Constant acceleration VS momentum shift

[Spacescifi]

This will be my final post, as I have to stop someday, and today is as good as any. Thank you for all your informative answers and being arguably the kindest forum on the internet (thank to whoever runs this site).

 

Now onto the final post....

If you had to choose, which sublight scifi engine method would you find most useful and why? Which is best, or does it depend if you use it for peace or war? Which is the least likely good choice?

Between the choices below?

 

M drive: Spaceship utilizes a fusion reactor powered rocket fueled with liquid methane. The M drive, when engaged, will allow the ship and crew to harmlesdly shift the vessel's forward momentum wherever the the ship's nose is pointed. In other words, to go backwards just flip over and engage the M drive. If you want to actually adjust your vessel's speed you need to either burn rocket fuel or use a planet for grav-assist.

Antimatter thermal rocket: with liquid hydrogen propellant. Can thrust for days at 1g or somehat less at higher g.

1g constant acceleration drive: Provides 1g forward momentum via a kilogram of antimatter. No rocket plume is created as antimatter is converted directly into forward acceleration without blowing exhaust out the back.

It would take a month at 1g to run out of a kilogram of antimatter.

The only catch is that the 1g drive only can do 1g or below. So to take off from Earth it needs to take off like a plane and build up enough forward velocity to enter orbit around earth. Simply flying vertical from a standstill won't work, it would only make your vessel hover in midair.

 

So... I await your answers. Thank you.

[KSK]

1g drive for the win.

Why? Because it implies that in your fictional setting, conservation of momentum does not apply. So, by Noether’s Theorem, the laws of physics vary depending on your location in space. And at that point, pretty much anything goes.

[Nightside]

Antimatter seems like it would only be used in experimental or military contexts.

I think the M drive could scale better, methane being easier to get and work with, so better the for common operations such as shipping and transport.

But I don’t understand how it can change momentum without changing velocity. Does it change its mass?

Also, why last post? What’s next?

[Guest]

My choice is antimatter thermal, but you already know that.  

[HebaruSan]

On 7/19/2019 at 6:14 AM, Spacescifi said:

M drive: Spaceship utilizes a fusion reactor powered rocket fueled with liquid methane. The M drive, when engaged, will allow the ship and crew to harmlesdly shift the vessel's forward momentum wherever the the ship's nose is pointed. In other words, to go backwards just flip over and engage the M drive. If you want to actually adjust your vessel's speed you need to either burn rocket fuel or use a planet for grav-assist.

It sounds like this assumes a preferred reference frame ("absolute space"), so this is probably an incoherent concept.

Suppose I'm in geosynchronous orbit around the Earth. Relative to the Earth, my momentum vector points tangent to my orbit around the Earth, with a magnitude of about mass * 3 km/s. But relative to the Sun, my momentum vector points tangent to Earth's orbit around the sun, with a magnitude of mass * 27-33 km/s (depending on where I am in my Earth orbit), since I am moving along with Earth in its orbit. So if I engage this fictional drive to "shift the vessel's forward momentum wherever the the ship's nose is pointed", that could be interpreted as up to a mass * 6 km/s change relative to Earth, or a mass * 60 km/s change relative to the sun (I could jump to a retrograde orbit opposite to Earth's). And so on up to bigger scales; if I can shift my momentum vector relative to the Andromeda galaxy, that would be on the order of mass * 300-600 km/s changes.

[Spacescifi]

21 minutes ago, HebaruSan said:

It sounds like this assumes a preferred reference frame ("absolute space"), so this is probably an incoherent concept.

Suppose I'm in geosynchronous orbit around the Earth. Relative to the Earth, my momentum vector points tangent to my orbit around the Earth, with a magnitude of about mass * 3 km/s. But relative to the Sun, my momentum vector points tangent to Earth's orbit around the sun, with a magnitude of mass * 27-33 km/s (depending on where I am in my Earth orbit), since I am moving along with Earth in its orbit. So if I engage this fictional drive to "shift the vessel's forward momentum wherever the the ship's nose is pointed", that could be interpreted as up to a mass * 6 km/s change relative to Earth, or a mass * 60 km/s change relative to the sun (I could jump to a retrograde orbit opposite to Earth's). And so on up to bigger scales; if I can shift my momentum vector relative to the Andromeda galaxy, that would be on the order of mass * 300-600 km/s changes.

 

Another thought just came to mind.

The m drive is far more dangerous than the the other ones here.

You could even use the m drive to repeat falling into a planet multiple times and then redirect all that velocity elsewhere.

Basically, you can virtually get free speed boosts quickly and easily with a planet/m drive combo.

Edited July 22, 2019 by Spacescifi
Planet

[KerikBalm]

M drive. breaks conservation of momentum (If I understand correctly), violates relativity. Big no from me

 

Antimatter thermal rocket: with liquid hydrogen propellant. Can thrust for days at 1g or somehat less at higher g.

^This one doesn't obviously violate any laws of physics (although you will need serious active cooling or seriously high efficiency in order for the waste heat to not vaporize the vessel in a short time), and thus wins it for me.

 

 

"1g constant acceleration drive: Provides 1g forward momentum via a kilogram of antimatter. No rocket plume is created as antimatter is converted directly into forward acceleration without blowing exhaust out the back.

It would take a month at 1g to run out of a kilogram of antimatter."

Ummm, violates conservation of momentum, and 1 month at 1G from 1 kg would violate conservation of energy unless the ship in question is really really small. So a big no from me.

[Cunjo Carl]

My honest answer would be to go for both! Maybe build in an interesting tradeoff between the two, so the crew needs to make difficult decisions when confronted with a new challenge or one of the drives going offline.

Safe travels, come back and say hi some time if the mood strikes!