Rocket engines - from speculations to successful flights

[fast]

Except that to land on an interstellar asteroid requires matching velocities, and doing so without crashing into it requires the same expenditure of rocket fuel as would be necessary without that asteroid. So that method does not get us anything.

There is also the difficulty of finding a suitable asteroid, because once it starts departing, it will not return. Traveling to such an asteroid is also a problem, since one has to reach it before it gets too far away, and one decides to chase it into interstellar space, one has to do what one would do if it was never present.

If such an asteroid was detected sufficiently early, then lassoing it could provide useful acceleration. You might need to use something stretchy, like a bungee-cord, to keep the payload acceleration down to survivable levels; And of course the cord will need to be super strong - perhaps a byproduct of the space elevator research will be a material that is suitable. But predicting its path around the sun, and waiting for it on the other side, shouldn't be too big a task, as long as you detect it (and are able to establish its exact velocity) early enough.

When you speak of survivable levels, you speak of payload. I guess that includes people.

From what I've gathered, two things are remarkable. Humans can both withstand very high acceleration, and humans cannot withstand even moderate acceration. Now that my apparent contradiction is out in the open, let me explain.

Again, from what I've gathered, it's remarkable just how poorly the human body can withstand acceleration. Sure, 2 or 3g's, but beyond that, training is required. Double that, and lots of training and equipment is paramount. 10 g's? Not a chance. Only the elite have a fighting chance.

However, for extremely short periods of time, we can easily withstand 20, 30, 40, 50 or more g's and walk away unscathed. That too is remarkable.

So, for a short moment, high acceration is survivable, but for more than a few seconds, it won't be pretty.

Now, I'm envisioning a whipping action in this bungie cord idea. Maybe there would be a way to harness an object such that tension will be controlled. That's fine, but for how long could we somewhat safely maintain even a 2g acceleration?

I would think we need interim modifications. For instance, 5g's for a period of time with intermittent 1g recuperation breaks.

I think it would be fascinating to solve the problem of surviving high acceleration. Is there (even in science fiction) an idea that teases at this? Altering the space adjacent to us. Something?

The only thing I can think of is for the destination to increase its velocity in our direction.

 

[bilby]

Except that to land on an interstellar asteroid requires matching velocities, and doing so without crashing into it requires the same expenditure of rocket fuel as would be necessary without that asteroid. So that method does not get us anything.

There is also the difficulty of finding a suitable asteroid, because once it starts departing, it will not return. Traveling to such an asteroid is also a problem, since one has to reach it before it gets too far away, and one decides to chase it into interstellar space, one has to do what one would do if it was never present.

If such an asteroid was detected sufficiently early, then lassoing it could provide useful acceleration. You might need to use something stretchy, like a bungee-cord, to keep the payload acceleration down to survivable levels; And of course the cord will need to be super strong - perhaps a byproduct of the space elevator research will be a material that is suitable. But predicting its path around the sun, and waiting for it on the other side, shouldn't be too big a task, as long as you detect it (and are able to establish its exact velocity) early enough.

When you speak of survivable levels, you speak of payload. I guess that includes people.

From what I've gathered, two things are remarkable. Humans can both withstand very high acceleration, and humans cannot withstand even moderate acceration. Now that my apparent contradiction is out in the open, let me explain.

Again, from what I've gathered, it's remarkable just how poorly the human body can withstand acceleration. Sure, 2 or 3g's, but beyond that, training is required. Double that, and lots of training and equipment is paramount. 10 g's? Not a chance. Only the elite have a fighting chance.

However, for extremely short periods of time, we can easily withstand 20, 30, 40, 50 or more g's and walk away unscathed. That too is remarkable.

So, for a short moment, high acceration is survivable, but for more than a few seconds, it won't be pretty.

Now, I'm envisioning a whipping action in this bungie cord idea. Maybe there would be a way to harness an object such that tension will be controlled. That's fine, but for how long could we somewhat safely maintain even a 2g acceleration?

I would think we need interim modifications. For instance, 5g's for a period of time with intermittent 1g recuperation breaks.

I think it would be fascinating to solve the problem of surviving high acceleration. Is there (even in science fiction) an idea that teases at this? Altering the space adjacent to us. Something?

The only thing I can think of is for the destination to increase its velocity in our direction.

If you want to go fast (and as space is big, you really do), there are two options - high acceleration for short periods, or moderate acceleration for long periods.

Sadly, the former seems to be much easier to arrange than the latter; I say 'sadly', because both high acceleration and prolonged exposure to very low acceleration are quite bad for humans (and to add insult to injury, high acceleration for short periods doesn't get you going very fast). A spacecraft that maintained a steady 1g acceleration solves both problems: It doesn't take long at 1g to get up to seriously high speeds; and you can spend as long as you like at 1g and not feel any ill effects that you wouldn't have felt if you had stayed at home (unless you hit a pebble - see below).

A continuous 1g trip to Alpha Centauri (4.3 light years away), turning round halfway, and decelerating at the same 1g, gives a journey time of just over three and a half years (although it looks like nearly six years to mission control back on Earth, and then the 'mission accomplished' signal takes another 4.3 years to get back to them, so the time from launch until hearing 'That's one small step for man...' is over ten years for mission control; while the time from launch to saying 'That's one small step for man...' is just three and a half years for Major Tom, the intrepid mission commander.

After 21 months of acceleration at 1g (as measured by his watch - mission control think he's been out there for three years at this point), Major Tom is traveling at 95% of light speed relative to Earth, having never experienced an acceleration greater than that which he felt when sitting on the couch in his home in Houston - other than for the trip to Earth orbit to join his spacecraft for its epic mission.

Of course, as the cosmic detritus in the vicinity of the Solar and Centauran systems is all moving at about 95% of light speed relative to Major Tom at the mid-point of his trip, he will need a pretty tough spacecraft, to withstand any impacts from interstellar dust particles. But that's the least of his engineering challenges - it's difficult to carry enough reaction mass and/or fuel to keep a 1g acceleration going for three and a half hours, much less three and a half years.

Solar sails try to overcome this by not carrying fuel at all, and using the Sun (or another nearby star) to provide the push, but a 1g acceleration is hard to achieve even close to the Sun with a large sail - the sail you would need at the halfway mark would be (literally) astronomically large - and as a result, would be certain to be torn apart by those .95c dust particles.

Ramjet designs (such as Bussard's) collect interstellar material to use as fuel and/or reaction mass - but they have a number of theoretical problems, not least of which is that the energy available from the interstellar medium may be somewhat lower than the energy needed to collect the stuff, as Loren Pechtel pointed out back on page 1 of this thread.

 

[fromderinside]

Maybe dark energy and energy will become known and be found to provide a solution to the problem.

 

[bilby]

Maybe dark energy and energy will become known and be found to provide a solution to the problem.

They don't appear to be significant at such small scales. If you want to travel between galaxies, maybe they will produce some effects, and maybe those effects will be useful. Maybe.

But a short trip to Alpha Centuri isn't a big enough distance for them to be likely to be helpful, for the same reason that a gravitational slingshot around Jupiter is good on a flight to Saturn, but you can't make effective use of a gravitational slingshot around the Empire State Building when driving a Yellow Cab through Manhattan (the documentary movie MiB notwithstanding).

 

[lpetrich]

If such an asteroid was detected sufficiently early, then lassoing it could provide useful acceleration. You might need to use something stretchy, like a bungee-cord, to keep the payload acceleration down to survivable levels; And of course the cord will need to be super strong - perhaps a byproduct of the space elevator research will be a material that is suitable. But predicting its path around the sun, and waiting for it on the other side, shouldn't be too big a task, as long as you detect it (and are able to establish its exact velocity) early enough.

That I'd like to see -- if it will ever be feasible. It should be easy to test this proposed scheme on Solar-System asteroids.

One would likely attach this cord to a winch, and that device will let it be gradually pulled out. For acceleration a and final velocity v, the necessary length is v²/(2*a). For 1 g and 30 km/s, this is about 46,000 km. That's more than enough to wrap around the Earth in a great circle.

 

[bilby]

If such an asteroid was detected sufficiently early, then lassoing it could provide useful acceleration. You might need to use something stretchy, like a bungee-cord, to keep the payload acceleration down to survivable levels; And of course the cord will need to be super strong - perhaps a byproduct of the space elevator research will be a material that is suitable. But predicting its path around the sun, and waiting for it on the other side, shouldn't be too big a task, as long as you detect it (and are able to establish its exact velocity) early enough.

That I'd like to see -- if it will ever be feasible. It should be easy to test this proposed scheme on Solar-System asteroids.

One would likely attach this cord to a winch, and that device will let it be gradually pulled out. For acceleration a and final velocity v, the necessary length is v²/(2*a). For 1 g and 30 km/s, this is about 46,000 km. That's more than enough to wrap around the Earth in a great circle.

Sure; but if your target acceleration is 10g (presumably for an instrument payload rather than a human one) then it's an order of magnitude easier - that's only a few thousand km of cable. Easy

 

[fast]

The problem is our solution. Eh, what I mean by that is we avoid problems instead of solving them. That's all fine and well when a mission's success trumps methodology, but avoiding problems leads us down a path that has us solving easier problems but nevertheless problems of a different kind. Idealistic goals become twisted and contorted. Granted, with monetary constraints and current technological impossibilities gets us moving in whatever discretionary direction most rewarding.

What we need, what we want, is to go outside and step into our vehicle. Our supremely nice, highly expensive, most advanced machine will take us where we want with the same ease as a Sunday drive. It will have vertical lift with pure unadulterated total control. We can leisurely and with utmost modest speed (we're in no hurry) ascend towards the sky. Do we need to reach escape velocity? Hell to the no! Easily we could with our awesome propulsion system, but escape velocity is not is not is not the velocity necessary to escape a massive bodies gravitational pull. It's the speed necessary only if no further impulse power will be used.

That being said, we'll head to space at a nice comfortable speed of snail pace leisure, just to show the world we can. Once we get to space, we'll fly over to the moon, land, take a gaze back home and realize we forgot something. We'll lift off, go back to earth, get our favorite music collection and head back out ... to outer space.

Unfortunately, we still haven't solved the problems to take a quick run out to our neighboring solar system. There are, well, other problems to be solved, but before we tackle those problems, let's look back at the problems that must of been solved just to do the things mentioned earlier.

Fuel. Oh my goodness, isn't that a stone age delimma. Let us for the moment imagine that energy issues and thus weight issues and storage issues have entered a new era. The days of the Commodore 64 is over, so to speak--and we have somehow been able to harness energy that could most easily and be compactly fitted such that energy is a well taken care of concern.

This brings me to mediums. Our vehicle (our quaint mid sized space craft) is not a submarine, so it need not traverse the depths of the ocean, but it does need to go through air, but alas, it must also travel through space where there is none.

That brings me to this thread where rocket engines is the discussion. There are, apparently a few. With fuel and energy and escape velocity not a concern, we need a propulsion system that will enable us to put us where we want to be.

And where is that? I'll introduce the idea now: our craft will need to lift off the ground and ascend (oh say) 25 miles up and hover with no motion relative to the earth. Then, it needs to gradually ascend to 50 miles and hover again. Then to 75.

What mechanism is our mechanism of choice? Not, what we must settle for instead.

 

[bilby]

The problem is our solution. Eh, what I mean by that is we avoid problems instead of solving them. That's all fine and well when a mission's success trumps methodology, but avoiding problems leads us down a path that has us solving easier problems but nevertheless problems of a different kind. Idealistic goals become twisted and contorted. Granted, with monetary constraints and current technological impossibilities gets us moving in whatever discretionary direction most rewarding.

What we need, what we want, is to go outside and step into our vehicle. Our supremely nice, highly expensive, most advanced machine will take us where we want with the same ease as a Sunday drive. It will have vertical lift with pure unadulterated total control. We can leisurely and with utmost modest speed (we're in no hurry) ascend towards the sky. Do we need to reach escape velocity? Hell to the no! Easily we could with our awesome propulsion system, but escape velocity is not is not is not the velocity necessary to escape a massive bodies gravitational pull. It's the speed necessary only if no further impulse power will be used.

That being said, we'll head to space at a nice comfortable speed of snail pace leisure, just to show the world we can. Once we get to space, we'll fly over to the moon, land, take a gaze back home and realize we forgot something. We'll lift off, go back to earth, get our favorite music collection and head back out ... to outer space.

Unfortunately, we still haven't solved the problems to take a quick run out to our neighboring solar system. There are, well, other problems to be solved, but before we tackle those problems, let's look back at the problems that must of been solved just to do the things mentioned earlier.

Fuel. Oh my goodness, isn't that a stone age delimma. Let us for the moment imagine that energy issues and thus weight issues and storage issues have entered a new era. The days of the Commodore 64 is over, so to speak--and we have somehow been able to harness energy that could most easily and be compactly fitted such that energy is a well taken care of concern.

This brings me to mediums. Our vehicle (our quaint mid sized space craft) is not a submarine, so it need not traverse the depths of the ocean, but it does need to go through air, but alas, it must also travel through space where there is none.

That brings me to this thread where rocket engines is the discussion. There are, apparently a few. With fuel and energy and escape velocity not a concern, we need a propulsion system that will enable us to put us where we want to be.

And where is that? I'll introduce the idea now: our craft will need to lift off the ground and ascend (oh say) 25 miles up and hover with no motion relative to the earth. Then, it needs to gradually ascend to 50 miles and hover again. Then to 75.

What mechanism is our mechanism of choice? Not, what we must settle for instead.

The reason it's hard to get to orbit isn't that space is high up.
It's hard to get to orbit because you have to go so fast.

https://what-if.xkcd.com/58/

 

[skepticalbip]

What mechanism is our mechanism of choice? Not, what we must settle for instead.

I think H.G. Wells hit on that "mechanism of choice" back in 1900... the solution is carborite.

 

[Treedbear]

What mechanism is our mechanism of choice? Not, what we must settle for instead.

I think H.G. Wells hit on that "mechanism of choice" back in 1900... the solution is carborite.

Or as Bullwinkle's uncle called it: Upsidaisium.

 

[James Brown]

The reason it's hard to get to orbit isn't that space is high up.
It's hard to get to orbit because you have to go so fast.

https://what-if.xkcd.com/58/

I miss those What if? posts. He seems to have stopped them after his book was published.

 

[fast]

The problem is our solution. Eh, what I mean by that is we avoid problems instead of solving them. That's all fine and well when a mission's success trumps methodology, but avoiding problems leads us down a path that has us solving easier problems but nevertheless problems of a different kind. Idealistic goals become twisted and contorted. Granted, with monetary constraints and current technological impossibilities gets us moving in whatever discretionary direction most rewarding.

What we need, what we want, is to go outside and step into our vehicle. Our supremely nice, highly expensive, most advanced machine will take us where we want with the same ease as a Sunday drive. It will have vertical lift with pure unadulterated total control. We can leisurely and with utmost modest speed (we're in no hurry) ascend towards the sky. Do we need to reach escape velocity? Hell to the no! Easily we could with our awesome propulsion system, but escape velocity is not is not is not the velocity necessary to escape a massive bodies gravitational pull. It's the speed necessary only if no further impulse power will be used.

That being said, we'll head to space at a nice comfortable speed of snail pace leisure, just to show the world we can. Once we get to space, we'll fly over to the moon, land, take a gaze back home and realize we forgot something. We'll lift off, go back to earth, get our favorite music collection and head back out ... to outer space.

Unfortunately, we still haven't solved the problems to take a quick run out to our neighboring solar system. There are, well, other problems to be solved, but before we tackle those problems, let's look back at the problems that must of been solved just to do the things mentioned earlier.

Fuel. Oh my goodness, isn't that a stone age delimma. Let us for the moment imagine that energy issues and thus weight issues and storage issues have entered a new era. The days of the Commodore 64 is over, so to speak--and we have somehow been able to harness energy that could most easily and be compactly fitted such that energy is a well taken care of concern.

This brings me to mediums. Our vehicle (our quaint mid sized space craft) is not a submarine, so it need not traverse the depths of the ocean, but it does need to go through air, but alas, it must also travel through space where there is none.

That brings me to this thread where rocket engines is the discussion. There are, apparently a few. With fuel and energy and escape velocity not a concern, we need a propulsion system that will enable us to put us where we want to be.

And where is that? I'll introduce the idea now: our craft will need to lift off the ground and ascend (oh say) 25 miles up and hover with no motion relative to the earth. Then, it needs to gradually ascend to 50 miles and hover again. Then to 75.

What mechanism is our mechanism of choice? Not, what we must settle for instead.

The reason it's hard to get to orbit isn't that space is high up.
It's hard to get to orbit because you have to go so fast.

https://what-if.xkcd.com/58/

Staying in space at a consistent distance from Earth after vertical ascent should be a sinch and find you with no need to accelerate such that you are orbiting the planet if (if) the beforementioned problems have been solved.

They haven't been solved. They've been sidestepped. That path brings with it new issues, issues that have been solved, but the solving of those problems leaves beforementioned problems not solved but sidestepped.

 

[bilby]

What mechanism is our mechanism of choice? Not, what we must settle for instead.

I think H.G. Wells hit on that "mechanism of choice" back in 1900... the solution is carborite.

Or as Bullwinkle's uncle called it: Upsidaisium.

Unobtainium is also frequently used for this purpose.

 

[fast]

What mechanism is our mechanism of choice? Not, what we must settle for instead.

I think H.G. Wells hit on that "mechanism of choice" back in 1900... the solution is carborite.

No, I mean what propulsion system would be optimal if used both at 10,000 feet and 1,000,000 feet? Is there such a technology available today that will allow us to both hover in an air atmosphere and allow us to gain forward momentum in space? Never mind energy. Seperate issue.

 

[skepticalbip]

What mechanism is our mechanism of choice? Not, what we must settle for instead.

I think H.G. Wells hit on that "mechanism of choice" back in 1900... the solution is carborite.

No, I mean what propulsion system would be optimal if used both at 10,000 feet and 1,000,000 feet?

Exactly... carborite. Anti-gravity material, the control of which is adjustable by simply adjusting the shutters between it and a gravitating mass.

Is there such a technology available today that will allow us to both hover in an air atmosphere and allow us to gain forward momentum in space? Never mind energy. Seperate issue.

Never mind the energy required? Then adjustable thrust rockets should do the trick. It has worked fine for Lunar and Mars landings.

 

[fast]

Or as Bullwinkle's uncle called it: Upsidaisium.

Unobtainium is also frequently used for this purpose.

Cute.

Okay, so let's say you use your cave man methods and send rockets into space at such escape velocity speeds that it is able to maintain orbit. Great. Now, point to the moon and step on the gas. What engine or means or system are you using to accelerate? Can THAT system be used on earth for take off if your craft is equipped with 16 car sized batteries where each one is capable of holding practically unimaginable energy that can be harnessed and utilized such that your alternative cave man methods would be rendered obsolete?

Unobtainium is on back order.

 

[fast]

No, I mean what propulsion system would be optimal if used both at 10,000 feet and 1,000,000 feet?

Exactly... carborite. Anti-gravity material, the control of which is adjustable by simply adjusting the shutters between it and a gravitating mass.

Is there such a technology available today that will allow us to both hover in an air atmosphere and allow us to gain forward momentum in space? Never mind energy. Seperate issue.

Never mind the energy required? Then adjustable thrust rockets should do the trick. It has worked fine for Lunar and Mars landings.

Well, never mind energy, to an extent. The problem we have, I gather, is that our supply is very limited. What I'm wanting you to suppose is that we have far more than enough. That being the case, there seems very little need for traditional rockets where parts are discarded along the journey.

 

[skepticalbip]

Exactly... carborite. Anti-gravity material, the control of which is adjustable by simply adjusting the shutters between it and a gravitating mass.

Never mind the energy required? Then adjustable thrust rockets should do the trick. It has worked fine for Lunar and Mars landings.

Well, never mind energy, to an extent. The problem we have, I gather, is that our supply is very limited. What I'm wanting you to suppose is that we have far more than enough. That being the case, there seems very little need for traditional rockets where parts are discarded along the journey.

It seems that you are looking for a way to get acceleration in space without using reaction mass. Newton has a bit of a problem with that idea. But then Newton never heard of carborite.

 

[Loren Pechtel]

Except that to land on an interstellar asteroid requires matching velocities, and doing so without crashing into it requires the same expenditure of rocket fuel as would be necessary without that asteroid. So that method does not get us anything.

There is also the difficulty of finding a suitable asteroid, because once it starts departing, it will not return. Traveling to such an asteroid is also a problem, since one has to reach it before it gets too far away, and one decides to chase it into interstellar space, one has to do what one would do if it was never present.

If such an asteroid was detected sufficiently early, then lassoing it could provide useful acceleration. You might need to use something stretchy, like a bungee-cord, to keep the payload acceleration down to survivable levels; And of course the cord will need to be super strong - perhaps a byproduct of the space elevator research will be a material that is suitable. But predicting its path around the sun, and waiting for it on the other side, shouldn't be too big a task, as long as you detect it (and are able to establish its exact velocity) early enough.

You realize how long that bungee must be?? And how tough?

You've got a minimum of 12 km/sec to absorb (that's assuming a perfect geometry we certainly don't have)--more velocity than going to orbit. Note that by the time you've reached orbit your energy of motion considerably exceeds the energy of the most powerful chemical explosives and you're talking about twice that energy.

You come along and somehow harpoon one end of the bungee to the rock. It's also got your velocity. The bungee stretches--absorbing several times the energy of TNT in the process. How can it store far more in stretching than the most energetic molecules can hold?? Assuming it somehow survives you wait until it's brought you to a stop then jettison it. It still has all that energy which is liberated throwing the bungee at the tether point. All the energy that was absorbed stopping you will be liberated in a big boom at the tether point. I don't want to be anywhere nearby when that happens!

 

[bilby]

Except that to land on an interstellar asteroid requires matching velocities, and doing so without crashing into it requires the same expenditure of rocket fuel as would be necessary without that asteroid. So that method does not get us anything.

There is also the difficulty of finding a suitable asteroid, because once it starts departing, it will not return. Traveling to such an asteroid is also a problem, since one has to reach it before it gets too far away, and one decides to chase it into interstellar space, one has to do what one would do if it was never present.

If such an asteroid was detected sufficiently early, then lassoing it could provide useful acceleration. You might need to use something stretchy, like a bungee-cord, to keep the payload acceleration down to survivable levels; And of course the cord will need to be super strong - perhaps a byproduct of the space elevator research will be a material that is suitable. But predicting its path around the sun, and waiting for it on the other side, shouldn't be too big a task, as long as you detect it (and are able to establish its exact velocity) early enough.

You realize how long that bungee must be?? And how tough?

Yes. Hence my comment about its being a byproduct of space elevator research. We are discussing options on the basis of materials not yet developed; why is this one any less credible than a space elevator?

You've got a minimum of 12 km/sec to absorb (that's assuming a perfect geometry we certainly don't have)--more velocity than going to orbit. Note that by the time you've reached orbit your energy of motion considerably exceeds the energy of the most powerful chemical explosives and you're talking about twice that energy.

You come along and somehow harpoon one end of the bungee to the rock. It's also got your velocity. The bungee stretches--absorbing several times the energy of TNT in the process. How can it store far more in stretching than the most energetic molecules can hold?? Assuming it somehow survives you wait until it's brought you to a stop then jettison it. It still has all that energy which is liberated throwing the bungee at the tether point. All the energy that was absorbed stopping you will be liberated in a big boom at the tether point. I don't want to be anywhere nearby when that happens!

I don't think you and I are considering the same objective here. Why are you jettisoning the tether? Why are you using a long tether, if not to allow you too spool it out from a winch, to translate some (but not all) of the tensile energy into kinetic energy for your spacecraft? In no way am I proposing that the entire energy transferred from the asteroid to the spacecraft needs to be stored in the cable at any one point in time. But even assuming that you DID want to do that - the potential energy stored in the stretched material is proportional to the mass (and therefore to the length) of the bungee cord; Your comparison with chemical explosives is meaningless without some consideration of quantity. A cable massing a hundred tonnes can hold the energy of a tonne of TNT if the material it is made from can hold 1% of the chemical energy stored in TNT, as elastic stress.