"Direct" burn to planet?

[DrunkenKerbalnaut]

2 minutes ago, SpacedInvader said:

Was actually thinking of the D-T Vista with its insane amount of thrust and respectable ISP.

The fun part: "Kills Nearby Kerbals"

You would probably want to go the beamed power route. That bad boy is thirsty at max output. A massive solar orbiter with microwave transmitters, and all that fun stuff. Maybe set it up at Kerbin L4, and another at L5 for the return trip. 

 

[SpacedInvader]

2 minutes ago, DrunkenKerbalnaut said:

The fun part: "Kills Nearby Kerbals"

You would probably want to go the beamed power route. That bad boy is thirsty at max output. A massive solar orbiter with microwave transmitters, and all that fun stuff. Maybe set it up at Kerbin L4, and another at L5 for the return trip. 

 

If only lagrange points were real in KSP... I know I can simulate them, but you have to baby satellites you put in those spots because they drift a lot.

[DrunkenKerbalnaut]

Just now, SpacedInvader said:

If only lagrange points were real in KSP... I know I can simulate them, but you have to baby satellites you put in those spots because they drift a lot.

Get it as close as you can, and then "set orbit" with Alt F12. It's not your fault the game can't hang.  should be good for a few decades, after that. 

[Guest]

3 hours ago, DrunkenKerbalnaut said:

How do you plan to provide 1g thrust for ~15hrs in KSP? 

In the most awesome way possible.

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Pulse--Orion

https://web.archive.org/web/20070704104944/http://ntrs.nasa.gov/archive/nasa/casi.ntrs.nasa.gov/19770085619_1977085619.pdf

 

[GoSlash27]

7 hours ago, HebaruSan said:

A vehicle capable of that is sometimes called a "torch ship," and its drive a "torch drive," but the only name I'm finding for the maneuver is "Brachistochrone trajectories."

The first place I heard of this "torch ship" concept was Robert Heinlein. He was a wise old bird...

Best,
-Slashy

[septemberWaves]

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).

[wumpus]

12 hours ago, DrunkenKerbalnaut said:

Here we go. 3 year flight. 1 year thrusting. So not constant thrust, but certainly not traditional Oberth maneuver:

http://www.astronautix.com/t/tmk-e.html

The big catch here is that you need ~3000m/s to escape Earth and only ~1000m/s more to get to Mars (more if you can't benefit from Oberth, but the ratio should stay the same).  So pretty much all the fuel used to "direct burn" isn't needed (i.e. once you are past the Clarke constellation, you have burned roughly 3/4 of the fuel to get to Mars).  If your "constant thrust" takes nearly a year to get to speed (like all known constant thrust engines not developed for Project Orion), don't count on any shorted trip thanks to burning on the way to Mars.

[DrunkenKerbalnaut]

24 minutes ago, eloquentJane said:

The Martian doesn't contain an example of a spacecraft that provides constant 1g thrust. ........

Never said it did. 2mm/s^2. 

25 minutes ago, wumpus said:

The big catch here is that you need ~3000m/s to escape Earth and only ~1000m/s more to get to Mars (more if you can't benefit from Oberth, but the ratio should stay the same).  So pretty much all the fuel used to "direct burn" isn't needed (i.e. once you are past the Clarke constellation, you have burned roughly 3/4 of the fuel to get to Mars).  If your "constant thrust" takes nearly a year to get to speed (like all known constant thrust engines not developed for Project Orion), don't count on any shorted trip thanks to burning on the way to Mars.

Good point. 

[razark]

Can someone clarify why these plans are always "thrust 1g until halfway there, then turn around and thrust 1g to slow down"?

Why is it never "thrust 1.5g halfway..." or "thrust 1g more than halfway, then turn and decelerate at 2g"?

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

[DrunkenKerbalnaut]

10 minutes ago, razark said:

Can someone clarify why these plans are always "thrust 1g until halfway there, then turn around and thrust 1g to slow down"?

Why is it never "thrust 1.5g halfway..." or "thrust 1g more than halfway, then turn and decelerate at 2g"?

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

I think the appeal is simulating gravity. 

[septemberWaves]

13 hours ago, DrunkenKerbalnaut said:

This burn method received recent notoriety due to it's use in the Book and Movie "The Martian".

Your phrasing implied that you did, given that this was the first post in response to the author discussing the 1g constant thrust maneuver. I'm sure you can see how it's easy to make the mistake of thinking that you said that.

[wumpus]

1 hour ago, razark said:

Can someone clarify why these plans are always "thrust 1g until halfway there, then turn around and thrust 1g to slow down"?

Why is it never "thrust 1.5g halfway..." or "thrust 1g more than halfway, then turn and decelerate at 2g"?

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

1g is entirely for human comfort.  This was a major plot point in L.E.Modesitt jr's "Solar Express" book*.  Unfortunately that book came out in 2015 (and I read it much more recently) so I basically had trouble with the completely botched orbital mechanics (I could have told him that a "partial space elevator" made zero sense even before playing KSP).

* I think the various characters had to figure out how much they (and the spaceship) could tolerate for trip near the orbit of Mercury.  I think it turned out to be around 2g, but it was considered risky and not recommended.  In real life Saturn V (and I think the shuttle) were limited to ~3g thanks to structural issues (and also so the astronauts could still reach the controls).

Edited January 22, 2017 by wumpus
footnote added

[Nathair]

12 minutes ago, razark said:

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

1

In fiction it's often about having a crew living in 1g for extended periods... comfort.

[HebaruSan]

 

5 minutes ago, razark said:

Can someone clarify why these plans are always "thrust 1g until halfway there, then turn around and thrust 1g to slow down"?

Why is it never "thrust 1.5g halfway..." or "thrust 1g more than halfway, then turn and decelerate at 2g"?

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

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.)

[septemberWaves]

11 minutes ago, razark said:

Can someone clarify why these plans are always "thrust 1g until halfway there, then turn around and thrust 1g to slow down"?

Why is it never "thrust 1.5g halfway..." or "thrust 1g more than halfway, then turn and decelerate at 2g"?

Is there a benefit to a symmetrical acceleration/deceleration, and why limit it to 1g instead of slightly higher accelerations?

Higher accelerations would allow the journey to be faster, but this becomes a problem if humans are on board. Humans evolved to survive in a gravitational field with 1g of acceleration, and so that is the acceleration that our bodies are best prepared to survive in. Higher accelerations at first will merely impact comfort, and make humans require more energy and thus more food/water intake in order to move around. Increase the accelerations beyond a certain threshold and crew members who are not physically prepared will struggle to move around at all. Under an acceleration of 2g, your movement becomes as though you have to carry round a duplicate of yourself wherever you go. Going straight from 1g surface acceleration to 2g or more under thrust without proper preparation could have serious medical consequences, and even more so if you've spent a while in the 0.376g environment of the Martian surface. If someone spends a year on Mars and then undergoes accelerations of 2g, they will be suddenly experiencing more than five times the acceleration that they've adjusted to, which is extremely dangerous if it continues for a long time. The higher acceleration would shorten the journey and make this less of a problem, but people returning from Mars under 2g acceleration could expect serious health issues.

[DrunkenKerbalnaut]

1 hour ago, eloquentJane said:

Your phrasing implied that you did, given that this was the first post in response to the author discussing the 1g constant thrust maneuver. I'm sure you can see how it's easy to make the mistake of thinking that you said that.

Sorry to have mislead you, or anyone else. The rest of the conversation after that post should hopefully serve to clarify my reasoning behind the reference. If it remains unclear to any passerby -having read the conversation in full and to this point- my comparison was not on the grounds of constant 1g thrust. The correlation was the use of constant thrust to reach Mars or Duna from Earth or Kerbin. 

EDIT: I've edited my reply to reflect the differences. 

Edited January 22, 2017 by DrunkenKerbalnaut

[SpacedInvader]

I gotta say, this is why I love this community... a relatively simple question of "what is this thing and can KSP do it" is now two pages of discussion of its benefits and feasibility in the real world. 

[DrunkenKerbalnaut]

On January 22, 2017 at 0:55 PM, SpacedInvader said:

I gotta say, this is why I love this community... a relatively simple question of "what is this thing and can KSP do it" is now two pages of discussion of its benefits and feasibility in the real world. 

KSP forums: Where whimsy, intellect, curiosity, and good will crash headlong into each-other at terminal velocity. The Kraken smiles on us, and then everybody has ice cream. 

http://www.demeterclarc.com/wp-content/uploads/images/2014/07/TAURUS1.gif

Have you been able to test your constant thrust flight plan yet?

Edited January 23, 2017 by DrunkenKerbalnaut
Reaction gif. -Frybert Link instead -DK

[SpacedInvader]

2 hours ago, DrunkenKerbalnaut said:

KSP forums: Where whimsy, intellect, curiosity, and good will crash headlong into each-other at terminal velocity. The Kraken smiles on us, and then everybody has ice cream. 

Have you been able to test your constant thrust flight plan yet?

*As long as everybody reads through the entirety of every 500 page thread before asking a question and never under any circumstances asks for an update from a mod author... 

As for my flight plan, I'm still setting up my install... i tend to run something like 200 mods for my careers so setting up the game from a fresh install takes a couple of days. Not to mention I'm going to be playing career mode so I probably won't get to the point where I'm going to try for some time.

[razark]

Thanks all.

I figured it was a crew comfort issue*, but was just wondering if there was any other reason.

I remember reading a Heinlein story involving an emergency torch ship mission to Pluto that used higher accelerations, killing one crewmember and reducing the other to a permanent bed-ridden life.

 

* That's why my question was kept to low levels of 1.5-2g.

[SpacedInvader]

1 minute ago, razark said:

Thanks all.

I figured it was a crew comfort issue*, but was just wondering if there was any other reason.

I remember reading a Heinlein story involving an emergency torch ship mission to Pluto that used higher accelerations, killing one crewmember and reducing the other to a permanent bed-ridden life.

 

* That's why my question was kept to low levels of 1.5-2g.

While I can imagine that there might be reasons to burn at accelerations higher than 1g for specific purposes like emergencies, it is still a considerable amount of thrust and nothing to be sneezed at. For example, at Pluto's furthest point from earth (7.5 billion KM), a ship capable of accelerating for a constant 1g could make the journey in about 2 weeks as opposed to the 10 years it just took New Horizons.

[DrunkenKerbalnaut]

13 minutes ago, SpacedInvader said:

While I can imagine that there might be reasons to burn at accelerations higher than 1g for specific purposes like emergencies, it is still a considerable amount of thrust and nothing to be sneezed at. For example, at Pluto's furthest point from earth (7.5 billion KM), a ship capable of accelerating for a constant 1g could make the journey in about 2 weeks as opposed to the 10 years it just took New Horizons.

Now if only those (lofty) design ideas that carry zero reactant mass were capable of this. I'm thinking solar sail, ram scoop, EM... That sort of thing. 

Didn't New Horizons spend 100s of days thrusting just to escape Earth? 

[mikegarrison]

1 hour ago, HebaruSan said:

Joe Haldeman, The Forever War. [...] The UN turns everyone gay.

Yes, but cloning gives them all a shared mind and they become straight again.

[SpacedInvader]

6 minutes ago, DrunkenKerbalnaut said:

Now if only those (lofty) design ideas that carry zero reactant mass were capable of this. I'm thinking solar sail, ram scoop, EM... That sort of thing. 

Didn't New Horizons spend 100s of days thrusting just to escape Earth? 

According to Wikipedia, New Horizons was the first spacecraft launched directly into a solar escape trajectory.

[DrunkenKerbalnaut]

6 minutes ago, SpacedInvader said:

According to Wikipedia, New Horizons was the first spacecraft launched directly into a solar escape trajectory.

Yeah was just reading the same. Any of you folks happen to know which satellite I'm thinking of?