Author Topic: LEO is halfway to anywhere in solar system. Fact or fiction ?  (Read 26801 times)

Offline Lee Jay

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When New Horizons separated from its Atlas V January 19, 2006, would you say it was "most of the way" to Pluto, since it wasn't going to get much more delta-V (well, from rockets anyway)?

The web page says, "The New Horizons spacecraft, now more than halfway between Earth and Pluto...".

I think when most people say "half way", they're thinking of time or distance, not energy or velocity.

Offline QuantumG

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I'd argue that easier to LEO makes the rest easier, as once you're in LEO there's no need to expend energy at the prodigious rate that a launch vehicle must, you can get to about anywhere in the solar system with far more conservative engineering.

Well, the same is true of getting above the atmosphere. If you had a way to "start" at 100 km, with no atmospheric penalty to worry about to get there, a much simpler rocket could get you to orbit than existing second stages of launch vehicles - because they have to go through supersonic transition in the lower atmosphere.

I can think of a few ways to do that, some more realistic than others, but I think that's a much more complicated argument than saying 100 km is half way to orbit.
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Offline jabe

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Another way of looking at it....As this kickstarter seems to suggest ..they can go "anywhere" on a modest budget..hard part for the project, if development is succesful, is getting to LEO.
jb

Offline savuporo

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So what you're saying is that there are different measures to delta v some of which make LEO far less than half way, others that make it far more than half way. Yep.
I'm saying that all the delta-v maps drawn up are missing many, and more important map layers.
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Offline Oli

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It's not only delta-v, but the fact that you don't need huge thrust to fight against gravity anymore. Thrust is expensive and heavy.

Quote from: Quantum
A typical two stage launch vehicle has a booster stage that essentially just throws the second stage above the atmosphere and gives it a little kick. In essence, it's close to the total delta-v of a suborbital tourism flight (about 4.5 km/s).

So, clearly, suborbital tourism is half way to orbit.

4.5km/s is Mach 13+, that's roughly 4x the delta-v of suborbital tourism.
« Last Edit: 01/04/2014 12:44 am by Oli »

Offline MATTBLAK

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"LEO is halfway to anywhere in solar system. Fact or fiction?" 

Sadly, it's been more fact than fiction since December 1972... :(
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Offline Patchouli

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It depends on your definition of half way and what the mission parameters are.

A space probe can take a slow trajectory though they don't always do, but with a crewed mission on the other hand you usually want to follow the fastest economically feasible trajectory.
« Last Edit: 01/04/2014 01:00 am by Patchouli »

Offline Oli

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4.5km/s is Mach 13+, that's roughly 4x the delta-v of suborbital tourism.

Gravity and aerodynamic losses eat 2 km/s, potential energy eats about 2 km/s. Suborbital vehicles that make it over 100 km, should they ever start flying, require about 4.5 km/s of delta-v. About the same as a first stage of a launch vehicle.

Apparently Space Ship One required a delta-v of roughly 1.4km/s...

http://en.wikipedia.org/wiki/Delta-v_budget

« Last Edit: 01/04/2014 01:41 am by Oli »

Offline QuantumG

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4.5km/s is Mach 13+, that's roughly 4x the delta-v of suborbital tourism.

Gravity and aerodynamic losses eat 2 km/s, potential energy eats about 2 km/s. Suborbital vehicles that make it over 100 km, should they ever start flying, require about 4.5 km/s of delta-v. About the same as a first stage of a launch vehicle.

Apparently Space Ship One required a delta-v of roughly 1.4km/s...

http://en.wikipedia.org/wiki/Delta-v_budget

Ahh, okay. My mistake. I think the point still stands: "100 km altitude with 2 km/s of velocity is halfway to orbit" is a misleading statement, even though it's technically correct in terms of delta-v.
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Offline sdsds

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Its not about the "first time" so much as about that a deep space craft has to keep working for a long long time in a very harsh environment. Barring magical leaps in propulsion technology, the launch windows to Jupiter come around only every so often, and even if launch to LEO was at its theoretical minimum, you would still have to design your hardware to last for years because you cant go after it and fix it.

I think your point presupposes that, upon reaching "anywhere in the solar system," your spacecraft isn't going to return to Earth. What would Elon say to that?
« Last Edit: 01/04/2014 02:02 am by sdsds »
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Offline aero

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Conversations such as these almost always confuse LEO, Low Earth Orbit, with LOA, Low Orbit Altitude. To stay in a stable LEO the vehicle must have both orbital altitude and orbital velocity.

For Earth, orbital velocity is 7,791.99 m/s at 185,000 meters altitude. At that altitude but no velocity your space vehicle will fall directly to earth under the influence of gravity. With insufficient velocity your vehicle will move a distance down range falling all the while before it encounters the earth. With exact orbital velocity your vehicle will fall past the side of the earth at exactly the same orbital altitude and with excess velocity it will pass the side of the earth at a higher altitude.

The "LEO" in LEO is halfway to anywhere includes both the altitude and the velocity needed for orbit. And note that if your vehicle is stationary at orbital altitude it has very little time to reach orbital velocity because it starts to fall immediately, and gravity will not wait for you to build your speed. Your vehicle needs to be ruggedly built in order to withstand the high accelerations needed to reach orbital velocity before gravity causes it to reach earth.

Two stage rockets overcome the need for extremely high acceleration of the second stage in part by giving the second stage a percentage of the horizontal orbital velocity and a very significant upward velocity. Gravity must overcome this upward velocity before it can accelerate the second stage downward hence the second stage has more time to accelerate through the remaining percentage of horizontal orbital velocity. With more time and less velocity increment the second stage acceleration can be lower and the vehicle somewhat less rugged. But note, as second stage fuel is burned off even the most wimpy ( but successful) second stage engines do provide the stage with high acceleration.
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Offline pathfinder_01

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Its not about the "first time" so much as about that a deep space craft has to keep working for a long long time in a very harsh environment. Barring magical leaps in propulsion technology, the launch windows to Jupiter come around only every so often, and even if launch to LEO was at its theoretical minimum, you would still have to design your hardware to last for years because you cant go after it and fix it.
Extra mass budget only helps with limited aspects of spacecraft engineering.

Every Flight including deep space flights to the moon by Apollo start in LEO and even flights to far places like Jupiter start from parking orbits. Sure you would have to design a craft to last a bit longer if assembling in LEO but there are limits to how much one can put on the top of an rocket no matter how big you make the thing.  Some assembly may be required no matter what(i.e. Even Apollo had to give up on Direct landing for various reasons. )

You don't have to design the hardware to last for years, it just needs to last long enough to pull the mission. This may push you towards different solutions but that is that. We have spacecraft that work without humans for years already. Probes that go to distant planets likewise and Voyager is still ticking at 36 years. There is only so much simplicity to be had by designing for short periods of time(i.e. One of the problems with Apollo is the fact that the LM was only built for short trip..i.e. you would require some redesign for it to serve as a craft that could support a lunar base.)

Offline Lars_J

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LEO is halfway to anywhere in solar system. Fact or fiction ?
« Reply #32 on: 01/04/2014 05:32 am »
Savuporo, I think you are fundamentally misunderstanding the "halfway to anywhere" idea.

You (and many others) seem to read it as "once you get to LEO, everything else is EASY". But that is NOT true, and not what the saying implies.

Getting to LEO is hard. Getting to the rest of the solar system from there is ALSO hard. A different set of challenges, but still very challenging.
« Last Edit: 01/04/2014 05:34 am by Lars_J »

Offline savuporo

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Getting to the rest of the solar system from there is ALSO hard. A different set of challenges, but still very challenging.
That's what i'm trying to call out here. In fact, getting to rest of the solar system is much, much harder in many ways, seeing as there are plenty of organizations that can put payloads to orbit, but only so few that have operated a spacecraft beyond earth orbits.

A lot of space enthusiasts seem to think its all about mass, delta-vee and orbits, whereas the real challenges are elsewhere.

Give you an example. If launch was free, you can probably build a decently working LEO cubesat with modest mission goals for about five figures.
If launch and insertion to lunar orbit was free, you could not build a working moon orbiting cubesat for a similar amount, you'd have to pay about 10x more.
« Last Edit: 01/04/2014 06:24 am by savuporo »
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Offline QuantumG

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Give you an example. If launch was free, you can probably build a decently working LEO cubesat with modest mission goals for about five figures.
If launch and insertion to lunar orbit was free, you could not build a working moon orbiting cubesat for a similar amount, you'd have to pay about 10x more.

Citation needed.
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Offline savuporo

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Offline Robotbeat

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The statement says absolutely nothing about what it's like to /operate/ anywhere in the solar system, just what it takes to get there.
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Offline savuporo

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The statement says absolutely nothing about what it's like to /operate/ anywhere in the solar system, just what it takes to get there.
Ah, ok - if thats what we are talking about then Heinlein is exactly correct - give or take a few km/s. LEO is halfway to anywhere in solar system as long as we are talking bricks or dead bodies.
« Last Edit: 01/04/2014 07:11 am by savuporo »
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Offline Robotbeat

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What kind of twisted understanding would it take to interpret the phrase as meaning it's just as easy to operate in LEO as it is, say, in Io orbit? The statement is about getting there, not about operating there.
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Offline Andrew_W

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Give you an example. If launch was free, you can probably build a decently working LEO cubesat with modest mission goals for about five figures.
If launch and insertion to lunar orbit was free, you could not build a working moon orbiting cubesat for a similar amount, you'd have to pay about 10x more.

If launch were free I'd be booking a ticket, along with millions of others, and all those billions of tourist dollars would push Manned activities far out into the solar system.

Imagine being able to launch from LEO without the cost of getting there, the cost of all interplanetary operations would plummet to a fraction of present costs. The several thousands of tons launching from the Earth's surface each year could be several thousands of tons launching from LEO, without the need to use high thrust chemical rockets those thousands of tons in LEO could, with higher Isp propulsion, launch far more mass to the planets than we currently put into LEO.
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