The VASMIR/"The Martian" tactic to getting to Duna or other planets

[davidy12]

I just re-read the Martian, and read about the VASMIR engine and I'm curious. Is there a way to simulate getting to Duna or other planets without doing a simple Trans-Dunan Injection burn? The Hermes got to Mars by accelerating constantly and I wanna know, has any one ever done that in KSP? Thank you.

[mikegarrison]

I just re-read the Martian, and read about the VASMIR engine and I'm curious. Is there a way to simulate getting to Duna or other planets without doing a simple Trans-Dunan Injection burn? The Hermes got to Mars by accelerating constantly and I wanna know, has any one ever done that in KSP? Thank you.

You want to go to Duna without timewarping? Go ahead. Come back in 2016 and tell us what it was like.

(That being said, there is a mod that allows "persistent thrust" while timewarping. It is intended exactly for this purpose, to allow modeling a long, low-thrust, sustained acceleration.)

[davidy12]

Exactly what I was talking about. But, has anyone done that in KSP?

[Guest]

I guess it´ll be hard stock and without cheats, because the amount of Delta-v you need is

INCREDIBLE HIGH.

Think that you already burn away whole orange tanks of fuel in a few minutes to accelerate at a reasonable rate for these minutes, and just think what you would need to burn all the way to Duna... However, with infinite fuel it should be possible. Scott Manley made a nice (and popular)

about reaching lightspeed, maybe this helps you a bit.

[mikegarrison]

I guess it´ll be hard stock and without cheats, because the amount of Delta-v you need is

INCREDIBLE HIGH.

Think that you already burn away whole orange tanks of fuel in a few minutes to accelerate at a reasonable rate for these minutes, and just think what you would need to burn all the way to Duna... However, with infinite fuel it should be possible. Scott Manley made a nice (and popular)

about reaching lightspeed, maybe this helps you a bit.

Apparently you have never used the ion thruster.

[problemecium]

Not to nitpick but: VASIMR, not "VASMIR." It doesn't roll off the tongue as easily, but the "SI" stands for "Specific Impulse."

[wumpus]

Apparently you have never used the ion thruster.

Neither the KSP nor real life working/tested ion thrusters provide the "INCREDIBLE HIGH" delta-v, at least once coupled with multi-passenger crew compartments. The other issue is that launchable/powerable ion thrusters would take more than 250 days to provide "incredible high" delta-v.

VASIMR could well provide the "HIGH" delta-v needed to get to Mars on a realistic schedule ~250 days. There is already another thread ridiculing the idea of using VASIMR for the high speed (can't find the difference between delta-V for the 39 day vs. 250 day, but it's a dozy) path. It isn't just that VASIMR isn't fully developed, it is that you would need a space 200kW capable nuclear reactor (all the ISS solar panels only cover about 10% of what is needed for the VASIMR that was planned for deployment) but the bigger issue is just how are you going to make a 200kW heat sink in space?

[Terwin]

The Near future mod family has a VASMIR engine, it takes 2K charge/s to power, but you can go from ~34kn to ~200kn using LH2 if I remember correctly (the ~34 kn setting has an ISP of 16K)

I use one powered by on on-board nuclear reactor to power my outer-plants Grand Tour ship(I shut it down between burns, takes about 25 min to get to full power and will run for 5 years at full power before needing a refuel)

I am also using one to help my inner plants Grand Tour vessel get back from Moho(acceleration is a paltry 0.22m/s/s so it will take a while...)

[mikegarrison]

Neither the KSP nor real life working/tested ion thrusters provide the "INCREDIBLE HIGH" delta-v, at least once coupled with multi-passenger crew compartments. The other issue is that launchable/powerable ion thrusters would take more than 250 days to provide "incredible high" delta-v.

VASIMR could well provide the "HIGH" delta-v needed to get to Mars on a realistic schedule ~250 days. There is already another thread ridiculing the idea of using VASIMR for the high speed (can't find the difference between delta-V for the 39 day vs. 250 day, but it's a dozy) path. It isn't just that VASIMR isn't fully developed, it is that you would need a space 200kW capable nuclear reactor (all the ISS solar panels only cover about 10% of what is needed for the VASIMR that was planned for deployment) but the bigger issue is just how are you going to make a 200kW heat sink in space?

Yes, there are technical challenges. But that wasn't the point (especially in the context of KSP).

The point is that "incredibly high" delta-v is actually attainable if you also have very high ISP. The tradeoff for very high ISP is usually very low thrust. But if you integrate low thrust over a long time, you achieve high speed. Basically you thrust halfway to Mars/Duna, then retrothrust the other half of the way. If your ISP is high enough, this works.

But this mechanism is basically unavailable in KSP because nobody wants to take months of real time to run a mission. EXCEPT that, like I mentioned, there is a mod that allows "persistent thrust" during timewarp, which at least in theory would allow this kind of mission in KSP.

[Motokid600]

Anything below a TWR of .5 for me in KSP is just awful.

How did the Hermes break from Earth's gravity? How did it brake into Martian orbit with such an abysmally low TWR?

I mean I know how.... Dozens of passes gradually raising the apoapsis until you break orbit. And braking into a hyperbolic orbit around Mars and do the opposite.

That's great and all for realism, but that sounds like a massive pain in KSP.

And then emulating a craft that thrusts for the entire duration of the trip... That's impossible. Unless you want the game to run for months at a time. So unfortunately doing exactly what the Hermes does in ksp is completely impractical.

[Laie]

How did the Hermes break from Earth's gravity? How did it brake into Martian orbit with such an abysmally low TWR?

a) as I understand the plot summary, it was still on it's way back to earth and could do a slingshot for another transfer to Mars.

the point of the VASIMR is that you can have high thrust/low isp or low thrust/high isp or anything in-between, as needed.

Now that slingshot maneuver has written "PLOT DEVICE" all over it, but it's not impossible, only quite incredible. Capture at Mars can be as cheap as 700m/s, another 1500 for low circular orbit; capture at earth and circularizing in LEO would be more like 4000m/s. This means that a vessel that's returning to earth with a 10% safety margin has just enough fuel to circularize in low Mars orbit and head back to Earth.

Apparently a supply vessel with snacks is brought up during the slingshot around earth. Which might conceivably also carry fuel, and it better should, as the above calculations only deal with a best-case scenario, and leave out the question of how they're supposed to stop at Earth when they eventually come back.

[monstah]

From what I remember, fuel was never an issue; Hermes was built to be multi-mission, and this was mission 3 of 5 planned. The thing was time: the maneuver got Hermes back at Mars before Watney died; if they had missed it, it would be too long to get there by the usual route.

[Torquemadus]

The Hermes as depicted in The Martian seems to have been conceived entirely as a plot device.

It takes six months to send crewed spacecraft to Mars using existing chemical propulsion. Cargo can be sent on lower energy trajectories such as an eight-and-a-half month Hohmann transfer. Using VASIMR thrusters and huge nuclear reactors drastically increases the mass, development time, flight time, and cost of the mission, while minimising the time spent actually exploring Mars.

Current long duration mission plans call for a six month type one fast ballistic transfer to Mars, with a year and a half spent exploring the surface, and a six month transfer back to Earth. Martian resources are leveraged to provide propellants for the return flight, fuels for surface vehicles, and life support consumables. Assets are concentrated on the Martian surface to establish a permanent base.

By contrast, the mission depicted in The Martian spends a few weeks (at best) on Mars and provides the astronauts with negligible mobility and power, reducing their mission to an effectively worthless "flags and footprints" exercise. The base they establish during their short stay is soon abandoned, never to be used again. By employing "MacGyver" techniques, Mark Watney is able to use the technology and resources available to him to survive for a year and a half. In other words, he is able to survive a standard length Mars mission by adapting inadequate hardware and supplies to perform a predictably more capable long-duration mission that should have been designed that way all along...!

In the book, NASA admit that Mark Watney has provided a far better mission and far better science return than could possibly have been provided with their existing programs.

[The Yellow Dart]

By contrast, the mission depicted in The Martian spends a few weeks (at best) on Mars and provides the astronauts with negligible mobility and power, reducing their mission to an effectively worthless "flags and footprints" exercise.

I'd hardly call 30 days on Mars worthless, and suggesting that a single man's last ditch 3200km drive across Mars with no backup or contingency plans be the "standard" that NASA should shoot for seems silly. Also, the 30 day time frame is based on the fact that the less time they spend after arriving, the less DV is required for return, and the shorter the trip is. That part isn't just a plot tool, but a side effect of a nuclear/VASIMR powered Mars mission.

[davidy12]

Okay so there is no way to imitate "Mars in 39 days" in KSP with ion thrusters or VASMIR?

[xebx]

Very high delta-V ? Ion Engine clusters for Mars manoeuvres and coming back + chemical for the first part and the lander are the best way to manage safe realistic Mars mission.

Ion and chemical exist and perform very well, nuke don't, it's outdated, overheats (2000K running !?), heavyweight, dirty as hell, boring, why losing our time and money on it ?

The real improvement are in life support equipment and storage (water/air recycling, etc) in this kind of mission.

@davidy12 : When KSP will allow stable time warp for low twr long time propulsion, it will be easier to simulate 'fast travel between planets'.

Note : for example in KSP my Ion manned racer for Eeloo (solar panels / fuel cells ~ 2 years total to and back) have 12000 m/s delta-V for the ion stage with 1h30 burn total duration. (3x30 min 'forum friendly' : from Eeloo capture to low elliptical orbit + manoeuvres + from low Eeloo orbit to ejection + Kerbin capture to low orbit; add to this another 12000 m/s delta-V from the LFO aspargus stage, total 24000 m/s manned rocket stock)

Edited September 18, 2015 by xebx

[DBowman]

Anything below a TWR of .5 for me in KSP is just awful.

How did the Hermes break from Earth's gravity? How did it brake into Martian orbit with such an abysmally low TWR?

I mean I know how.... Dozens of passes gradually raising the apoapsis until you break orbit. And braking into a hyperbolic orbit around Mars and do the opposite.

Hermes aerocaptures at Mars (and Earth on a regular return home). The film version must do what you say though as it sure doesn't look aerocapture capable.

[TheAurora]

I just finished a mission to duna with ion engines in SKY. All four payloads were sent with ion tugs, and the crew rode in a Hermes like transfer vehicle. Each payload had about 16 to 20km/s Delta V. I didn't do it, but with my experience, you could do a 6km/s boost there to get there very fast and 6km/s to stop. You would need to dock with a fuel module for a similar quick trip back. With this though, the mission time is still limited by the window back, but there may be some funky maneuver to come back I don't know about. In conclusion, a ridiculously fast time there is plausible, just with patience and 4x warp. I couldn't do it without you 4x warp!

[selfish_meme]

The real question is why does hermes ever go into orbit, its more like a cycler than a transfer vehicle

- - - Updated - - -

I guess because of emergencies like the one that happened

[Norcalplanner]

There's a video where the author shows the exact path the Hermes takes, although I agree that the whole "breaking earth orbit" part is a bit hand-wavy.

Here it is. The interesting part starts at 12 minutes in.

[selfish_meme]

There's a video where the author shows the exact path the Hermes takes, although I agree that the whole "breaking earth orbit" part is a bit hand-wavy.

Here it is. The interesting part starts at 12 minutes in.

He actually wrote a program to calculate it

[Norcalplanner]

So I just did some back of the envelope calcs. Delta V for a Mars transfer burn is 4.3 km/sec, assuming a single transfer burn using the Oberth effect. The stated acceleration of the Hermes is 2 mm/sec, which translates into 172.8 m/s per full day of thrusting. It would take 24.8 days of thrusting in order to get 4.3 km/sec of delta V.

Of course, that assumes that the 24.8 days of thrusting would be broken up into hundreds or thousands of periapsis kicks, assuming that the system can reliably cycle on and off that many times. Otherwise, you're looking at a slow orbital spiral outward that loses any Oberth effect boost and will require even more delta V.

When I read the book, they don't really talk about circling around the earth for more than a month before actually setting off. I like the author's program showing the trajectories for the interplanetary parts of the trip, but I'd really appreciate seeing better detail of what happens near the Earth and near Mars.

Then again, it may be a bit "hand-wavy" (the author's term) because glossing over it serves the story. The example that the author gives of "hand-waviness" is the original storm that causes them to abort. With Mars' atmosphere being so thin, he readily acknowledges that there's no way it would have been strong enough to tilt over the MAV or cause a lot of the other damage that occurs. But it sure makes a good setup for the rest of the story.

[mikegarrison]

So I just did some back of the envelope calcs. Delta V for a Mars transfer burn is 4.3 km/sec, assuming a single transfer burn using the Oberth effect. The stated acceleration of the Hermes is 2 mm/sec, which translates into 172.8 m/s per full day of thrusting. It would take 24.8 days of thrusting in order to get 4.3 km/sec of delta V.

Of course, that assumes that the 24.8 days of thrusting would be broken up into hundreds or thousands of periapsis kicks, assuming that the system can reliably cycle on and off that many times. Otherwise, you're looking at a slow orbital spiral outward that loses any Oberth effect boost and will require even more delta V.

When I read the book, they don't really talk about circling around the earth for more than a month before actually setting off. I like the author's program showing the trajectories for the interplanetary parts of the trip, but I'd really appreciate seeing better detail of what happens near the Earth and near Mars.

Then again, it may be a bit "hand-wavy" (the author's term) because glossing over it serves the story. The example that the author gives of "hand-waviness" is the original storm that causes them to abort. With Mars' atmosphere being so thin, he readily acknowledges that there's no way it would have been strong enough to tilt over the MAV or cause a lot of the other damage that occurs. But it sure makes a good setup for the rest of the story.

I think you are missing something. You get the Oberth effect because you are moving faster at your periapsis than at your apoapsis. But in a long sustained burn, you get the same kind of effect as your burn continues, because you get faster and faster and faster. It would still be best to add all that velocity instantaneously. But given that you can't, at least you do get some compensation for losing your gravitational Oberth effect by gaining a sustained thrust effect.

Edited September 18, 2015 by mikegarrison

[Norcalplanner]

Found another video with just the paths taken by the Hermes. This is the distilled essence of the earlier video:

http://www.galactanet.com/martian/hermes.mp4

Edited September 18, 2015 by Norcalplanner

[Streetwind]

Okay so there is no way to imitate "Mars in 39 days" in KSP with ion thrusters or VASMIR?

You can do it in KSP just fine, thanks to the absurdly overpowered ion drives and tiny solar system with tiny dV costs. Flying from low Kerbin orbit to Duna in 39 Earth days (156 Kerbin days) requires only ~1130 m/s (about 50 more than a Hohmann transfer). Doing it in 39 Kerbin days requires 5,150 m/s, assuming you aerobrake, and that's possible (if inefficient) even with a single-stage chemical rocket. With nukes or ion drives, it's effortless. Of course, that kind of aerocapture will probably incinerate your craft, so let's do it without: 11,900 m/s. That barely still works with nukes, and would be quite comfortable with a two-stage rocket (one chemical, one nuclear). Ion drives could still do it effortlessly though. Add in mods like Near Future, offering advanced electric drives and power sources to go with them, and that kind of dV would be considered "low-ish"

It's IRL that it's completely impossible, VASIMR yes or no. There is no power source existing today that's even within an order of magnitude of the power/weight ratio required to pull off such a feat - and nothing in that direction is being actively developed either.