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#26 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2012-01-05 10:31:43
Hop wrote:JonClarke wrote:Some perhaps, but very few.
This paper http://arxiv.org/abs/0907.3010 calls imapctors below 11 km/s "slow", Figure 3a shows that there are effectively none below 10 km/s,
No, figure 3a on page 7 does not show there are effectively no lunar impacts below 10 km/s. A significant portion of the the lunar bell curve lies to the left of 10 km/s. What you say is true of the dotted line (earth's impactors).
There are a indeed a few below 10/s, but bulk of the are all above that km/s,
Using Photoshop to count pixels...
9%. 9% of impacts is effectively no impacts?
Hop wrote:JonClarke wrote:The text of this paper has slightly different numbers - average of 17 km/s and a lower limit of 12 km/s www.sciencemag.org/content/309/5742/1847.full.pdf .
Behind a pay wall. I'll note a 17 km/s average doesn't mean there aren't slower impacts. I am skeptical of the 12 km/s lower limit. I suspect you're misinterpreting this paper as you did the paper by Ito and Malhotra.
Go to a library. Not all knowledge is on the internet. It's a basic part of research. Misinterpreted the paper, no, I have not.
From a histogram showing 9% of impacts are below 10 km/s you conclude there are effectively no such impacts. I would call that a misinterpretation.
Furthermore, the Ito and Malhotra said that the evidence may indicate they're under estimating the slow impacts.
You can draw all the cartoons you like but it won't alter the fact that while they do exist (I have never denied this)
If you're saying 12 km/s is a lower limit, you are denying they do exist.
Are you saying that low veolcity impacts are the norm?
Absolutely not.
Velocity of a hyperbola is sqrt(Vesc^2 + Vinf^2). For most asteroids, Vinf is quite high and this is the quantity that dominates. But there are are number of asteroids with low Vinf, some have a Vinf of nearly zero. Hence I said lunar impacts can be as slow as 2.4 km/s. Which is entirely correct.
Somehow you have morphed this into slow impacts are the norm, a position I've never taken.
#27 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2012-01-04 10:36:45
But complete irrelevant to the existence of Au and Hg at the lunar poles, wafting about in the lunar breeze.
Still waiting for a cite about your mercury silicates.
Native mercury and HgS are much more common.
Baking HgS will cause the mercury vapor to separate out. So you would expect the lunar crust to mercury poor, just as it's poor in other volatiles.
Likewise, you would expect what surface mercury there is to accumulate in the cold traps, just as with the other volatiles.
#28 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2012-01-04 10:24:18
Hop wrote:JonClarke wrote:Even a metallic asteroid is so dispersed during the impact process that ores do not result, not even of the most adundant elements.
Cite?
Lunar meteorite impacts can be as slow as 2.4 km/s.
Some perhaps, but very few.
This paper http://arxiv.org/abs/0907.3010 calls imapctors below 11 km/s "slow", Figure 3a shows that there are effectively none below 10 km/s,
No, figure 3a on page 7 does not show there are effectively no lunar impacts below 10 km/s. A significant portion of the the lunar bell curve lies to the left of 10 km/s. What you say is true of the dotted line (earth's impactors).
Furthermore, on page 12 the authors note their models don't match the lunar impact record. "One possible explanation is related to the impact velocity distributions. The leading/trailing asymmetry becomes more prominent when the average relative velocity between the Moon and the projectiles is low. The NEA-like particles are, by their dynamical definition, the “slowest” (relative to Earth) among all the known small body populations in the solar system. That even these slow particles may not fully account for the observed asymmetric distribution in the lunar crater record suggests that there may exist a presently-unobserved population of small objects near the Earth’s orbit that have even lower average relative velocity than the currently known near-Earth asteroids do."
The text of this paper has slightly different numbers - average of 17 km/s and a lower limit of 12 km/s www.sciencemag.org/content/309/5742/1847.full.pdf .
Behind a pay wall. I'll note a 17 km/s average doesn't mean there aren't slower impacts. I am skeptical of the 12 km/s lower limit. I suspect you're misinterpreting this paper as you did the paper by Ito and Malhotra.
Yet another paper http://128.97.36.172/Warren%20et%20al-s … aradox.pdf suggests 16 km/s as the average and suggests that this number has not changed much throughout the history of the Moon.
Again, an 16 km/s average doesn't demonstrate the nonexistence of slower impacts.
#29 Re: Human missions » The Myth of the trillion dollar mission » 2012-01-03 19:47:04
Mark Friedenbach wrote:$17 billion, but that's a pie split many, many ways.
I think you're misunderstanding me. What I'm advocating is a phased approach, I just don't think your Phase 1 is enough.
Whoops - what's $10billion among friends...actually $19 billion now I see.
Mark is correct. It's about 17.8 billion split many ways. See the page labeled 252 of of the Conference Report accompanying House Resolution 2112
"Exploration" includes Orion, SLS, Commercial crew and exploration R&D. This is about $3.8 billion. $1.8 for SLS, $1.2 billion for Orion. $.4 billion for Commercial Crew.
"Space Operations" includes Space Shuttle, I.S.S., and Space and Launch Support. this is about $4.2 billion.
The manned space station, as well as well as developing transportation for humans is being given about 8 billion.
#30 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2012-01-03 07:49:36
Even a metallic asteroid is so dispersed during the impact process that ores do not result, not even of the most adundant elements.
Cite?
Lunar meteorite impacts can be as slow as 2.4 km/s.
#31 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2012-01-03 07:39:21
Only in the metallic form. Not when it is locked up in silicates.
Mercury locked up in silicates? Cite? So as I know it occurs as a native metal or as HgS.
And lunar rocks have very low Hg - less than 0.3 parts per billion. That is less than 1% of the average abunance in terrestrial cust.
You heat cinnabar and the mercury vapor separates out. Given that the lower lunar latitudes get quite hot and it's vacuum, you'd expect the Mercury vapor to be baked out of the lunar crust.
You'd expect mercury to be scarce in the lunar crust, for the same reason you'd expect scarcity of other volatiles.
And those volatile vapors that don't escape to outer space move about the moon's surface. If their travels take them to a cold trap, there they will stay.
Your assertion that there's no mechanism to concentrate mercury in the cold traps is wrong.
Is the same true of gold? I don't know. Off hand, I can't imagine a mechanism that would concentrate gold at the lunar poles. But just because I can't think of one, doesn't mean such a mechanism can't exist. We don't know the history of the minerals in the cold traps. While I'd agree the gold finding isn't conclusive, I reject the notion that it should be disregarded just because it doesn't meet our expectations.
#32 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2012-01-03 07:14:33
I understand what you wrote.
Not demonstrated. You still haven't answered my objections with numbers or equations.
Scratch that, you did toss out 9.4 km/s. If you did understand what I wrote, you'd know that was a straw man.
If you deliberately used a dishonest straw man argument, that would harm your credibility more than mere lack of understanding.
What I don't understand is why you continue to talk about it when its relevance to the thread is minimal at best.
(sigh....) Once again, ascent trajectory models are relevant to designing reusable rockets to orbit. They are the heart of matter.
Moreover your original post started with an attempt to model ascent.
What is irrelevant is an an avalanche of fan boy cheer leading. Cheerleading without valid equations or sound models. Such discussions will bring us no closer to achieving re-usable rockets.
Willfully ignoring the numbers actually subtracts from the credibility of such cheerleaders. If you want this New Mars to be thought of as an ineffective fringe group, ignoring math is a good way to achieve that.
#33 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2012-01-02 20:13:38
2) Concentration of gold and mercury in shadowed craters is extremely unlikely -there is not mechanism to do so.
Mercury can easily sublimate into a gas. Therefore it amenable to accumulation in the cold traps.
#34 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2012-01-02 19:41:20
Hop, are you seriously not willing to accept an estimate of 9400 m/s +~100 m/s?
Of course I'm willing to accept that estimate. That isn't the result I differ with.
This is what I object to:
The blue arrow is the length of the Green arrow minus the Red arrow. The integral of this arrow with respect to time from launch until orbit is going to be equal to 2,370 m/s for a 300 km orbit. 2,370 m/s is 2.81 MJ/kg expressed as a velocity. If you separate the two different vertical ⌂V components in this manner, the methods you would have to use to calculate both become much clearer.
I've seen several models I like that arrive at around 9.4 km/s.
GW has described two good models. One is taking small time slices and re-evaluating the thrust vector each slice, accounting for gravity and drag loss on each slice.
The other is the quick and dirty (but effective) jiggered rocket equation.
John Shilling describes his methods of figuring ascent penalty here.
Depending on vehicle specifics, most of all these can give you about 9.4 km/s for reaching a 300 km altitude circular orbit?
I don't even know what your method is. So far it very vague and nebulous. How do you plug in the above 2.37 km/s to get 9.4 km/s?
Given the models you've volunteered so far, I don't give even odds you can write useful code to model ascent.
Seriously?
Seriously? Seriously -- I'm not contesting 9.4 km/s. That you think I am demonstrates you didn't bother to even glance at what I actually wrote.
#35 Re: Human missions » Developing the cis-Lunar economy and infrastructure » 2011-12-31 16:14:07
LCROSS detected 100kg of water in a total of 30 million kg of debris. That's a concentration of 3 parts per million. The instruments on Lunar Prospector could only detect soil contents 1 metre deep, but the impact of LCROSS dug several metres deep. And this concentration of 3 ppm was the most concentrated spot on the Moon.
Conclusion: there isn't any water on the Moon. Not any worth harvesting.
The October 2010 issue of Science had some articles on the LCROSS ejecta. Here are the published results:
N 6.6000%
CO 5.7000%
H2O 5.5000%
Zn 3.1000%
V 2.4000%
Ca 1.6000%
Au 1.6000%
Mn 1.3000%
Hg 1.2000%
Co 1.0000%
H2S 0.9213%
Fe 0.5000%
Mg 0.4000%
NH3 0.3317%
Cl 0.2000%
SO2 0.1755%
C2H4 0.1716%
CO2 0.1194%
C 0.0900%
Sc 0.0900%
CH3OH 0.0853%
S 0.0600%
B 0.0400%
P 0.0400%
CH4 0.0366%
O 0.0200%
Si 0.0200%
As 0.0200%
Al 0.0090%
OH 0.0017%
Lots more water than you indicate. Also a lot of carbon and nitrogen compounds.
But LCROSS isn't the most exciting find. The elevated CPR of India's Chandrayaan-1 lunar orbiter seems to indicate sheets of ice at least two meters thick. See: NASA Radar Finds Ice Deposits at Moon's North Pole.
#36 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-31 16:01:25
I don't recall even ine post of yours in this thread on reusable rockets, rather you were critiquing a model
Modeling ascent trajectories is relevant to a discussion of reusable rockets.
that was mentioned as support of a point that has since been mostly vindicated)
I was hoping someone would notice your erroneous model gives wrong results. But evidently I am the only one to plug some numbers into your integral. Please see post above. Your point hasn't been vindicated.
#37 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-31 15:56:00
The blue arrow is the length of the Green arrow minus the Red arrow. The integral of this arrow with respect to time from launch until orbit is going to be equal to 2,370 m/s for a 300 km orbit. 2,370 m/s is 2.81 MJ/kg expressed as a velocity. If you separate the two different vertical ⌂V components in this manner, the methods you would have to use to calculate both become much clearer.
You seem to state this as a general rule. If so, a single counter example suffices.
Call the net vertical acceleration a(t). If a(t) is a constant it is quite easy to integrate. Call it a.
/
| a dt = at + c
/
Where c is constant of integration. This is typically initial velocity, Vo. At the beginning of launch, vertical velocity is zero so we can ignore the Vo
I'll set the constant a = 8 m/sec^2.
/296
| 8 m/sec^2 dt = 2.37 km/sec.
/0
Altitude reached is an integral of vertical velocity over time:
/
| a t dt = 1/2 a t^2
/
Over 296 seconds 1/2 a t^2 is 351 kilometers. Already your conjecture is proven false as a general rule.
But it gets worse. At an altitude of 351 kilometers it will have a vertical velocity of 2.37 km/s. This will take it to an altitude of about 700 kilometers.
#38 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-31 15:41:13
I know how to figure out hyperbolas since I was 17. Doing that wouldn't have answered louis' question, since he gave no specific departure point or date, and a hyperbola isn't a transfer orbit.
A Hohmann path is an elliptical orbit with regards to the sun.
However departing earth to enter this solar orbit, you are in an hyperbolic orbit with regard to the earth.
On arrival at Mars, you are in a hyperbolic orbit with regard to Mars until you do a deceleration to enter mars capture orbit.
Knowledge of hyperbolic orbits is a needed if you want to be be competent in orbital mechanics.
The answer you gave Louis remains very wrong. Until you show a little time and effort, I will not bother investing time and effort explaining why.
#39 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-13 22:27:43
With regards to the question at hand, I would advise that people not do the calculations for Louis (though of course I advise this as a fellow forumgoer as opposed to a moderator) given that he should be perfectly capable of plugging into the calculator which I have linked to previously.
Yes, Louis has been given the rocket equation many times.
By the same token, you and Rune have been shown how to figure the velocity of a hyperbola. Many, many times.
So perhaps you can understand my frustration when Rune gives Louis this spectacularly bad information. And you watch Rune dispense this misinformation, evidently clueless how wrong it is. Then Rune goes on to say there's no math behind it, just a gut feeling. And why should he invest time and effort doing math? After all, it's Hop and Louis he's talking to.
Know this:
You and Rune are no better than Louis.
Your smug arrogance and condescension towards him are completely unwarranted.
... you can say so in a respectful manner. In the future, if you feel the need to make such a comment, I would advise thinking again.
[/mod]
Thou hypocrite, pull the beam from thine own eye first.
I am not going to waste any more time here.
#40 Re: Human missions » Mission One: a one way ticket to Mars? » 2011-12-13 19:51:59
To be fair, we're not really talking about the initial mission anymore
Whether it's the initial or the 20th mission, delivering humans to Mars will require lots of mass to LEO.
I offered one assessment of what's needed. It's a starting point for a conversation.
Their estimate was 694 tonnes and 4 people. About 170 tonnes per person.
Louis thinks 75 tonnes to LEO per person is ludicrous, much less 170.
Do you concur? If so, please show me an itemized mass budget as Willson and Clarke have done. So far I have seen nothing indicating you've invested and time and effort learning what it'd take to keep the passengers alive for the Mars trip.
If 75 tonnes is needed, launch prices need to drop to $14 per kilogram to meet your million dollar a person tickets to Mars.
#41 Re: Human missions » Mission One: a one way ticket to Mars? » 2011-12-13 04:24:09
75 tonnes per person is in my view a ludicrously high figure.
They give itemized lists at the Mars Semi Direct paper. What would you cut?
A lot of Mars supplies can in any case be taken to Mars robotically in small loads - in the same way that Curiosity is going to be landed.
This wouldn't lessen the mass you need to launch to LEO.
Of course, what one has to factor in is the incredibly high value of the material returned from Mars - which will be worth far more than $20,000 per kg.
You envision return trips? Then the ascent vehicle can't be cut. Nor can a lot of the other stuff I cut assuming a one way trip. Back to 694 tonnes if you have Mars Ascent vehicles, Earth Return Vehicles, etc. Now it's back to 170 tonnes per person And if astronauts are coming back as well as going there, the population growth rate will be slower.
You're hoping collectors of rare asteroids will pay for this. I don't think this niche market can provide enough revenue. And when you flood the market, asteroids are no longer rare.
#42 Re: Human missions » Mission One: a one way ticket to Mars? » 2011-12-13 02:04:06
You wrote:So a small population could sit down on Mars and come up with multi-trillion dollar ideas?
Sounds like trillions of dollars per year to me.
No "per year" in what I said. "Trillions of dollars per year" is a complete fabrication on your part.
I am assuming subsidized development up to 500 people as a scientific base/outpost.
Mars semi-direct assumes 4 people each launch window. 500 people would be 125 missions. To be less than 1 trillion, each mission would have to cost less than 8 billion.
I think we're looking at about .1-.2 kg of imports per person per year. For a 500 person colony, perhaps $250,000/year. Maybe a little more, but not all that much more.
The South Pole base is in a much more hospitable environment. Smaller than 500 people. And the transportation is much less expensive. $.25 million per year is short even for this much more modest endeavor.
In the long(ish) run, I think we're looking at a cost somewhere below a million dollars per person to emigrate. Keep in mind that Apollo was not in the least cost-focused. The minimum launch price per kilo for a payload in the range of tonnes went down from $10,000/kg to $15,000/kg to $5,500/kg (Falcon 9) in a very short period of time. We can anticipate that they will go down significantly more in the long run. The private space age is really just beginning.
In this Mars Semi Direct architecture it takes 694 tonnes to LEO to deliver 4 people to Mars. But Mars Semi Direct provides for a return trip and we're talking one way. So 300 tonnes instead of 694. That's 75 tonnes per person. 1 million dollars per person? That comes to about $14 per kilogram to LEO. And that's assuming all the payloads are free. You're being a tad optimistic here.
#43 Re: Human missions » Mission One: a one way ticket to Mars? » 2011-12-13 00:51:19
First of all, a colony is not going to need trillions of dollars per year. Nowhere close.
I didn't say trillions per year. Please retract that.
I don't know where you got a figure of trillions of dollars from, but it's completely ridiculous.
Apollo launches were about ten billion a pop. For each Mars Semi-Direct mission you'd need 4 or 5 HLV launches.
Some of the payloads would be extremely costly. In some ways an Mars Transfer Vehicle is more challenging than the I.S.S. The I.S.S. keeps people alive beneath the protective Van Allen belts. The I.S.S. also enjoys frequent supply and maintenance. And the I.S.S. cost about 100 billion. You also need to land nuclear reactors, CO2 cracking plants.
50 billion each 2.14 years is optimistic. 20 such missions would be a trillion. And I don't think a Mars colony would be self sufficient after 40 years. Multi trillions to establish a Mars colony is not ridiculous at all.
For the handful of Mars colonists to export ideas at a profit, they'd need to do better than Elon Musk, Bill Gates, Steve Jobs, Jeff Bezos, and Mark Zuckerberg combined. You still haven't explained how the Mars environment could stimulate such profitable creativity.
Trillions of dollars per year? Now that is completely ridiculous. However that figure comes from you, not me.
#44 Re: Human missions » Mission One: a one way ticket to Mars? » 2011-12-13 00:03:44
Hop-
One way to convince me would be to demonstrate your optimism is plausible.
For example, by spending a significant proportion of my time and thought looking into the issues facing a colony and devising economical solutions to these problems in the best way that I am able?
An avalanche of words demonstrates nothing.
I mean do it.
You say it's possible to take small number of people, equipment of moderate mass and establish a self sustaining, growing community in a forbidding environment.
So you and some friends should establish your own Walden pond in Siberia. Not only grow your own food, but do your own mining from resources within reach of the base. Manufacture your own plumbing. Your own backhoes. Your own grow lights. And so on.
Actually, this would be many time easier than establishing such a base on Mars.
#45 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-12 22:18:31
And 5.8km/s faster than C3 speed? That would puts you at Mars orbital speed.
Let's see. Assuming already traveling 10.9 km/s at a 300 km altitude, then doing a 5.8 km/s burn... That gives you about 12 km/s Vinf.
There are a multitude of possible directions and therefore a multitude of possible orbits.
Depending on direction, that could take you to a perihelion inside Mercury's orbit. Or it could take you out past Pluto.
Pay me if you want a perfectly correct answer
You have been paid what your answer's worth.
#46 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-12 11:15:37
How much more fuel and propellant would be required for a direct shot, in percentage terms, compared with a "sling shot approach" (is it at "opposition"?).
"Brute force" trajectories would take about as much delta-v as is the difference between the orbital speeds of mars and earth, so about 29.8km/s (for earth) - 24km/s (for mars) = 5.8km/s, without taking into account entering and leaving either planet's orbit (whichever orbit you choose to park in, the delta-v requirements vary).
This sounds like a Hohmann orbit. For injection into a Earth to Mars Hohman orbit, Vinfinity is about 3 km/s. At the end of the transfer path the hyperbola with regard to Mars has about 2.6 km/s.
Total V infinity for a Hohmann is about 5.6 km/s.
If the transfer orbit intersects Mars' or Earth's orbit at a healthy angle, the Vinfinity speeds could be a lot higher.
For comparison, the "standard" minimum energy Hohmann is about 1.5km/s, again without the departure and capture taken into account.
That is about the Hohmann delta V if your departure and arrival orbits are just under parabolic -- highly eccentric elliptical capture orbits.
But if you're departing from LEO? Then Trans Mars Injection is about 3.6 km/s for a Hohmann transfer.
#47 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-12 10:51:47
Wow, lots of posts to respond to. First, a quick question to Hop that would clear things up for me a lot:
Rune says that Gravity Drag is a function of the time it takes to get to orbit. You'll see no argument on this one from me.
Good.
You say that you have to consider orbital velocity when calculating gravity drag.
My objection was your use of potential energy alone for energy between altitudes. A vehicle acquires kinetic energy along the way so it is wrong to say the difference between altitudes is 2.81 MJkg.
Also as a vehicle picks up horizontal velocity it acquires so called centrifugal force. This centrifugal lessens the gravity pull. This has an effect on gravity loss over time.
Is your argument that because the velocity required to be in orbit is the strongest determining factor, you have to take into account the velocity required to get into orbit when calculating gravity drag?
No.
A calculator for the delta-V required to get to orbit sounds extremely useful, though.
Here's such a calculator.
If your choice of launch vehicle is user defined vehicle, you'll note Schilling asks for thrust as well as ISP.
#48 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-10 16:46:43
Rune. But wait, haven't I already said that? Like in the post you actually quote.
Which quote? There's this:
Oh, and T/W of an engine, for the same engine, IS proportional to the fuel's density
And then there's this:
Actually, the molecular weight you have to look at is the one of the combustion products.
I've agreed with the latter.
#49 Re: Human missions » Is space within our reach? » 2011-12-10 13:53:08
The guy seems to be confusing several different issues and coming up with wrong answers.
I very much doubt that the Earth will "run out of resources". There is v. little sign of that happening. World economic growth continues at some huge figure like 4-5% (despite the problems of Euro-America). We are recycling materials more and more efficiently. We are on the brink of limitless energy, if not yet cheap energy through green energy technologies. We don't have to rely on oil - it's just rather convenient for a lot of things. But electric cars will perform just as well as petrol/gas cars within a few years (Toyota have just developed a 1000 mile range battery that will cost 25% of current batteries - and even if that didn't work, you could have battery changing stations to extend range). So that's ticks for energy and transport. Aircraft can fly on bio fuels. Plants can produce polymers. Once you have limitless energy, you can put a lot of energy into growing food in special facilities e.g. polytunnels, farm towers, and so on. Energy can also be used to extract water from the atmosphere in dry aeas and irrigate the land.
Even mild exponential growth like 4 or 5% will lead to dramatic quantities over time.
Human cleverness has increased amount of resources at our disposal. But this also has limits. If you read Murphy's blogs you will see he talks about thermodynamic limits to car efficiency, maximum amount of sunlight we can harvest, etc.
Earth is a finite body of resources. Murphy is absolutely correct that it can't sustain exponential growth forever. Sooner or later the growth curve will flatten and our growth will resemble logistic growth rather than exponential.
I have never thought space mining will be a major industry in the next 100 years or so. But certainly Mars and the asteroids can provide supplies of the most valuable metals and stones e.g. gold, platinum and diamonds.
PGMs from asteroids is a possibility. I don't see mineral exports from Mars to earth ever making a profit.
I think that old adage from the Apollo/Hippy era is needed now: "Just do it!"
If we drop enough LSD and chant a mantra, that will levitate a Noah's Ark to another planet.
#50 Re: Interplanetary transportation » Reusable Rockets to Orbit » 2011-12-10 13:37:23
But Hop, as soon as those liquids hit the combustion chamber, they stop being liquids. Actually, the molecular weight you have to look at is the one of the combustion products.
Yes, the combustion product is a hot gas. H20 when you burn hydrogen and oxygen. A mixture of H20 and CO2 when you burn hydrocarbons and oxygen.
With longer hydrocarbon chains you have more CO2 and less H20. Thus better thrust but lower ISP.
What that has to do with the density of liquid propellants, you still haven't shown.