almost always the better deal.

But, when are trying to leave earths surface and get into orbit, thrust to wieght ratio is important too.

Imagine a rocket with a really high ISP, but a terrible thrust to weight ratio. The high ISP implies it needs less fuel. But, if the thrust to weight ratio is really bad, like say 1.1 pounds of thrust to 1 pound of wieght, that means two things. First, to start with, its going to accelerate at 0.1 G, (from the extra .1 pound) which is terrible. It also means most of the energy, the thrust contributing the other 1 pound (and one G) is just used for “hovering”.

An extreme example would be where the thrust to weight ratio is 1 to 1. The rocket does nothing but hovering (till it runs out of fuel).

Yeah, if you added up the pounds of LOX and kerosense and compared it to the pounds of LOX and liquid H2, the LOX/H2 total would be significantly less. But the much smaller (and less pain in the ass kerosene vs cold ass pain in the ass H2) kerosene tanks more than make up for it in other words.

8

I think I get it too, kerosene has higher energy density per volume but less per mass. Kerosene can be fed into a rocket engine faster to burn quicker. Those are some good design advantages. Ultimately tho hydrogen has higher energy per mass.

9

Btw, energy density of kerosene is ~45 MJ/kg, while hydrogen is ~130 MJ/kg. Big difference.

10

Incidentally, the canceled shuttle replacement craft (the X-33) used only hydrogen as fuel.

11

Ok… I hope everyone is not entirely tired of this.
This is what I’m getting. Hydrogen has more energy per MASS… but is not very dense and requires large tanks. Hydrogen has less energy per volume.
So this means to me, that the reason the first stage is not hydrogen is that the rocket would be way too big, correct? Since hydrogen is less dense, you’re talking about a huge tank with much air resistance.
Now as to part II, why don’t we make all the stages kerosene, I think you could, but you would end up with less power for the weight of the rocket, as opposed to the volume.
In other words, getting of the ground is structurally limited by size, and once you are in a
light enough atmosphere, your goal is strictly power for mass…

Everyone more or less agree?

Thanks.

12

OK, from what I can tell, LH is a really good fuel for the mass, but because it isn’t dense, it burns slow (because it takes a long time to squish the thin fuel into the engine) and it requires a lot more heavy bulkhead, etc. So when you’re in gravity/atmosphere, you want the faster-burning kerosene to get you off the ground – even if it takes more mass. Once the fast-burning, mass-inefficient kerosene gets you quickly away from the heaviest gravity and atmosphere of sea level, you can burn the rather more leisurely but efficient LH2 without worrying about gravity and atmo, because now much less of the rocket’s power is necessary just to maintain position.

Also, I have to imagine there’s just some historical legacy involved. The Jupiter burned kerosene. When they created the original S-1 stage for the first Saturn 1, they literally took Jupiter rockets and strapped on some extra propellant tanks, so it burned kerosene too, while the S-IV second stage (designed and built expressly for the Saturn 1) burned LH2. Then, when they upgraded to the Saturn 1B, they just made more powerful versions of the extant stages, and basically the same thing happened in the upgrade to the Saturn V. But of course, that legacy decision only made sense because the kerosene and hydrogen fuels worked in their differing roles in the first place.

–Cliffy

13

Anyone care to comment on the last 2 posts? Hydrogen burns slow? I would think you oxidize hydrogen you get a pretty phenomenal reaction - water as a by product? My impression was that hydrogen would have a bit more kick but would take up more room. Anyone?

14

First of all, a distinction needs to be made between propellant specific impulse (I[sub]sp/sub) and vehicle specific impulse (I[sub]sp/sub). Propellant specific impulse is just accounts for the impulse per unit mass of propellant products. Vehicle specific impulse has to factor in the weight of the vehicle, including the dead weight of tankage and propulsion hardware (mass fraction). Because liquid hydrogen is less dense it requires more tankage, which both increases weight and the vehicle outer diameter, or length, or both. Note that thrust is related to specific impulse by a ratio of change in mass over change in time; hence, it is more efficient to have a rocket with higher thrust for a shorter period of time than one with lower thrust for a longer action time for the same I[sub]sp[/sub], even if the specific impulse of the propellant is lower, and especially if there is a significant difference in vehicle specific impulse. So it makes sense to use kerosene or some other massy but energy dense hydrocarbon fuel on a lower stage, but use cryogenic propellants on an upper stage where the total volume is lower and therefore the mass ratio is not as unfavorable.

Another reason for using high molecular weight fuels on the lower stage is the thermodynamics; in the atmosphere, heavier molecular products will deviate from ideal gas behavior and retain momentum through the exit plane o the nozzle longer than low molecular weight propellants. However, in the upper atmosphere and vacuum where p[sub]3[/sub]~0, those effects disappear and low molecular weight products have more momentum per unit energy, giving more impulse.

This difference is so pronounced that most “strap on” or zero-stage boosters are solid rocket motors (SRM) rather than liquid fuels. Although the performance of these motors is only 60-70% in terms of I[sub]sp[/sub] of a comparable liquid propellant rocket engine, the high thrust over shorter action time gives superior performance, and the compactness of the motor provides greater overall performance. Similarly, most modern ICBMs are SRMs (except for the post-boost maneuvering systems which require throttling and high precision) in part because they can be stored indefinitely without fear of leakage or corrosion, but also because of how compact they are per unit thrust.

Large rockets that use cryogenic fuels attempt to reduce structure as much as possible. The SM-65 ‘Atlas’ missile actually used the tank pressurization to support the tank walls and prevent them from buckling (balloon tanks), while modern designs use a lightweight machined aluminum web structure. The X-33 used an ultra-lightweight composite honeycomb design to provide adequate strength and stiffness, and also tried to adopt linear aerospike engines to optimize performance at varying altitude, ultimately without success. The STS uses hydrogen fuel in the main engines with the Solid Rocket Boosters; however, while both propulsion systems are ignited nearly simultaneously, it is the SRBs which offer the bulk of the energy to achieve orbit, even though they are a fraction of the volume of the External Tank.

As for hydrogen burning slower than petrofuels, consider this; for a given aperture, only a certain volume of essentially incompressible liquid fuel can be forced. While hydrogen has more energy per unit mass, it is significantly less per unit volume, which means that you either have to increase the flow rate by spinning turbopumps faster or adding more pumps. If you want more thrust than can be provided, the answer is to go to a fuel with a much higher energy density, even if its combustion performance is actually lower. Also note that most liquid engines (and all solid motors) to not burn at stoichiometric ratios; rather, they tend to run richer, expending some uncombined propellant that is heated by combustion. This turns out to be more thermodynamically efficient than complete combustion, and is sometimes necessary to prevent excessive heating of the nozzle throat or combustion chamber.

Stranger

15

Some fascinating detail from “Stranger on a Train”. But I am still stuck on a couple concepts. I see that there are two kinds of specific impulses, one involving the fuel and
one taking into account the tank or work that needs to be done external to the fuel.
Few questions

  1. kerosene has a lower specific impulse than a cryogenic propellant? Or it has lower specific
    impulse than a cryogenic propellant at a certain altitude - Which?
  2. the mass ratio is unfavorable in the first stage, requiring more thrust. Thrust or power, can be created by having massy hydrocarbon fuels such as kerosene that have more of an affect in denser atmosphere. But, as you get higher in the atmosphere, lower weight products give more momentum per unit energy, whereas lower in the atmosphere, higher weight products give more momentum per unit energy?
    When we get to lighter atmosphere, it is better to use a fuel with better specific impulse.
    It still sounds to me like we want more power to deal with weight and air resistance at the
    bottom of the flight. But this is lower specific impulse fuel? if so, we must use it because something else is at work than specific impulse to derive more thrust - is it the non-ideal gas
    behavior? Or simply the fact that it doesn’t require a giant tank because it is denser?

What would happen if my 2nd stage rocket used kerosene. What if a rocket was designed to do that - what would it look like?
Also, this section confused me

"
While hydrogen has more energy per unit mass, it is significantly less per unit volume, which means that you either have to increase the flow rate by spinning turbopumps faster or adding more pumps. If you want more thrust than can be provided, the answer is to go to a fuel with a much higher energy density, even if its combustion performance is actually lower."

Well, hydrogen has higher energy density than kerosene as mentioned by a previous poster.
What would be an example of a fuel we would use in that situation? Something with higher
energy density than hydrogen and lower combustion performance? What is “combustion performance” ?

16

First of all, when comparing propellant specific impulse (also called theoretical specific impulse) you need to consider the combination of fuel and oxidizer. Assuming a common oxidizer–say, liquid oxygen–you can generate specific impulse curves based on steady-state combustion temperature and chamber pressure versus nozzle throat pressure. So it’s not a specific number of a family of curves. However, comparing hydrogen to RP-1 (rocket grade kerosene) gives higher I[sub]s[/sub] for LH2/LOX than RP-1/LOX, provided that you can achieve the same chamber pressure. With liquid propellant engines, attaining optimal sustained tank pressure is difficult, especially in high thrust engines, just because of the amount of fuel and oxidizer that has to be throughput to keep the pressure high. Using low energy density liquid hydrogen makes this more difficult than higher energy density RP-1; while hydrogen has more energy per unit mass (or mol) it has less energy per unit (incompressible) volume, so you have to have faster pumps, larger injectors, et cetera to maintain high throughput compared to petrofuels. From Sutton’s Rocket Propulsion Elements:
For high performance a high content of chemical energy per unit of propellant mixture is desirable because it permits a high chamber temperature [and pressure]. A low molecular mass of the product gases of the propellant combination is also desirable.
So while the low molecular weight of hydrogen is desirable, the energy density of petrofuels allows higher thermodynamic performance in the engine chamber for high thrust, high performance applications. There is actually a parameter called density specific impulse, I[sub]d[/sub], which is the specific impulse multiplied by the average specific gravity of the propellants. Note that all of this just pertains to properties within the combustion chamber; actual vehicle specific impulse is determined by propellant flow rates (for liquid engines) or from thrust-time profiles (solid motors).

First of all, realize that specific impulse and thrust have nothing to do with each other. Indeed, specific impulse is a performance parameter in which thrust is essentially normalized, and all existing really high I[sub]s[/sub] propulsion systems have very low thrust (on the order of a few newtons or less) and are actually extremely energy inefficient. Using propellants with high molecular weight products are more inefficient at high altitudes and in vacuum because regardless of how large you make the nozzle they don’t expand well and push against the sides of the nozzle; instead, they fly out retaining a large fraction of heat energy, whereas lightweight gases will expand more completely and will also develop higher momentum per unit energy. This is especially true of solid propellants which produce particulate matter in the exhaust. In thicker atmosphere this effect is less pronounced and the compactness and high thrust/shorter action time of the heavier propellant is more important than high specific impulse.

In fact, there are a number of upper stage rockets that use storable (non-cryogenic) propellants. The SpaceX Falcon 1 and Falcon 9 use LOX/RP-1 for both the first and second stages, and the Angara rocket system developed by the Russian KSRPSC will use kerosene and LOX on all stages in its RD-191 engine, which are developed from the RD-170/-171 engine used for the Zenit boosters on the Soviet Buran shuttle program. Most American storable systems have used NiTeT and UDMH or Aerozine-50 on upper stages, like the famed Agena. This is most generally done for restartability or throttle control.

Stranger

17

This is very interesting stuff and I can’t thank you enough. It is much more complicated than I thought. Where can one learn more about this? I minored in Chemistry so I understand some of the terminology but not all of it. Is Sutton’s “Rocket Propulsion Elements” that you mentioned worth cracking open? Is there anywhere in the U.S. where people experiment in model rocketry that is not “Estes” prefab solid rocket motors? I guess it’s a fairly well understood science by now and probably the only people that have fun are the military?
Just asking.

18

Well, it IS rocket science. :slight_smile:

Another set of many thanks to Stranger on a Train. It has been a fascinating read in a topic where my interest greatly exceeds my ability to understand.

19

George Sutton’s Rocket Propulsion Elements is the basic ‘Rockets 101’ text; it’s not something I reference on a daily basis, but it does have comprehensive basic propulsion theory for solid and liquid motors as well as touching on advanced technologies (hydrid motors, gelled propellants, electric and nuclear propulsion). Sutton has cribbed from pretty much everybody in the rocket industry, often without credit, so he’s pissed a lot of people off but it’s a good starting point and is accessible to anyone with college physics and chemistry. I think Humble’s Space Propulsion Analysis and Design is actually a better book (from a theory standpoint) but it requires more understanding of thermodynamics and calculus, and is lamentably out of print. *Modern Engineering for Design of Liquid-Propellant Rocket Engines* is a good review of liquid rocket engines (with which I am only passingly familiar) but it gets to some advanced combustion theory that is beyond dilettante interest.

I’m not a rocket hobbyist so I can’t really speak to recreational experimentation, but the National Association of Rocketry is the hobbyist organization for rocket enthusiasts. It looks like they approve and certify only solid propellant motors up to Class O (Heavy Sport). Manufacturing your own solid motors would get into recreational pyrotechnics. Liquid motors are another area entirely and probably outside the domain of the recreational hobbyist, but there is no doubt people who do this, so poke around. Depending on the propellants used you may need to get BATFE approval, plus adhere to state and local regulations regarding pyrotechnics, explosives, and destructive devices. This is certainly an activity that requires knowledge and caution.

Stranger

20

Thanks Stranger!
Pricey books unfortunately. All the good books are like that.
Thanks for your help.
-Random