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Lecture 2: Archive Notes February 10, 2010

Posted by drspaceshow in Uncategorized.

Space Show Classroom Lesson 2:  The Rocket Equation

Tuesday, Feb. 9, 2010

Archive Notes and Program Information


The Space Show Classroom Lesson 2 The Rocket Equation can be downloaded or heard from:


Guests:  Classroom guest panelist Paul Breed with co-hosts Dr. Livingston, Dr. John Jurist, Dr. Jim Logan.  Topics:  The Rocket Equation and why it matters.  Welcome to Lesson 2 The Rocket Equation in our Space Show Classroom series.  This nearly two hour program focused on The Rocket Equation, how to understand it, use it, and why we build and launch rockets the way we do.  In the first of two segments, Paul Breed began our discussion using his presentation materials posted on The Space Show Classroom blog, https://spaceshowclassroom.wordpress.com.  This is a very comprehensive discussion and explanation of the rocket equation and the physics and engineering governing the launching of chemical rockets.  Paul and our co-hosts talked at length about the difference between suborbital and orbital, the amount of energetic differences in the two and why going to orbit is 25 times harder than suborbital flight.  We talked about fuel, mass rations for the rocket full of fuel and payload as well as the rocket’s dry weight mass ratios.  We talked about the need to accelerate to a minimum of 17,000 mph and the 1,000 mph boost the rocket gets from launching west to east at the equator which diminishes to only about a 300 mph if you launch from a polar site.  The Classroom rocket equation team also described  and explained the penalty for launching east to west and used Israel as the example for this type of launch.  Comparisons were made to the Saturn V rocket and jets.  We talked about why a big rocket has less of a launch penalty than a small rocket, the SSTO was examined so that all of us would understand why it is so difficult and illusive to build.  We talked about why wings, landing gear and reusable robustness work against the orbital vehicle but are better for the suborbital vehicles.   Air-breathing rocket engines were discussed along with jet engines, and toward the end of this segment, we got into the rocket’ equation significance for costs.  In our second segment, Paul took us through the rocket characteristics, mass, and costs for the typical Lunar Lander Challenger rocket.  Listen carefully to what he says about this and how the rocket equation dictates the scaling up for this type of vehicle to orbital status.  Paul said such a vehicle would have a gross takeoff mass of 3,000,000 pounds!  Air drag was discussed in both segments of the show but again applied to small rockets in this segment, thus within certain limits, an air launch of a smaller rocket is useful.  Paul explained why in his opinion the ramjets, scramjets, and hypersonics don’t make sense other than for the military or only for upper atmospheric operation because at best they can only produce about 16% of the needed energy for orbit. But again, for suborbital, its a different story.  During this segment, we explained rocket staging.  As you will hear, each stage is planned and designed using the rocket equation but builds upon the performance of the preceding stage.  Later in this segment, Dr. Logan used the Space Shuttle and the SMEs as an example of mass ratios for payload.  We had support on what Jim was saying with a Space Shuttle scientist from Houston who offered many comments and facts regarding the Space Shuttle example.  This example dramatically describes what Paul, John, and Jim talked about during this rocket equation program.  Near the end of Lesson 2, we discussed possible propulsion and space access from other means that would allow significantly better performance given the rocket equation constraints or even not be subject to the rocket equation including nuclear, the space elevator, fusion, and non-chemical rockets.  We also talked about the difference in fantasy and reality and all our participants said that the fantasy while important, does not work for space exploration.  Space exploration requires reality and reality is not interchangeable with fantasy.  The issue of changing orbits came up, carrying more fuel on an emergency Space Shuttle flight, and why its not practical or doable and even insignificant given the ratios for the shuttle.  If you have a comment or question for any of the participants on this Classroom program, please post it on the blog. E-mail addresses for the three co-hosts are listed on the blog. For Paul Breed please use paul@rasdoc.com.


1. drspaceshow - February 10, 2010

I want to thank Paul Breed, Dr. John Jurist, and Dr. Jim Logan for an excellent, informative, and important discussion regarding the rocket equation. On behalf of The Space Show and its global family of listeners, you have all provided an important service to us and helped in furthering important space education. I know that I learned a lot during this discussion.

Listeners, your comments and questions are welcome. Feel free to agree or disagree with civility. Please post your comments regarding the rocket equation Classroom program here on this blog. Comments I receive directly will be posted to this blog.

I look forward to the continued discussion about the rocket equation and hope to hear from all of you.

Dr. David Livingston, Host
The Space Show

2. Jim Davis - February 12, 2010

This lecture was very well done even though the material really demands a visual presentation.

Some comments and questions:

1. Commercial airline engines have Isps in the 6000 s range at cruise and substantially higher at lower speeds.

2. The discussion of safety margins was interesting. It has been claimed that Soviet/Russian rockets are designed to higher safety margins. Is this the case? Can this be quantified?

3. The discussion of RLVs, both one and two stage, reminded me again of claims made online. The claim is often made that fully reusable launch vehicles have been well within the state of the art for almost a half century and the reason we have yet to see one is because “the missile lobby is too powerful” or “that no one has really tried”, etc. It would be interesting to hear the panel’s view of such claims.

Thanks again. I am looking forward to future shows.

3. Drspace - February 12, 2010

Jim, I will defer the technical aspects of your comments to Paul, John, and Jim ,but I suggest you read the paper that Drs. Jurist, Dinkin and I wrote a few years back, “When Physics and Economics Collide” as we did a thorough analysis of costs associated with RLVs and much more. Our good friend Tom Olson has this paper posted on one of his websites, http://www.colonyfund.com/Reading/papers/phys_econ_leo.html so you should be able to read or even download it. As John commented during the program, this paper is still very current.

And since The Space Show and The Space Show Classroom are radio/audio, yes, I concur, visuals would be a plus but connecting us all visually and streaming it for example would be costly and beyond the means right now of The Space Show and guests. FYI, The Space Show is working on an option that might allow visuals from my end but still connecting guests with visuals would be spotty at best. It is something we all know would be valuable and maybe some day we can make it happen. Your suggestion is appreciated and noted.

Thanks for your comments.

4. Markus Voelter - February 12, 2010

Hey guys,

This was one of the best space shows ever, maybe the best one.


5. drspaceshow - February 12, 2010

Thank you Markus. All of us appreciate your comments.


6. Jerry Volland - February 13, 2010

Thank you very much for starting this Classroom series. I listened to the Rocket Equation program three times. I did know about this equation but one important aspect I’ve missed is that Isp goes up directly with an increase in exhaust velocity. This is where the turbo pumps add to the efficiency, something I didn’t know. Perhaps there are other ingenious ways of doing this?

7. on behalf of Dr. Jim Dewar - February 24, 2010

The following post comes from Dr. Jim Dewar regarding the Rocket Equation Space Show Classroom. He email his comments to me and as I have said, I post all such comments here on the blog.


From Dr. Jim Dewar:

I liked your presentation on the rocket equations to Space Show listeners, but felt you got somewhat sidetracked in your discussion of it. For on the air discussions, it is quite common to go off on a tangent and never get back to the main point; heaven knows I’ve done it all to often and I know I should get back to the main point, but never do.

Specifically, the heart of the rocket equations is that the exhaust velocity (V) of any rocket is proportional to the square root of the temperature (T) of the hot gases just before they enter the nozzle throat over the molecular weight (M) of those exhaust gases. Basically, if you drop the square root sign for simplicity’s sake, it’s just T over M and to get the maximum performance, you want T to be as hot as possible and M to be as low as possible. So you want to pump T up while pushing M down.

In your discussion, the example of throwing a baseball from a rowboat was used and this is fine, but I think it left the impression with the listeners that all rocket fuels are baseballs in terms of their molecular weight. I’m sure you realize this isn’t the case. Some propellants have heavy molecular weights, which leave them more like bowling balls, and some gunpowder rockets might be viewed as trying to throw old cannonballs from the rowboat. Also the temperature part of the equation, I feel, didn’t come through to the average listener who might think that all rocket fuels burn at the same temperature. Again, some fuels and oxidizers have more BTUs in them than others, the same way that soft woods such as pine and poplar don’t burn as hot or as long in your fireplace as hardwoods such as oak and locust.

Overall, these are not major points for I feel you did a fine job in explaining the tyranny of the rocket equations to the average listener and debunking many of the popular solutions for getting into space easily and inexpensively. No, our gravity well is deep and the rocket equations and specific impulse are quite real and limiting. And as long as we keep with chemically propelled rockets, we will be pursuing pipe dreams. Payload fractions will be quite tiny. And for the space elevator crowd, I view this as the new alchemy. The rocket equations are really quite tyrannical.

There is a way out: Nuclear rockets. Here T is theoretically unlimited, to the temperature of the sun, while M mostly is hydrogen, the lightest element with a molecular weight of 2.

You briefly mentioned the old NERVA with an isp of 825 seconds, but I hope you are familiar with my two books on the subject. “To the End of the Solar System: The Story of the Nuclear Rocket” traces the political and technical history of Project Rover/NERVA from 1955-1973. If you haven’t read it, I would urge doing so, as this history will give a completely different view on the subject and shatter many of the myths that have evolved in popular culture. My second book “The Nuclear Rocket: Making Our Planet Green, Peaceful and Prosperous” examines what type of a space program we would have if we broke the taboo against using nuclear rockets to reach LEO and then to go beyond. Bear in mind that NASA prohibited in 1960 using a nuclear rocket to reach LEO, out of fear of adverse publicity on the image conscious agency. Technically, the second book argues this fear of “radioactivity” is vastly overstated and that using nuclear rockets to reach LEO will be much safer both for the payload/astronauts and to the general public than a rocket launched from the Cape or other fixed sites. And cost wise, we’re talking $100 per pound to LEO.

I’m going to be giving a talk at the ISDC in May in Chicago where I explore this $100 figure in greater detail and will show, I hope, that it is both right and wrong at the same time. There are at least six technical and one quasi-technical factors that will drive the cost down to this level and perhaps well below. Let me just mention one: you indicated NERVA would have 825 seconds of isp, well, even back then they knew a second generation NERVA would have 925 seconds. Today a 1000+ seconds is possible from carbon/graphite fuels and perhaps 1200 seconds from carbides, though that would be quite hard. But it might not end there. At one point early in the program’s history, some speculated 1600 seconds might be reached by adding on different bells and whistles to the engine; today, some say that might be 1700 to 1800 seconds. But who knows now, all that can be said is that once you start an R and D effort, many things become apparent that were not so at the start and you can end up with an engine that is 2x or 3x the best chemical system, and perhaps even 4x or 5xs.

8. Oldspacecadet - February 25, 2010

Dr. Dewar is correct in that high exhaust gas temperature and low molecular weight of the exhaust both drive up exhaust velocity. This is the rationale for running hydrogen through a reactor as in the old Rover/NERVA program.

The problem here is that politics, not technology, becomes the showstopper. Contrast public reaction to Three Mile Island to that when a refinery has an explosion killing several people. The highly useful medical technology of magnetic resonance imaging (MRI) used to be called nuclear magnetic resonance imaging (NMR) but public radiophobia drove the name change — and NMR didn’t even involve ionizing nuclear radiation. Performance and safety are irrelevant to this issue. Can anyone even conceive of the public reaction to a nuclear powered rocket launch? The chattering classes freak out at the thought of a radioisotope-powered thermoelectric generator being launched into space or the thought of spent reactor fuel rods being transported in armored casks to a safe central storage facility as opposed to on-site (and less secure) storage by the reactor containment. I firmly believe that until rationality enters public discourse, nuclear rocket motors are a dead issue.


9. Andy Hill - February 26, 2010

I would have liked to have heard more about different fuels and why a RP-1 propellant would be chosen in preference to something like methane which the Russians have designed engines for with Isps in the region of 380. Methane is much denser than hydrogen so wouldn’t suffer the problem of huge tank size and would give a significant improvement over RP-1. It would have been interesting to hear about the pros and cons of different fuels. Perhaps such a discussion could be included in a future classroom programme?

Really interesting progamme that answered a few questions I didn’t know I had, I look forward to more in the series.

10. Paul Breed - February 26, 2010

Methane is basically half way between RP-1 and H2, To get decent density it is mildly cryogenic , its density is better than H2 and poorer than RP-1, its ISP is also partway between the two.

RP-1 is a denser fuel so its probably better for first stage motors.

As for the nuclear rocket, the problems are 1% technical and 99% political. The people that won’t let you transport subcrital waste in a train proof cask won’t ever stand for a flight weight nuclear rocket. I fear this will be done first by Russia or China, where they are better at suppressing dissent. (No I would not want to trade systems)

11. James Dewar - February 27, 2010

With due deference to John and Paul, I must comment forcefully for the other listeners/readers on two items. First, I must reemphasize my point on the rocket equations, specifically the heart of them which is the exhaust velocity (V) of any rocket is proportionate to the square root of the temperature of the gas (T) over its molecular weight (M). I’ll drop the square root for simplicity’s sake, so its just T/M. This is for all rockets, not just nuclear ones or not just chemical ones. Within this simple formula of T/M is a great tyranny for chemically propelled rockets. With T, all fuels and oxidizers, such as alcohol and oxygen, kerosene and oxygen and hydrogen and oxygen, have a BTU content. It’s a ceiling that cannot be exceeded, they can burn only so hot and not hotter. This is a permanent limiting factor for chemically propelled rockets. Period. With M, all fuels/oxidizers have heavy molecular weights, the lightest practical one is LOX/LH2 at 18. This means your pushing fat, heavy molecules out the nozzle and pushing fat, heavy molecules takes a lot of energy or heat. So, using specific impulse as a measuring stick, alcohol/oxygen has a specific impulse of 230 seconds, kerosene/oxygen at 330 seconds and LOX/LH2 at 450. This is the best that can be attained – forever – with chemical propellants, though some exotic propellants might go to around 500 or so seconds, but they are either extremely dangerous or toxic or both.

Second, this has implications. It means most of a rocket must be fuel and oxidizer – Paul and John mentioned 90% and that’s about right though it differs slightly from rocket to rocket. That leaves about 10% for the rocket’s structure – its tanks, turbopumps, pipes, valves and nozzles – and after that the payload – which is roughly 1-2%. This cannot be changed unless some new elements on the periodic table are discovered that have a better BTU value and molecular weight than LOX/LH2. I don’t see these new elements coming along; the periodic table seems pretty set except at the heavy end where new ones are being found. It also means the new space pioneers who are spending their own money on “new” rockets are really throwing their money down a rat hole. They are hoping that by tinkering with the manufacturing processes for the tanks, pumps, nozzles they can achieve greater efficiency and lower costs and they’re hoping that razzle-dazzle business models and streamlined organizations can also lower costs. All this, they hope, can shave some dollars off the costs of taking a pound of payload to LEO, however that is defined. They’ve been at it for about a decade and the big boys of space have been at it since WWII and for both, it’s not happening. It still costs thousands of dollars to take a pound of payload to LEO and you still have payload fractions about 1-2%. In other words, the space program you have now is the space program you will have a century from now. Chemical rockets are dead end.

The time has come to put an end to this type of thinking that seeks to polish up the tanks and turbopumps and develop razzle-dazzle business models and organization charts and focus on the problem staring everyone in the face: it’s T/M. It’s time to pump up T while pushing down M.

That means nuclear, as the energy within a U-235 atom can reach the temperature of stars and as most nuclear rockets use hydrogen, the lightest element which has a molecular weight of 2, and that means using a nuclear rocket to reach LEO. Now an image conscious NASA banned this use of a nuclear rocket in 1960, fearing public protests, but before that many studies existed. It was no big deal technically; (the Navy ran into the same problem with its nuc subs and some cities banned them from their harbors, fearing radiation or them blowing up, and some welcomed them and got a big economic boost for decades). In fact, Rocketdyne did a study in 1968 that concluded by replacing the J-2 LOX/LH2 third stage of the Saturn V with a NERVA engine (75k thrust at 825 seconds of isp) the payload to LEO would double, from 250k for the all-chemical Saturn to 500k for the Saturn-NERVA. That’s with a first generation system. It can get even better with later generation systems, as there is plenty of growth potential even with the humble solid core. In other words, it means a dramatic lowering of the costs of reaching LEO and going beyond.

Now where I differ with John and Paul most strongly is with their belief the political system will prohibit this. This may be unfair, but both seem to view Washington as our political masters who must be approached on bended knee or think the anti-nuclear and environmental movement people would never allow it. No, absolutely not! Such a psychological attitude must end; we the people are sovereign, not the President or Congress or NASA, and the antis and environmental people only have a voice in the democratic process, not a veto. We are still a democracy and so what must happen is to get off our psychological knees, organize, do the studies on this launch profile, which really is benign and safer than with chemical rockets, and come up with cost estimates, which I believe will look at $100/pound to LEO. From then on it’s public relations, it’s building a movement much the way von Braun, Arthur C. Clarke and Willy Ley did in the early ’50s, it’s building public awareness that each man, woman and child can now have personal access to space. I subscribe to the view that if arguments are made that show the average person can profit personally from this launch profile, he or she will back a nuclear rocket program.

I hope after John and Paul read my second book, we can continue this dialogue and involve other Space Show listeners. It’s time to pump up T and push down M and stop polishing tanks and turbopumps or indulging in flights of fancy on razzle-dazzle business models and streamlined organizations.

Oldspacecadet - March 4, 2010

Paul and I both undertand the T/M concept and we both agreed with you. We part company on a practical issue involving both political and public acceptance. Whether or not a nuclear rocket is actually safe is not relevant. “We the people” may be soverign, but we the public have a negative mindset about things associated with the word “nuclear.” How do we change that particular mindset when a significant fraction of the US public (and our leadership) is so ignorant and technically illiterate?

12. James Dewar - March 4, 2010

That starts with debate and dialogue to build public awareness and acceptance. “Space” in the 1950s just didn’t happen out of thin air, it was the result of books, speeches, and movies and magazines that publicized “space” to the general public and if you remember guys such as von Braun, Willy Ley and Arthur C. Clarke did it so well that cars started to sprout fins in 1955, imitating a rocket’s fins. So when Sputnik hit, the public was outraged that the US was “last in the space race.”

So I propose that after I return from two months overseas, we ask David to host a two hour debate. I’ll spend a half hour describing the launch profile for using nuclear rockets to reach LEO and return and then you and the listening public can rebut and explain where the technical show-stoppers are — if any. Then in the second hour I’ll explain how I came up with a $100/pound to LEO and why that figure is both wrong and right at the same time. I’ll take a half an hour and then you and the listening public can rebut for the other half hour and explain why it is wrong — if it is. I’m prepared to justify my positions.

In doing this, I hope you will have at least read my second book on the nuclear rocket (The Nuclear Rocket: Making Our Planet Green, Peaceful and Prosperous) and hopefully the first.

A debate such as this would be how we start changing the mindset and building a movement of people who want a real space program where everyone can both participate in and profit from it. When a movement hits Washington, politicians respond. The remaining options are continuing to beat the dead chemical rocket horse or indulging in flights of fancy on “advanced” means of reaching LEO such as space elevators, space cannons or other such nonsense.

Those dogs won’t fly; meanwhile, the humble solid core nuclear rocket sits patiently on the shelf, it needs little development and can fly within five years after the program restarts. And if my argument is correct, it needs little to no federal funding to do so.

13. Daniel L - March 8, 2010

Commenting on the idea that asteroids are easier destinations than the Moon or Mars: the absence of a gravity well at an asteroid is the *only* factor favoring this opinion. Compared to a planet (or a moon), much less science can be done at an asteroid. Consider the vast geological complexity of Mars, or the Moon – it is a whole universe of geology compared to an asteroid. Consider also, that public opinion (or that portion of it which is pro-space) wants to visit and explore new *worlds* – and not be confined to islands in space. Consider also, your own arguments that asteroids present greater opportunities for exploitation. How difficult would it be to transport mining equipment to an asteroid and set it up for operations? What would be the cost of that, under a chemical-rocket regime which allows only a couple of percent deployable payload? How about the difficulties and cost of transporting the mined materials back to Earth? Obviously you would need a completely new kind of rocketry (eg. nuclear) to enable asteroid exploitation. And so then, if we had that, would it not render your point moot, as we would then have easy capabilities to Mars and the Moon anyway? Finally: the idea that asteroids are best suited to ISRU seems wrong, in light of emerging plans to construct tools out of Lunar regolith with robots, or for that matter the old idea of generating rocket fuel with hydrogen feedstock and Martian CO2 atmosphere. – Anyway, thanks to all for a wonderful and fascinating program. Highly educational, really worthwhile and all your efforts are very much appreciated.

14. Daniel L - March 8, 2010

In clarifying, I should have written “Commenting on the idea that asteroids are BETTER destinations than the Moon or Mars” – they are certainly easier desinations; my post was trying to argue why they are not better ones. DL

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