"Direct" burn to planet?

[HebaruSan]

2 minutes ago, mikegarrison said:

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

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?

1 minute ago, DrunkenKerbalnaut said:

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

Was it possibly the Indian probe to Mars?

https://en.wikipedia.org/wiki/Mars_Orbiter_Mission

Quote

Several orbit raising operations were conducted from the Spacecraft Control Centre (SCC) at the ISRO Telemetry, Tracking and Command Network (ISTRAC) at Peenya, Bangalore on 6, 7, 8, 10, 12 and 16 November

[...]

On 30 November 2013 at 19:19 UTC, a 23-minute engine firing initiated the transfer of MOM away from Earth orbit and on heliocentric trajectory toward Mars.

Or maybe IKAROS? Wikipedia doesn't describe its ejection trajectory, but it's a solar sail in solar orbit.

https://en.wikipedia.org/wiki/IKAROS

[DrunkenKerbalnaut]

1 minute ago, 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?

Was it possibly the Indian probe to Mars?

https://en.wikipedia.org/wiki/Mars_Orbiter_Mission

Or maybe IKAROS? Wikipedia doesn't describe its ejection trajectory, but it's a solar sail in solar orbit.

https://en.wikipedia.org/wiki/IKAROS

I think MOM was the one I was thinking of. Thank you, most honorable Hebaru. 

[TheEpicSquared]

I've seen many vids where people burn straight at the Mun, and then a few hundred kilometers above its surface, they flip around and perform a suicide burn until they reach the surface. So yeah, I guess it's possible in KSP, at least in the relatively small distances of the Kerbin system. Not sure how well it would work when going to another planet, though. 

[Carl]

42 minutes ago, SpacedInvader said:

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

I'm pretty sure The Voyager probes where on escape trajectories well before that.  In fact they're officially outside the solar system and in interstellar space as of a few years ago AFAIK. One of the Pioneer probes may be on an escape trajectory too, i forget.

[SpacedInvader]

2 minutes ago, Carl said:

I'm pretty sure The Voyager probes where on escape trajectories well before that.  In fact they're officially outside the solar system and in interstellar space as of a few years ago AFAIK. One of the Pioneer probes may be on an escape trajectory too, i forget.

Right, but the Voyagers used several gravity slingshots to achieve their final velocities whereas New Horizons was basically a straight shot out from Earth.

[1101]

8 hours ago, String Witch said:

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

 

The funny thing is, that list contains an even better, even more Kerbal way of doing things, and I'm sort of surprised it isn't in KSPIE:

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Gas_Core--Open_Cycle--Nuclear_Salt_Water

The amazing Nuclear Salt Water Rocket - Originally proposed by Robert Zubrin, with 8,000 Isp.  Though there is a more insane version listed - 479,103s Isp, 3kg/s mass flow, 13MN thrust.

Disadvantage:  Your fuel tanks are literally one leak away from being an enormous nuclear bomb.

 

This makes me want to learn how to mod the game....

[Carl]

Quote

Right, but the Voyagers used several gravity slingshots to achieve their final velocities whereas New Horizons was basically a straight shot out from Earth.

 Like this

Yeah but a trajectory isn't determined by how you get it, it just is.

[SpacedInvader]

19 minutes ago, Carl said:

Yeah but a trajectory isn't determined by how you get it, it just is.

I think the point they are making is that, in this case, how they got the trajectory is the noteworthy fact.

[Guest]

10 hours ago, 1101 said:

The funny thing is, that list contains an even better, even more Kerbal way of doing things, and I'm sort of surprised it isn't in KSPIE:

http://www.projectrho.com/public_html/rocket/enginelist.php#id--Nuclear_Thermal--Gas_Core--Open_Cycle--Nuclear_Salt_Water

The amazing Nuclear Salt Water Rocket - Originally proposed by Robert Zubrin, with 8,000 Isp.  Though there is a more insane version listed - 479,103s Isp, 3kg/s mass flow, 13MN thrust.

Disadvantage:  Your fuel tanks are literally one leak away from being an enormous nuclear bomb.

 

This makes me want to learn how to mod the game....

Mmm I think the SWR is a bit too elegant to call especially Kerbal. I mean, NPP is literally just setting off shaped bombs under your ship; it's as incredibly rudimentary as it is effective.

Edited January 23, 2017 by Guest

[1101]

2 hours ago, String Witch said:

Mmm I think the SWR is a bit too elegant to call especially Kerbal. I mean, NPP is literally just setting off shaped bombs under your ship; it's as incredibly rudimentary as it is effective.

Kerbal Space Program - Where riding through space on a constant pillar of nuclear fire is too elegant.

 

This is a great game.

[Sharpy]

"Dark Drive" from OPT

Thrust 1500kN, ISp 3000s, mass - not sure, quite heavy but not excessive, 2.5m profile, rather long but not excessively so (way shorter than Twin Boar ) LF+Ox.

Interestingly, parameters entirely unaffected by atmosphere.

[Lunar Sea]

On 1/21/2017 at 10:31 PM, SpacedInvader said:

I actually don't remember seeing it in The Martian (I admit I haven't found time to read the book sadly). The two sources I've gotten the idea from are the old tabletop RPG games Battletech, Mechwarrior, and Aerotech 

How is that? Can't say I remember the concept coming up in any of those old FASA game books. 

[KAL 9000]

7 hours ago, 1101 said:

Kerbal Space Program - Where riding through space on a constant pillar of nuclear fire is too elegant.

Mind if I put this in my sig?

[MinimalMinmus]

It remembers me of Tintin: they go to the mun moon with a rocket equipped with a huge "nuclear engine" that seems to provide about 1g of acceleration.

The comic doesn't make it clear if Calculus invented what's basically a huge NERV or an Orion drive.

[Ten Key]

3 hours ago, Lunar Sea said:

How is that? Can't say I remember the concept coming up in any of those old FASA game books. 

 

Quote

Most commonly, a ship performing system transit accelerates toward its destination, usually at 1 G of thrust, for most of the first half of the trip. Shortly before reaching the halfway point, the vessel ceases acceleration and turn (sic) 180 degrees to (sic) that it is traveling stern first. Then it expends approximately 1 G of thrust to decelerate for the second half of the journey. Though this maneuver consumes relatively large amounts of fuel, it provides the fastest means of system transit and allows the crew to operate in the equivalent of Terran gravity for the majority of the voyage. 

-- Aerotech 2 Revised Edition, pg 122

That was harder to find than I thought it'd be. Although writers almost always use continuous thrust in stories set in the BattleTech universe, the technical manuals seem to be a bit more vague on the issue. Almost as if someone did the math and realized how impractical continuous acceleration at 1 G really is.  

[Lunar Sea]

1 hour ago, Ten Key said:

 

That was harder to find than I thought it'd be. Although writers almost always use continuous thrust in stories set in the BattleTech universe, the technical manuals seem to be a bit more vague on the issue. Almost as if someone did the math and realized how impractical continuous acceleration at 1 G really is.  

Ah, AeroTech 2... I didn't get around that far. Not sure why they needed that one concession to realism when they have JumpShips and the like. But we digress!

[SpacedInvader]

4 hours ago, Lunar Sea said:

Ah, AeroTech 2... I didn't get around that far. Not sure why they needed that one concession to realism when they have JumpShips and the like. But we digress!

I would say they went reasonably far trying to square their advanced tech with reality. In the technical readout that covers many of the jumpships, for example, they described how the most popular models were the ones with the largest rotating grav decks to keep the crew comfortable. In many of the supplemental books they also spent time describing how the tech actually worked rather than just having it be magic.

[legoclone09]

On 1/21/2017 at 8:36 PM, SpacedInvader said:

Its always saddened me that the only real reason we managed to put boots on the moon was because we wanted to "show the Reds" rather than really wanting to do it for the furtherance of scientific and technological understanding. That said, considering that we've had nearly 50 years of further development since then and still probably won't have another human on the surface of another orbital body for 20 more years, I can only imagine how far we'd be from that original goal without the cold war.

Well, the majority of humanity's scientific advancement is because of war and being better than other countries. It's how humans are.

[DrunkenKerbalnaut]

3 minutes ago, legoclone09 said:

Well, the majority of humanity's scientific advancement is because of war and being better than other countries. It's how humans are.

It's good to be Kerbal. 

[Kyrt Malthorn]

I mostly seen to hear about constant thrust trajectories with regard to interstellar travel, on which scale star systems move rather little relative to one another over the "short" span of centuries it takes to travel between them.  I guess by the time that's practical, traveling from planet to planet in the same system with that method would be cake.  But economic?  If you're sending people with valuable time and limited quantities of food, then yes, but for many things I expect slowboat intercepts could still be in use, assuming no reactionless drive at which point it doesn't cost any more.

I really wonder just how far we'll get in exploring our solar system before a practical method of FTL becomes available, if one ever does.   

 

[Nathair]

On 2017-01-22 at 2:30 PM, mikegarrison said:

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

Er, not exactly. As Man tells them "I make no distinction between heterosexual play and homosexual."

On 2017-01-22 at 2:51 PM, HebaruSan said:

I love when a sci fi novel reveals the author's wacky ideas for future cultural developments.

1

Then you should read yourself some Samuel R. Delaney.

I don't really think that most of the "wacky ideas", like Haldeman's, are predictions or expectations. They're often more like extended metaphors. The Forever War, for example, is about a war that seems to go on forever while meanwhile, back at home, society changes out of all recognition. Not terribly surprising themes from Haldeman, a Vietnam vet who had trouble reintegrating into civilian life...

Edited January 24, 2017 by Nathair

[Birdco_Space]

On ‎1‎/‎22‎/‎2017 at 9:29 PM, 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? 

Its the Dawn mission to Vesta and Ceres.  It spent 100s of days thrusting and coasting on Ion engines, getting a mars gravity assist.  It was basically just slowly raising its solar orbit until it matched the target asteroid when it arrived, avoiding the need for large deceleration burns

[KerikBalm]

On 1/22/2017 at 9:52 PM, Carl said:

Yeah but a trajectory isn't determined by how you get it, it just is.

well, this is a case where imprecise wording causes problems, and both ways can be interpreted to be correct.

Its also a case where one can consider abstraction, or reality for practical purposes.

From the moment it left Earth's Hill sphere, it was on a trajectory that would lead to solar escape.

The moment it left Earth's Hill sphere, it was not at solar escape velocity.

Its instantaneous velocity relative to the sun would not be an escape trajectory if there was just it and the Sun - doing a single body calculation would show its not going to escape. Doing an N body simulation with Jupiter there would lead to an escape trajectory.

That same trajectory relative to the Earth-Sun, or sun-milky way center... or whatever your preferred point of reference would not be an escape trajectory except at specific times (ie specific launch windows).

One was a direct solar escape trajectory, the other was an indirect escape trajectory that required the probes to enter another hill sphere.

On 1/22/2017 at 6:21 PM, 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?

Constant thrust trajectories with these magnitudes of acceleration require ludicrous amounts of dV. This in turn requires ludicrous Isp's, and this leads to ludicrous power outputs.

So your ship's powerplant is going to have a maximum power output, and a relatively fixed maximum acceleration (assuming fixed Isp, and a low fuel fraction). The mass fraction of the engine/powerplant would be relatively high. The question is then why would you design a ship, lugging around that enormously powerful powerplant, if you're going to spend most of the time only making use of a fraction of its capacity?

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.

Throwing comfort out the window, and accepting high accelerations, it would be most efficient to "front load" the acceleration, but again, for the high dV and low transit times that we are talking about, the engine power output required is enormous.

Often with these torch ships.... if the engine was 99% efficient, and 1% of the output energy was dumped into the ship as waste heat, the ship would vaporize itself in less than a second.

1 G seems to be the Sci-Fi compromise point where they realize that its fast enough to get acceptable travel times, its convenient for explaining shipboard operations and getting around the effects of long term weightlessness, while still seeming (very) remotely plausible.

[HebaruSan]

4 hours ago, Nathair said:

Then you should read yourself some Samuel R. Delaney.

I'll add him to the list, thanks!

4 hours ago, Nathair said:

I don't really think that most of the "wacky ideas", like Haldeman's, are predictions or expectations. They're often more like extended metaphors.

I think of it as mostly branding, and something to give the reader that "we're not in Kansas anymore" feeling.

[DrunkenKerbalnaut]

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.