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[10ebbor10]

Perhaps one of the new aerospace start-ups will go mad scientist and build the Sea Dragon (or another BDB).  It was estimated to have a payload of 450000 kg at $667 per kg.

Why would anyone invest in such a monstrosity?  To be the source of all space development for the foreseeable future. With payloads that heavy, you could lift entire industries into orbit.

You would inevitably run into upscaling problems. Such a big single engine will have serious design repercussions.

[wierd]

most likely it would have tortional stress and shear issues that would make rapid acceleration and deceleration very dangerous.

there are finite limits to what our current materials can handle, and the stresses are magnified when the vehicle is very long or very wide, due to parts of the vessel acting as a lever.

that's the issue i see with the "huge scoop on the front" idea for sustained nuke propulsion. the thrust is not constant, so the stress wave of the explosion behind the vessel will travel through the ship, and the wider/longer the vessel, the greater the "leveraging effect" will be. if that deformation exceeds the young modulus, the ship will crinkle up like aluminium foil.

[Urist Tilaturist]

The square-cube rule is very important here too. The volume of an object increases vastly quicker than its surface area or length as it becomes bigger. This would likely lead to far greater masses and strains than the framework of the craft could support.

[Rez]

There are BDB designs that use many small engines instead.  The bigger issue is that such heavy lifting is quite expensive and ultimately pointless if you're looking at the long-term.  Once you can get water, electronics, and metals from raw materials in space, you don't need to lift anything other than luxury goods and people.

It's even more pointless if you think we're going to be able to build a space elevator of some sort in the near future.

[Sheb]

What's a BDB?

[10ebbor10]

http://en.wikipedia.org/wiki/Big_dumb_booster

[Sergarr]

They quite obviously can't simply leave nothing afterwards, since conservation of momentum is an unbreakable law of nature.

More likely, there's some virtual-virtual processes going on there that moves that momentum through space until it hits something and then transfers that impulse back into the real world... somewhere.

And, wierd? The idea that you can convert momentum to energy is one of the most weird ones I've heard.

No, not weird at all.  Think about what heat energy is. It's just diffuse kinetic energy, related to the chaotic motion of the constituent atoms, all of which have discrete vectors of momentum, all jostling around together.  Now, throw in black body radiation. photons are emitted from the matter as long as it is hotter than absolute zero, and this emission results in the "cooling" of the substance. Likewise, absorption of photons increases the temperature.  Since the atoms stop jostling with as much momentum as they emit photons, it is possible to describe momentum as an energy quanta.

There's also the wide world of radiation pressure, and the math behind why particle accelerators need fast moving particles to get greater collision energies. (the momentum of the particles adds to the energy value of the collision)

So, no. not a strange thing at all.

but momentum is a vector and energy is a scalar

how can you convert a vector into a scalar

[Urist Tilaturist]

Kinetic energy is momentum integrated.

KE=1/2*mass*velocity^2
Momentum=mass*velocity

[Sergarr]

There's a law about conservation of momentum. There's a law about conservation of energy.

There's no law that says that you can simply turn momentum into energy, or in reverse. It simply cannot happen.

[Graknorke]

Well momentum isn't a measure of energy. Kinetic energy is.
And you totally can convert kinetic energy into thermal energy.

[10ebbor10]

The friction you get is the hydrogen you gather.

[Urist Tilaturist]

Well momentum isn't a measure of energy. Kinetic energy is.
And you totally can convert kinetic energy into thermal energy.

This is true; from momentum it is possible to calculate kinetic energy, which can of course convert into other forms of energy.

[Sheb]

What? No, you need momentum and mass or velocity to calculate kinetic energy.

[UXLZ]

Any kilometer long scoop would have to be built in space (there's simply no way we're getting it into space from Earth without something going wrong), so we'll need a good method of doing that first if you want to go that way. A space elevator might be feasible to bring the construction materials up.

Isn't momentum velocity*mass?
I seem to remember P equaling mv.

The friction you get is the hydrogen you gather.

...and?

As I said, you don't gather enough hydrogen to offset the friction. Not if you're wanting to go at high speeds, at least.

I'm pretty sure the point is to maintain a speed. You might be slower than you otherwise would without that collection, but you will be able to accelerate to a certain point and the difference between 50000 m/s and 49050 m/s is negligible.

[Sheb]

Yes, p=mv. And kinetic energy = mv²/2. So if you only have momentum, you have no way to compute kinetc energy.