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HH -- what manned rocket speeds possible?(31 posts)

HH -- what manned rocket speeds possible?DougSloan
Jun 3, 2003 1:50 PM
HH, since you are the resident rocket scientist -- what speeds are foreseeably possible for manned flight to mars? I assume it's purely a matter of burn time, as rate of accelleration is not nearly as important as length of it. Even at 1 G, you could get going pretty fast with a continous burn. I've looked at some NASA info regarding ion (not applicable) and plasma propulsion, but I assume you know stuff that isn't even published, yet.


they went to the moon at 6 miles per second.nmniteschaos
Jun 3, 2003 5:14 PM
Closer to 7 ...Humma Hah
Jun 3, 2003 6:24 PM
25,000 mph terminal velocity of the third stage, but it lost speed rapidly, getting down to around 2000 mph around 80% of the way there. It took about 3 days to cross about 280,000 miles, about one mile per second average speed.
It's published. Fanciful, but published ...Humma Hah
Jun 3, 2003 6:20 PM
Off the top of my head, the fastest manned rocket so far was the Saturn V (notice, so old the numerals are Roman, like the Radio Shack Model III computer). That one had a first stage specific impulse (pound-seconds thrust per pound of rocket at takeoff) of 360 seconds. The third stage, starting from 17,500 mph, reached about 25,000 miles an hour during a burn of about 15 minutes. That propelled the Apollo craft to the Moon, and on the way the speed dropped to something like 2000 mph as the craft climbed out of Earth's gravity well.

A faster speed is needed to get to Mars, as it must climb both Earth's full gravity well and much of the Sun's. Again, most of the velocity of any chemical rocket would be lost climbing from Earth orbit to around the Moon's, so whatever is left over after that is what has to take you to Mars. Typical trip times they've been talking about would be on the order of 9 months or so, and the payload fraction of what leaves the surface of Earth that actually gets to Mars is some pathetically small number like half a percent. Unmanned interplanetary rockets so far have hit speeds like maybe 45,000 mph, accomplished by reducing the payload fraction.

The ultimate speed of rockets is calculated by "the rocket formula", a log function calculated from fully-fueled weight, the empty weight, and the specific impulse or exhaust velocity of the fuel.

The upper end of proven chemical rocket propulsion is 460 seconds Isp, for the Shuttle's hydrogen-oxygen engines that run hydrogen-rich to boost exhaust velocity. That's probably enough margin to get the extra speed needed to make a manned mission to Mars with chemical fuel at least possible, just barely. A friend of mine who passed away recently claimed to have a fuel system that might exceed 800 seconds Isp, but never demonstrated it before he died, and there are a lot of skeptics that it can be done.

Nuclear propulsion: my old boss, Dr. Bussard, designed an engine which was actually built and tested, capable of 860 seconds Isp. It passed hydrogen gas over a fission reactor core to heat it, achieving very high exhaust velocity. The final version, NERVA (1972), was Mars-capable, but never flown. The technology exists.

Dr. Bussard's present dream is an electric rocket powered by proton/Boron-11 fusion, a technology he is actively pursuing. That power source would allow relativistic-electron-beam heating of reaction mass to give an Isp of 4000 seconds or so, enough to get a manned rocket to Mars in under 3 months with a payload fraction of 20% delivered payload, if I recall the numbers correctly.

Going for gentler thrust but longer burn, fusion-powered rockets of up to half a million seconds Isp might one day reach speeds of up to 5% lightspeed on round trips.
thanks for infoDougSloan
Jun 4, 2003 6:37 AM
This stuff is fascinating to me.

I was under the impression that the Saturn V reached escape velocity, and therefore would no longer be pulled back or slowed by Earth's gravity. I realize that gravity acts on all objects, but I thought that's what "escape velocity" meant. Did they intentionally slow to 2,000 mph to orbit the Moon? Seems like a big problem going to Mars, if not. Would take a while at that speed.

Also, is the return trip from Mars easier, as you are approaching the Sun? Any idea how much so?

J got ya thinking, huh? Planning your next vacation? :) (nm)TNSquared
Jun 4, 2003 9:39 AM
sure, but the travel expense appear to be prohibitive (nm)DougSloan
Jun 4, 2003 9:47 AM
At age 16, R W Bussard concluded the same thing ...Humma Hah
Jun 5, 2003 7:45 AM
... he had been building and flying rockets, had calculated performance with all the chemial fuels known or proposed, and had reached the depressing conclusion that, while it was just possible to REACH Mars with chemical rockets, only at huge cost, long trips, and teensy payload fractions. Round-trips at reasonable cost, and therefor commerce, colonization, etc, (in other words, Flash Gordon/Buck Rogers stuff) were not possible.

He was pretty depressed.

About that time, there was a huge explosion in Hiroshima, followed closely by a second, and he realized that nuclear energy was more than just a speculative possibility. He made a sudden change in career direction that set him up, 8 years later, to propose the fission-powered rocket concept, which he intended expressly as a Mars engine.

His calculations for a p-B11-powered system, which I have partly checked and believe are reasonable, gave costs to put about 1200 colonists on Mars, each with some huge amount of supplies (50 tonnes?), and frequent round trips to Earth, at something in the $16 billion range. With chemical rockets, you'd maybe get one mission to Mars for a handful of people for that amount.

Cost to Low Earth Orbit with a p-B11 system would drop from about $3 million per person for the Shuttle down to about a quarter of the cost of a ticket to France on the Concorde.
well, let's go for it, thenDougSloan
Jun 5, 2003 7:52 AM
I assume lots of people would object to fission power, fearful of a Chernobyl from space. Didn't people object to the itty bitty nuclear power generators on Galileo?

Otherwise, what's the hold-up?

They're trying!Humma Hah
Jun 5, 2003 8:17 AM
... Shucks, I just got a call from 'em this morning, asking me how we hooked up a magnetron on an earlier experiment.

Who knows ... maybe they're building the machine that does it right this minute?

p-B11 fusion is pretty neat. It is not a chain reaction so it can't explode. It produces no neutrons so it won't make other materials radioactive. The byproduct is helium. No radioactive waste, in fact the fuel is component of borax and boric acid, which is somewhat toxic, so it removes a toxin from the environment. The process allows 80% or better conversion of fusion energy to electricity, so it even makes less waste heat than other processes.
The Cassini probe, actually ...Humma Hah
Jun 5, 2003 8:21 AM
... I think the objection was to the plutonium-fueled thermoelectric power generator on the Cassini Saturn probe. The same power sources have been used on the Apollo moon landings and almost every deep space mission and nobody complained ... flew right under most people's "nuclear radar."
Mars uninhabitable. nmKristin
Jun 5, 2003 8:29 AM
Barely worse than Antarctica ...Humma Hah
Jun 5, 2003 8:39 AM
... Technically, low earth orbit is "uninhabitable", but there are people living there continuously right now. Mars is easier.

The research stations in Antarctica are a very good model for what it would take to live on Mars. The only additional complicating issue is the need for pressurization of the habitat.

Either one is far less technically challenging, and less dangerous, than living on the floor of a deep ocean.

Living in any of these environments is just an exercise in engineering.
Jun 5, 2003 10:28 AM
I imagine the real challenge for Mars is not living there, but getting there (and back). Antarctica is what, maybe a 1,000 miles from help? The ocean bottom, 2-3 miles (up)? Plus, surfacing from the ocean bottom is a free ride, under the right conditions.

Probably the only places that are truly "uninhabitable" are places with conditions that our known structural materials could not survive (i.e., the sun), or with no energy source whatsover, long term.

yesHumma Hah
Jun 5, 2003 3:45 PM
Antarctica is 1000 miles from help, but cut off for half the year. In antarctic winter, the temps are so cold and the weather so bad, they can't operate aircraft. Remember the doctor stranded down there a few years back?

Space pressure problems: In the vacuum of space, 14.7 pounds per square inch are all the walls of the space station must endure. Contrary to Disney's cartoon, if you get a small hole in the space station, you can plug it with your hand, slap some duct tape over it, fix it with chewing gum, etc. It would take a pretty good-sized hole to let all the air out before you could fix it.

For every 32-ft down, you pick up that much external pressure in the ocean. The average depth is about 2 miles so that gives something on the order of 5000 pounds per square inch of water pressing against the habitat at that depth, and far higher than that in the really deep parts. The walls must be incredibly thick and strong or it will collapse faster than you can think. Even small distortions can be a problem: the hatches won't open. A pinhole leak would let a jet of water come thru that would cut you in two, and repairing it would be nearly impossible from the inside. It is cold, pitch-black, you can't see more than a few tens of meters.

Escape from the deep: Yes, providing your submersible escape vehicle is either built bouyant (thus unable to return to the bottom) or equipped with ballast pressure tanks that can force the ballast water out at that depth. If you lose the pressure required to blow ballast, you're stuck. There is a depth below which conventional submarines cannot surface again, but sink to the bottom, and this is the reason.
Gravity wells ...Humma Hah
Jun 5, 2003 7:01 AM
Picture gravity around a planet as a dimple in a sheet or surface, sort of an reverse pimple. The slope near the planet is very steep, but gets gentler the further away you go (by the inverse square law). The more massive the planet, the deeper and steeper the dimple.

A body orbiting the planet works like one of those stunt motorcycles riding on the walls of a barrel-shaped daredevil acts at the county fair. Ride fast enough and you can ride up vertical walls. A ship orbiting the planet must cancel out gravity by going at the speed at which centrifugal force equals gravity. That's faster near the surface of the planet, very slow far away.

But climbing out of the dimple is like climbing a hill. You have to either claw your way out with fairly steady effort, or get a running start and coast out. Chemical rockets usually use the second method. From 17,500 mph at the low orbit, they accelerate 2 miles per second (7 miles per second total) or more over some short burn, then coast out. 7 miles per second just barely gets the job done ... about 80% of the way to the Moon the speed is down to around 2000 mph just due to climbing. Then they start feeling the Moon's gravity and speed up again. The Moon is smaller and by the time they arrive they're doing around 4500-ish miles per hour. They burn a rocket to slow down a little and thus enter orbit around the Moon at something around 4000 mph.

That loss of almost 7 miles per second is most of the energy required to leave Earth to go anywhere nearby. To totally leave the Solar system, or go further out in the solar system, takes a little more to overcome the Sun's gravity (we're far enough away that the Sun's gravity well is not very steep, but it's large, so there's still a lot of gentle climbing to do). So if you want to go any place fast, you've got to have a rocket that can get you up to well above 7 miles per second, knowing you'll lose 7 miles per second just leaving the close environs of Earth.

I have a notebook at home somewhere where I have the calculations for Mars. It is easier coming back, except you have to get rid of the extra speed you get from coming downhill at the end of the trip. Some of that is done by rocket burn, some can be done by hitting the atmosphere and "aerobraking".

All of this involves physics that are actually conceptually pretty simple when you think of these "gravity well" dimples, something to think about while bicycling ... taking a banked turn or climbing or diving on hills, and suddenly hitting a big headwind (aerobraking) ... cycling gives you a real personal feel for the effort to accelerate, climb, etc.
great info; thanksDougSloan
Jun 5, 2003 7:38 AM
You should teach. Great job of putting in lay terms.

re: HH -- what manned rocket speeds possible?Miklos
Jun 5, 2003 10:43 AM
There was an artical in the Coulmbian a month or two back about a "Whiz Kid" in Portland Oregon. The kid wanted to try nuclear fussion in a lab expirement. He got the idea studying some of the work done by a "Tom Ligon".

Ring a bell?
Dinnnnnng!Humma Hah
Jun 5, 2003 11:29 AM
Look up Tom Ligon on the internet, find hits for Farnsworth Fusor, and Analog Science Fiction and Fact magazine. Look up Farnsworth Fusor, find Tom Ligon.

Ignore references to films, acting, and "Paint Your Wagon". That's another guy.
that's an open-ended invitation to us internet junkies, you knowJS Haiku Shop
Jun 5, 2003 12:03 PM
In the best spirit of amateur science, Tom Ligon sets up a demonstration of his fusion machine at the November 1998 meeting of the Philadelphia Science Fiction Society. Though the reactor operates four or five orders of magnitude below break-even (which means it doesn't produce energy), it does demonstrate the fusion of hydrogen into helium nucleii. See Tom's article in the December 1998 issue of Analog.
Durn, thanks! I've been wanting a copy of that pic ...Humma Hah
Jun 5, 2003 12:30 PM
... I remember the guy asking if he could take that picture, but never hooked up with him to get him to send me a copy.
so, working on a fusion powered bike? nmDougSloan
Jun 5, 2003 12:40 PM
Lee Iacocca is welcome to it ...Humma Hah
Jun 5, 2003 2:16 PM
I don't have to resort to an e-bike yet. I'd like to improve the present power source, but I kinda figured glycogen-powered makes more sense.
But what ifKristin
Jun 6, 2003 5:51 AM
What if there were some way to build a bike that could travel faster; but not require less effort from the rider? One that could cover twice the distance with the same amount of effort. I can see some benefit to being able to complete a double century in the same time and with the effort required for a single century. Weekend getaways that really get you away!!
technology already existsDougSloan
Jun 6, 2003 6:27 AM
Some riders do doubles as fast as 8 1/2 hours. That's about the same some do centuries. They just train more. There you go. ;-)

HummaHah. You are quite possiblyKristin
Jun 5, 2003 3:39 PM
the coolest guy I ever kinda met. And I thought I'd die without ever having known a real, live rocket scientist. Honestly though, the stuff you guys do is really something.
HummaHah. You are quite possiblyDougSloan
Jun 5, 2003 7:39 PM
HH, probably the smartest, humblest, nicest person here. Beard like Abe Lincoln to boot. Too bad he's married, Kristin. :-)

Neither humble nor the nicest ...Humma Hah
Jun 6, 2003 8:50 AM
... the nicest is Doug Sloan, which is why he was made the moderator.

As for rocket science, it ain't what most folks think of as rocket science. It's really a fairly simple field, just brute force and lots of it. I'm not that good at it, I just hung around someone who WAS for about 6 years.
too kind, but misguidedDougSloan
Jun 6, 2003 9:40 AM
You are of course too kind and humble to accept the labels of each, my friend. If we took a poll, I'd have no doubt of the results...

Things always seem simple when you really understand them. Getting to that understanding may not be simple. Even E=mc^2 is simple, now isn't it?

re: HH -- what manned rocket speeds possible?aliensporebomb
Jun 11, 2003 12:11 PM
Humma Hah actually KNOWS Dr. Bussard, the inventor of the
Bussard Ramjet, the projenitor of NERVA.


I swear, that guy is like a LEGEND to some of us.

Meeting him would be like, I don't know, like meeting a
rock star I guess.
maybe he'll meet Zefram Cochrane someday ;-) nmDougSloan
Jun 11, 2003 2:18 PM