The Martian and the MMU

[GarrisonChisholm]

Greetings everyone- just a short question here, perhaps due to my being "kerbally informed" regarding EVAs.  I know in the book it was not an issue, but in the movie they decided to add the drama of Mark bridging that final 60-70 meters on the desperate EVA gambit of breaching his suit's glove.  However, the range was only 60-70 meters.  Now certainly, no-one (except Kerbals) wants to go untethered into space, but I have to believe an MMU would have enough dV to reach 70m, change velocity with Watney (12 m/s), and return to the Hermes.  The crew though clearly saw this as a desperate last-gasp venture.  Would it have really been so difficult/perilous?

[magnemoe]

Shutte MMU typicaly had 25 m/s https://en.wikipedia.org/wiki/Manned_Maneuvering_Unit 
Add corrections and intercept and you would run out. This was probably the reason why she used an line, you usually don't use lines with MMU. 
If the MMU had better dV it would be possible. An second astronaut with MMU would also be able to drag them in.
You also has the option to move the ship, you can survive hours in an spacesuit if you don't cut holes in it

[fredinno]

7 minutes ago, magnemoe said:

Shutte MMU typicaly had 25 m/s https://en.wikipedia.org/wiki/Manned_Maneuvering_Unit 
Add corrections and intercept and you would run out. This was probably the reason why she used an line, you usually don't use lines with MMU. 
If the MMU had better dV it would be possible. An second astronaut with MMU would also be able to drag them in.
You also has the option to move the ship, you can survive hours in an spacesuit if you don't cut holes in it

Yeah, I doubt you can't move the "mothership" 70m/s and wait a few hours.

[sevenperforce]

It seemed to be suggested that the Hermes lacked any further maneuvering fuel and would not be able to successfully return to Earth if it burned any more propellant.

[GarrisonChisholm]

In the movie that was my recollection as well, though in the book not using the ship's main drive was due to the long span of time it would take to decel 40 m/s.  Well, I wasn't aware our kerbals were blessed with such a miraculous maneuvering unit!  Vastly more capable than its real-world counterpart it seems.

 

Thank you for chiming in with the answers I sought.

 

[cubinator]

Yeah, the ship would have taken way too long to do the burn using "conventional" propulsion systems, so they blew the airlock instead. It had enough to slow down enough and get in reasonable range, but they still had to reel him in because it would be impossible for the pod to magically dock by itself.

[sgt_flyer]

4 hours ago, GarrisonChisholm said:

In the movie that was my recollection as well, though in the book not using the ship's main drive was due to the long span of time it would take to decel 40 m/s.  Well, I wasn't aware our kerbals were blessed with such a miraculous maneuvering unit!  Vastly more capable than its real-world counterpart it seems.

 

Thank you for chiming in with the answers I sought.

 

i think the kerbal's MMU RCS ISP is way much higher than real world ones - MMU's use nitrogen cold gas thrusters - with a miserable ISP of around 73s ! (but it allows them to go without risking contamination of the science equipments they fly around ;)) contrary to KSP, real RCS exhausts don't magically pass through anything in their path ^^

 

[kerbiloid]

Tried to measure Kerbal suit engine params.

Hyperedited a kerbal onto a high circular orbit (surface speed = 2513 m/s), positioned him "face to the Kerbin, head to the North" and burned out all his EVA propellant, keeping the RCS engaged - in every direction sequentially (W,A,S,D,Shift,Ctrl).

Average burn time ≈ 200 s.

	Surface velocity		dV calculated
Direction	Initial	Final	formula	value
from Kerbin	2513	2565	sqrt(abs(25652-25132))	514
to Kerbin	2513	2617	sqrt(abs(26172-25132))	730
to North	2513	2593	sqrt(abs(25932-25132))	639
to South	2513	2593	sqrt(abs(25932-25132))	639
retrograde	2513	1878	abs(1878-2513)	635
prograde	2513	flown away from Kerbin
Average				631

I.e. dV of a kerbal suit is ≈ 630 m/s

Acceleration = 630/200 = 3.15 m/s².

Kerbal+suit mass = 95 kg.

Thrust force = 3.15 * 95 = 300 N.

EVA propellant amount = 5 (units)
EVA propellant density (according to Community Resource Pack definition) = 0.

Say, density is ~1 kg/l. So, propellant amount ≈ 5 kg.

630 = Vexhaust * ln((95 + 5)/95);

Vexhaust = 630 / ln((95 + 5)/95) ≈ 12280 m/s;
ISP = Vexhaust / 9.81 = 1250 s.

Power = 300 N * 12280 m/s = 3.7 MW.

I.e. Kerbal EVA backpack contains a tiny gas core reactor, with zero heat waste, producing ISP at least 4 times greater than the best of the Kerbal rocket engines.

So, an orbital galley equipped with a team/pack of Kerbal paddlers looks very useful. 
Or at least a “rickshaw” class orbital thug, as only Kerbal can be currently active at once.

[sgt_flyer]

actually, KSP's monoprop density has been calculated as 4kg /l  - if ksp's units are indeed liters   - still, even if ksp's units are not liters, you would end up with 20kg of monopropellant

with this weight, you end up with an isp of 336s (if the 20kg are in addition of the 95kg of the kerbal)

besides, what is the kerbal's weight before and after expending it's propellant ?

unless, of course, if the kerbals modify the monoprop fuel into eva fuel before putting it in their suits.

Edited February 12, 2016 by sgt_flyer

[kerbiloid]

20 minutes ago, sgt_flyer said:

actually, KSP's monoprop density has been calculated as 4kg /l  - if ksp's units are indeed liters  

http://forum.kerbalspaceprogram.com/index.php?/topic/25965-fuel-density-ksp-vs-science/

besides, what is the kerbal's weight before and after expending it's propellant ?

Not sure exactly aboy EVA propellant units, but CommunityResourcePack uses 0.001 as Water density, and this is "tons per unit" afaik.
IIRC the CRP discussion they agreed to use 1 l as a unit for most of resources. Of course, I can mistake.

So, if 4 kg/l, the suit contains 20 kg of propellant.

Not sure at the moment, is "95+20" or "75+20=95", but in this case the exhaust velocity is ~3000 m/s, i.e. more clear.

Less clear: if the liquid is 4 kg/l dense, it's a not a "burnable" fuel, it's a molten metal or a heavy organic substance.
So, to achieve 3000 m/s its temperature would be ~ 3000²*(0.03..0.1)/(3*8.3144) ~=10000..35000 kK.

Ergo, it's a tiny gas core reactor using a vapourized titanium.
 

Edited February 12, 2016 by kerbiloid

[sgt_flyer]

just tested, given the kerbal's weight do not change, either the backpack is not taken into the calculations, (so 95kg + 20kg) or the EVA fuel does not weight anything - which would completely throw off any calculations

still regarding densities, we're on kerbin - a planet largely smaller than earth - yet with the same surface gravity

Edited February 12, 2016 by sgt_flyer

[kerbiloid]

Not "do not change", but "its change is under the measurable limit".
So, probably it's an antimatter reactor (with amount in micrograms or so).

(Or even an ectoplasm)

Edited February 12, 2016 by kerbiloid

[sgt_flyer]

that, or they can transfer the fuel through a wormhole - in that case, why have a 5 units limit ^^

Edited February 12, 2016 by sgt_flyer

[kerbiloid]

Maybe they use Mark Watney's way: a set of holes - not only in a glove, but in different places, patched with a scotch tape.
So, the suit RCS is omnidirectional.

I'm stupid: it's a mistery goo. It's produced by Kerbals.

Edited February 12, 2016 by kerbiloid

[fredinno]

7 hours ago, kerbiloid said:

Maybe they use Mark Watney's way: a set of holes - not only in a glove, but in different places, patched with a scotch tape.
So, the suit RCS is omnidirectional.

I'm stupid: it's a mistery goo. It's produced by Kerbals.

Mystery goo is actually coconut cream pie.