"Direct" burn to planet?

[DoctorDavinci]

Something that hasn't been mentioned here is that experiencing prolonged G forces above 1G could likely cause cardiac arrest in regards to the time scale in question here ... also, the cerebral/spinal fluid in your body that transports nutrients and removes waste from your brain would likely not work in the manner that it does 

The human body is a fragile thing ... nobody is superman/superwoman

Edited January 24, 2017 by DoctorDavinci

[suicidejunkie]

15 minutes ago, DrunkenKerbalnaut said:

An interesting side effect of a constant thrust trip: less time exposed to space-borne radiation.

I'd say that's more a result of spending absurd amounts of dV for your trip over and above the minimum requirements.

 

All else being equal, spreading the thrust out hurts you.  If you expend the same amount of dV in a shorter time at the start and end, you'll go much faster on average and spend even less time exposed in space as you travel.

[Leafbaron]

interesting read relating to @DoctorDavinci point about extended periods of higher than normal g loads. 

http://space.stackexchange.com/questions/6154/maximum-survivable-long-term-g-forces

[KerikBalm]

Well, assuming one lies down in an "acceleration couch", or something similar, there would be minimal strain on the heart and ciculatory systems.

People certainly wouldn't be up and about walking around on the ship for very long in those conditions.

In the past, on another forum, I participated in a forum sci fi game... my "ships" used constant thrust, but I was much more conservative: they were only capable of about 0.1 G acceleration - such a decrease in acceleration results in a decreased dV expenditure as well, and thus a truly massive decrease in required energy output relative to a 1 G acceleration. The other forum members more or less accepted the idea of fusion powered spacecraft accelerating at 0.1Gs (which were also capable of much higher accelerations at much lower Isps with simple impulse trajectories by good ol' NTR propulsion... but of course the long coast times meant travel would take months longer)

2 minutes ago, suicidejunkie said:

All else being equal, spreading the thrust out hurts you.  If you expend the same amount of dV in a shorter time at the start and end, you'll go much faster on average and spend even less time exposed in space as you travel.

Yes, Impulse trajectories are more efficient than brachistichrone trajectories. The problem is that when you start to look at shorter and shorter travel times, that dV budget gets really high, so you need high Isps.

High Isp and high acceleration means ludicrous energy requirements. Most writers figured that this results in a power output limitation, and ships would simply operate at maximum reactor output throughout the journey.

Orion drives though... those are a case of high thrust and high Isp... the reactors being nuclear bombs... However, even those aren't really up to the task of a torchship drive... which are often matter-antimatter powered.

I suppose an orion drive with antimatter bombs could work- most writers assume the reactor is internal, but an external bomb reaction could work -> still the heat buildup from those bombs would be a problem - there are often 2 designs cited: with and without an ablative pusher plate. The ablative plate wouldn't work for a torchship because the ablator would be gone in short order. The non ablative one would quickly absorb too much heat, and couldn't sustain constant thrust either.

The main limitation of all torchships is the power output required, and the corresponding heat buildup from the slightest of inefficiencies.

When looking at those problems, the anatomical problems of humans at 2G constant acceleration seem trivial - besides, its so hard to get to even 1 G, why are we talking about 2 or 3?

[DoctorDavinci]

9 minutes ago, KerikBalm said:

Well, assuming one lies down in an "acceleration couch", or something similar, there would be minimal strain on the heart and ciculatory systems.

Yeah, acceleration couch ... is that something like an inertial dampener?

No such technology exists as of this post (and is not likely to exist in the forseeable future)

I think you give too much credit to the human body ... I spent 2 years on my back and it sure affected me after I started walking again

Even to this day I have to be careful not to exert myself due to the risk of an exploding heart

Your body is not able to sustain high amounts of G forces over the period of time being discussed here without having some form of possible medical complications

 

[Tyko]

On 1/22/2017 at 9:39 AM, HebaruSan said:

 

Passenger health and comfort, most likely. And we have a lot of experience building structures to withstand 1g. But it's not a hard limit...

E.g., if we're plugging sci fi stories that have actual, legit torch ships...

Joe Haldeman, The Forever War. America transitions from fighting North Vietnam to fighting aliens, and time dilation makes a mess of the main character's personal history. Interstellar travel is via wormholes, but the ships still have to accelerate and decelerate normally using fusion drives. They often need to burn at accelerations far beyond human survivability for reasons of military necessity, sometimes for days or weeks at a time, and lacking any form of artificial gravity, they resort to drugging the crew and submersing them in big tanks of liquid to hold their bodies together while the computer takes care of the piloting. The UN turns everyone gay.

Larry Niven and Jerry Pournelle, The Mote in God's Eye. A distant-future Second Empire of Man fields a fleet of fusion-drive ships across hundreds of worlds. Interstellar travel is via instantaneous point-to-point jump drives, but navigation within solar systems is necessary to get to and from those points. Normal burns are 1g, up to 3.5g for message sloops when the flight surgeon certifies all aboard (no fancy liquid tanks, you just have to sit or lie down and wait). One of the ship's officers is responsible for announcing any impending acceleration changes over the intercom. During long periods of coasting or orbiting with the engines off, the ships are spun up for gravity. I really liked the characters, and the culture of the Empire is interesting as well as significant to the plot.

(I hope I succeeded in avoiding spoilers there. I've been looking for excuses to recommend these, in case anyone in this audience missed them.)

Another interesting take is presented in Dan Simmons' "Endymion". No real spoilers here, but out of respect for those that hate them, I'll hide my comment below.

Spoiler

They've discovered a means to "resurrect" the dead, so they've designed ships that don't even try to stay within human limits. Acceleration couches are more like bathtubs - designed to catch all of a crew members' bits as they are squashed into something the consistency of raspberry jam by the intense acceleration...kind of a morbid, but effective approach.

 

Edited January 24, 2017 by Tyko

[DrunkenKerbalnaut]

38 minutes ago, DoctorDavinci said:

Yeah, acceleration couch ... is that something like an inertial dampener?

No such technology exists as of this post (and is not likely to exist in the forseeable future)

Perhaps he's referring to couches already in use, like Soyuz. But that that craft wasnt the first to do so.  They are made to support the body in the most optimal g-loaded position to prevent injury and blackout - helping crew survive the eventuality of forces well in excess of 10g. Likely not a good candidate for prolonged multi-g trips, but they do exist. 

And then there is the g-suits that fighter pilots use. Don't they constrict the extremities to promote blood-flow to the core and head? Once again, probably not ideal for extended use, but a proven technology that exists. 

Edited January 24, 2017 by DrunkenKerbalnaut
Typo

[DoctorDavinci]

2 minutes ago, DrunkenKerbalnaut said:

Perhaps he's referring to couches already in use, like Soyuz. But that that craft wasnt the first to do so.  They are made to support the body in the most optimal g-loaded position to prevent injury and blackout - helping crew survive the eventuality of forces well in excess of 10g. Likely not a good candidate for prolonged multi-g trips, but they do exist. 

And then there is the g-suits that fighter pilots use. Don't they constrict the extremities to promote blood-flow to the core and head? Once again, probably not ideal for extended use, but a proven technology that exists. 

Both of those do exist, however consider that everything inside your body in not being held in place.

Your body consists of individual parts such as your muscles, skeleton, blood and organs and all these organs have mass and some are heavier than others ... Think about the individual weights of each organ and how it would be compressed

I see bruised livers, bruised hearts, bruised kidneys, possible concussion like symptoms, increased strain on the lungs, blood system going all out of whack due to restriction of capillary flow, possible nervous system effects, stressed ligaments, strained or bruised muscles, spinal disc degeneration .... the list goes on

[Nathair]

8 minutes ago, DoctorDavinci said:

Both of those do exist, however consider that everything inside your body in not being held in place.

Your body consists of individual parts such as your muscles, skeleton, blood and organs and all these organs have mass and some are heavier than others ... Think about the individual weights of each organ and how it would be compressed

I see bruised livers, bruised hearts, bruised kidneys, possible concussion like symptoms, increased strain on the lungs, blood system going all out of whack due to restriction of capillary flow, possible nervous system effects, stressed ligaments, strained or bruised muscles, spinal disc degeneration .... the list goes on

 

 

Which brings us back to Haldeman's Acceleration Tanks/Shells. A combination of liquid breathing (oxygenated PFC) and full body immersion with compression...

Of course the whole point of that was to deal with relatively short periods of very high acceleration.

Edited January 24, 2017 by Nathair
Misspeling

[regex]

On 1/22/2017 at 11:51 AM, HebaruSan said:

I love when a sci fi novel reveals the author's wacky ideas for future cultural developments. Was that mostly a 60's and 70's thing?

From what I've read, yes. That was the "New Wave" science fiction, more about exploring developments in humanity and their reaction to technology. Some of my favorites came from that era, short stories from Dangerous Visions to pretty much the entirety of Herbert's catalog.

Edited January 24, 2017 by regex

[DrunkenKerbalnaut]

Just now, DoctorDavinci said:

Both of those do exist, however consider that everything inside your body in not being held in place.

Your body consists of individual parts such as your muscles, skeleton, blood and organs and all these organs have mass and some are heavier than others ... Think about the individual weights of each organ and how it would be compressed

I see bruised livers, bruised hearts, bruised kidneys, possible concussion like symptoms, increased strain on the lungs, blood system going all out of whack due to restriction of capillary flow, possible nervous system effects, stressed ligaments, strained or bruised muscles, spinal disc degeneration .... the list goes on

Yeah, for sure. And with your experience with your previous affliction combined with your interest in this sort of discussion, Its a safe assumption that you're better versed on these sorts of injuries than I am. 

I was just pointing out, for any who read this, that we squishy humans have methods of counteracting these harmful forces. As I stated, though, they aren't likely to do much good for a body at 3g for days on end, let alone months. 

Side note: I saw a preview of a movie recently. First human born on Mars, travels to Earth, has problems with organs (namely heart) being to big for prolonged exposure to 1g. Let's hope the movie has plausible sci, as it looks like it might be another teen heart-throb flick. 

http://m.imdb.com/title/tt3922818/

I think it's the kid from Ender. 

[MichaelPoole]

On 22. 1. 2017 at 5:39 PM, eloquentJane said:

The Martian doesn't contain an example of a spacecraft that provides constant 1g thrust. The Hermes, the interplanetary spacecraft in The Martian, uses an ion drive to achieve non-Keplerian orbits because it's a very mass-efficient way of doing it, but an ion drive like that doesn't have enough thrust to provide 1g throughout any part of the journey. The Hermes included centrifuges to simulate gravity, and would not have them if such a high acceleration could be provided by the engines for the entire journey. Additionally, the interplanetary transfers in The Martian took months to complete. With constant acceleration of 1g halfway to the destination and then the same in the other direction for the rest of the journey, getting between Earth and Mars would take mere days. However, such a maneuver would require the expenditure of enough fuel to equal hundreds of kilometers per second of delta-v, or possibly even millions of meters per second depending on how far apart the two planets are. This isn't yet realistically achievable even with VASIMR engines. The only possible way it could be done based on currently proposed technology is with fission/fusion propulsion, but even that would be at a push (and it would be much more efficient to use a high-energy transfer and stick to Keplerian orbits, and just add a centrifuge or two onto the spacecraft to simulate gravity).

This way of inter-system travel was already used by the Dawn probe to Ceres and Vesta btw.

[DoctorDavinci]

Just now, MichaelPoole said:

This way of inter-system travel was already used by the Dawn probe to Ceres and Vesta btw.

True, however it had no large biological entities onboard ... metals, ceramics and other compunds used in the design and construction of spacecraft can sustain much more stress than us 'squishy' types

[regex]

2 minutes ago, DoctorDavinci said:

True, however it had no large biological entities onboard ... metals, ceramics and other compunds used in the design and construction of spacecraft can sustain much more stress than us 'squishy' types

Right, but the Dawn spacecraft didn't make any high-G burns, aside from at launch.

[MichaelPoole]

3 minutes ago, DoctorDavinci said:

True, however it had no large biological entities onboard ... metals, ceramics and other compunds used in the design and construction of spacecraft can sustain much more stress than us 'squishy' types

You realize it accelerated at milliGees? The thrust it used was less strong than a touch of a feather. I was not talking about the type of "constant thrust" that would involved accelerating half of the way and braking the half, but the ion drive spiral type.

This:

Quote

The Martian doesn't contain an example of a spacecraft that provides constant 1g thrust. The Hermes, the interplanetary spacecraft in The Martian, uses an ion drive to achieve non-Keplerian orbits because it's a very mass-efficient way of doing it, but an ion drive like that doesn't have enough thrust to provide 1g throughout any part of the journey. 

I wanted to say Dawn had a similiar system like in the Martian.

Edited January 24, 2017 by MichaelPoole

[Nathair]

Just now, DoctorDavinci said:

True, however it had no large biological entities onboard ... metals, ceramics and other compunds used in the design and construction of spacecraft can sustain much more stress than us 'squishy' types

Stress? To put it in familiar terms Dawn went from 0-60 in four days. That kind of acceleration "stress" would be almost imperceptible.

[Hesp]

2 hours ago, KerikBalm said:

In this scenario: "thrust 1g more than halfway, then turn and decelerate at 2g" what is the benefit?

Apparently you've got a ship capable of 2G of acceleration... so why would you do this where the end result is that you go slower, and spend more than half the time lugging around twice the engine that you need?

You could get the same travel time by just accelerating halfway there at 1.33 G, and then decelerating at 1.333 G

Also its less fuel/time efficient. The acceleration you do just before reversing the thrust direction contributes very little to a reduction in travel time.

If you could accelerate at 2 G, it would be more efficient (in time and fuel/propellant) to spend 1/4 of your time accelerating at 2Gs, half your time coasting, and then 1/4 of your time decelerating.

Going at a lower acceleration than the ship is built to be able to achieve is just wasteful. If you target 1 G for comfort, then that answers the question why you don't decelerate at 2 Gs, or why you don't do a high impulse trajectory where you accelerate at way more than 1 G, then coast, then decelerate at way more than 1 G again.

True, but usually acceleration capabilities of the ship improve when the propellant gets used (assuming it is a significant fraction of the ship weight, of course)

[septemberWaves]

12 minutes ago, MichaelPoole said:

This way of inter-system travel was already used by the Dawn probe to Ceres and Vesta btw.

Continuous thrust has been used, but not continuous thrust with high acceleration. The Dawn probe uses an ion engine, which has extremely low thrust. A continuous thrust system with acceleration high enough to simulate gravity is far beyond what we are currently capable of.

[regex]

7 minutes ago, eloquentJane said:

Continuous thrust has been used, but not continuous thrust with high acceleration. The Dawn probe uses an ion engine, which has extremely low thrust. A continuous thrust system with acceleration high enough to simulate gravity is far beyond what we are currently capable of.

And might even be physically impossible. A NSWR might do the trick, but I don't think even its (theoretical) isp is up to the task, you'd still do long periods of coasting.

Edited January 24, 2017 by regex

[SpacedInvader]

2 hours ago, KerikBalm said:

well, this is a case...etc

I think an important factor to keep in mind here is that we're talking about a future where such drives are possible. While I expect there would be a period where technology could only "barely" achieve a constant 1g acceleration, that period would probably be relatively short. Instead I would expect vehicles such as this to be capable of much more than 1g of constant acceleration for use in emergency situations or military operations, much in the same way we have cars capable of driving over 100mph even though the speed limit for much of the US is 75mph. You are correct in stating that travel in this method is far less efficient than many other approaches, but this leads into my point in response to the next quote:

2 hours ago, DrunkenKerbalnaut said:

An interesting side effect of a constant thrust trip: less time exposed to space-borne radiation. 

Don't ask me to quote exact numbers, but I remember an interview with Cmdr. Chris Hadfield where he talked about radiation exposure, and how it effects extended stays in space. For a vessel to be protected, he referred to a rule of thumb for shielding it. Something like 5 inches of lead or 5 feet of water. Actively googling to find the exact numbers, bare with me. Whatever the numbers were, that's serious weight to lug around.

And if this protective jacket only surrounded the living quarters of the vessel, it could end up being a considerable fraction of the dry mass. Therefore, there's gotta be some trade-off economy in choosing a motor that gets you there in less time, using more fuel. Maybe not 1 for 1, but this lack of need for massive shielding frees up some mass budget for this faster propulsion method. 

Bringing interplanetary travel into line with a necessary human time frames is the whole point of a concept like constant thrust. Not only would such a propulsion system reduce the overall exposure to radiation and thus lower the needs for heavy shielding, but it would allow for routine travel between orbital bodies thanks to its ability to bring them within a few days travel time. Sure there are always going to be more fuel efficient ways to travel, but fuel is not the only efficiency we need to care about... After all, if we really only cared about how fuel efficient something is, we'd all be walking or sailing everywhere and more advanced forms of transport would never be used for anything.

[DrunkenKerbalnaut]

Just to satisfy my own curiosity, I'll take a whack at the numbers for a constant 1g trip from Earth to The Moon. Google says:

Spoiler

At perigee — its closest approach — the moon comes as close as 225,623 miles (363,104 kilometers). At apogee — the farthest away it gets — the moon is 252,088 miles (405,696 km) from Earth. On average, the distance from Earth to the moon is about 238,855 miles (384,400 km).

So let's call it 380,000km. Halfway is 190,000km. 

So playing around with this calculator:

http://keisan.casio.com/exec/system/1224829579

We find that accelerating at 9.82m/s^2 for 1 hr, 43 mins, 43.1274 seconds will take you to roughly a distance of 190,151.11513414 km. we will have attained maximum velocity of 220,000km/h. 

Our ship, existing only in my cartoonish mind, pivots instantaneously to apply thrust in the other direction. Upon arriving at the surface of the moon, it finishes the suicide burn to end all suicide burns, and lands approximately 3hr 26 minutes after departing Earth. 

 

EDIT: in this example, we completely ignore the fact that we aren't accelerating fast enough to escape Earth gravity. The distance used is just that: a known distance. So to add, we also ignore any gravitational handoff experienced when the Moon has greater effect on the ship around Earth L1, whatever that may be. 

Edited January 24, 2017 by DrunkenKerbalnaut

[SpacedInvader]

15 minutes ago, regex said:

And might even be physically impossible. A NSWR might do the trick, but I don't think even its (theoretical) isp is up to the task, you'd still do long periods of coasting.

It might be physically impossible with current or near-future technology, but I highly doubt the concept itself is physically impossible. As it stands it seems mostly like a problem of fuel energy density.. We have plenty of powerplants capable of 1g acceleration, but to do it for long periods of time would require impossible amounts of fuel. Given time, new fuels and/or new propulsion systems will most likely arise to allow for this sort of constant thrust. Basically, just because we haven't envisioned it yet doesn't mean its not possible.

[Nathair]

8 minutes ago, SpacedInvader said:

new propulsion systems will most likely arise to allow for this sort of constant thrust. Basically, just because we haven't envisioned it yet doesn't mean its not possible.

 

Bussard Ramjet. Obligatory SF reference: A World Out of Time featuring a 1g acceleration trip all the way to the galactic core.

[regex]

25 minutes ago, SpacedInvader said:

It might be physically impossible with current or near-future technology, but I highly doubt the concept itself is physically impossible. As it stands it seems mostly like a problem of fuel energy density.. We have plenty of powerplants capable of 1g acceleration, but to do it for long periods of time would require impossible amounts of fuel. Given time, new fuels and/or new propulsion systems will most likely arise to allow for this sort of constant thrust. Basically, just because we haven't envisioned it yet doesn't mean its not possible.

I don't feel like physical laws really support that sort of engine, at least regarding the theoretical work I've been exposed to and what I know of physics. I'd believe in negative mass "reactionless" drives like the Alcubierre drive before I'd believe in a high-isp, high-thrust physical-fuel-based engine. NSWR is about the closest I've seen in terms of theoretically possible. An antimatter pion drive would meet the isp requirements but not the thrust requirements.

Edited January 24, 2017 by regex

[HebaruSan]

9 minutes ago, Nathair said:

Bussard Ramjet. Obligatory SF reference: A World Out of Time featuring a 1g acceleration trip all the way to the galactic core.

As much as I love that book, we should note that it was later proved that such a craft has a speed limit of 0.12c due to drag from the interstellar medium. If I'm doing the math right, that means your 1g would run out after about 42 days (maybe longer if it winds down gradually).