Jovian System has better colony sites than moon&mars

[marcus]

Originally posted by Cecil
Is the second 3km/s necessary?...

I wonder about this too. The Apollo missions used atmospheric braking on the return and I don't know to what extent thrust played a role, if it did at all. I think they may have approached the Earth essentially at escape velocity and made a dive (at the right angle) thru the atmosphere. This seems pretty extreme, but I don't seem to recall that they went into LEO first or anything.

It seems conceivable to me that humans could do the Jove-Earth leg with the 1 km/s delta-vee to get out and onto an earth-crossing orbit, as you suggest. Just need the right kind of re-entry vehicle and the slowing down can happen in the atmosphere.

So maybe the minimum round trip delta-vee is 3+1+1
a lot less than what we thought when we began discussing it.


That said, suppose the bus that takes people on the J. tour is some big rotating hotel. If you don't spend the fuel to put that in LEO or some orbit when you get back, then that tour-bus is expended.
You can either let the bus go on by and have the people ride back in a little re-entry vehicle. Or you can slow the bus down and store it in LEO---hitch it to a space station or leave it for some future use. So the decisions make the problem more complicated.

I guess I am partial to the simplest form of the problem where you just say what is the minimum delta-vee for a round trip, not attempting to re-use anything.

If some people actually wanted to go live on Callisto, one would not be thinking so much of the problem of getting them home (although return could be provided for). Instead one would probably be thinking of a number of robot-ship one-way trips for machinery and supplies, setting down at some base on Callisto.

Only after a lot of unmanned tonnage had made the trip out there and was waiting on the ground, would you finally let the people travel.
And you might use more delta-vee for them to make the trip faster
(as well as sending them in a rotating cabin to keep them healthy).

But the unmanned supply trips out could be slow and use only the 3+1 km/s which we mentioned----or whatever it is Galileo used---plus whatever is needed to land on Callisto.

It does not seem a heck of a lot different from setting supplies and equipment down on the moon, delta-vee-wise, although the voyage takes a lot longer.

 

[enigma]

Originally posted by marcus
I wonder about this too. The Apollo missions used atmospheric braking on the return and I don't know to what extent thrust played a role, if it did at all. I think they may have approached the Earth essentially at escape velocity and made a dive (at the right angle) thru the atmosphere. This seems pretty extreme, but I don't seem to recall that they went into LEO first or anything.

Apollo didn't do a re-orbit maneuver once they got to Earth. They did a small maneuver to get themselves into the right path for the ballistic re-entry.

They screamed into the atmosphere at over Mach 25.

That's the reason why the half-angle of the Apollo re-entry module was around 45 degrees compared to Gemini's 20. It needed the flow from the front of the heat shield to separate almost immediately so it wouldn't heat up the nadir sides too much.

 

[Janus]

Originally posted by marcus


Who can calculate the payload ratio if you want 10 km/s and are using Tet and Monomethyl. How much of the truck has to be fuel and how much can be payload. It would be nice to know that, I think.

From a quick search on the performance of such engines I come up with a mass ratio of 25. (fully fueled ship vs fueled depleted ship)

I'll have to run some numbers to see if that can be theoretically brought down any by engine design parameters (combustion chamber pressure etc.

 

[enigma]

Originally posted by marcus
Who can calculate the payload ratio if you want 10 km/s and are using Tet and Monomethyl. How much of the truck has to be fuel and how much can be payload. It would be nice to know that, I think.

MMH/N204 has a vacuum Isp of ~333. N204/Hydrazine is slightly better at 340, but has tighter thermal restrictions.

Source:
http://www.astronautix.com

10,000 = - Isp * g_0 * ln \frac{M_0}{M_0 + M_P}

M_P = 19.1 * M_0

Not very good.

 

[marcus]

Originally posted by Janus
From a quick search on the performance of such engines I come up with a mass ratio of 25. (fully fueled ship vs fueled depleted ship)

Janus, this is very welcome input! Both you and Enigma have used the
rocket equation in this thread. Others of us might well be interested in a mini-tutorial showing how to do the payload fraction calculation.

Would you be willing to provide a basic explanation? As I recall one needs only to know the exhaust velocity (from Tet and Monomethyl in this case) and the desired delta-vee.

This set of problems is only gradually getting into focus for me, and perhaps others. I see now that
1.Callisto would be an interesting moon to explore and that
2.even if one imagines eventual manned missions the interesting case to look at, for propulsion requirements, is the one-way unmanned trip, and
3. the experience with Galileo suggests two parts: the initial boost out of low-earth-orbit into transjupiter orbit (or some indirect path involving gravity assists) which could use some less storable fuel, and then a second part for JOI, maneuvering and landing.
--------------
Nereid originally suggested Callisto might be interesting because undifferentiated. Richer chemistry maybe. risk of carbon monoxide?
well let's start by considering unmanned missions
Even if one imagines manned missions might eventually occur they would presumably be preceded by unmanned shipment of equipment and supplies. I am intrigued by the thought that a shipment might be done with something like 3+1+2 where the initial 3 is with non-storable fuel.

In any case, roughly what exhaust velocity should one assume for the fuel and oxidant used in Galileo

 

[marcus]

Well OK, Eric Weisstein's Mathworld
http://scienceworld.wolfram.com/physics/RocketEquation.html
gives it

\Delta v = u*ln(M_0/M)

where u is the exhaust velocity and M_0/M is the ratio
of the initial to the final mass

The exhaust velocity for MMH/N2O4 is about
3100 meters per second.

For JOI and maneuvering in the system suppose one allows
2000 m/s (twice what Galileo apparently got from its main engine)

2000 = 3100*ln(M_0/M)

I get a mass ratio of about 1.9, in that 2 km/s case,

that is, you boost the thing out of LEO and after it does its Jupiter-capture and all its maneuvering it weighs about half of what it did when it left low-earth-orbit.

 

[marcus]

some sources
http://dutlsisa.lr.tudelft.nl/Propulsion/Data/Rocket_motor_data.htm
http://fti.neep.wisc.edu/~jfs/neep533_lect41_chemRkt_99.html

for liquid hydrogen and oxygen the exhaust velocity seems to
be around 4400 meters per second

does anyone have different figures. If that is right for LH2/LOX
and if 3100 is right for MMH/N2O4, then I have to say I am
impressed with the latter pair of chemicals-----storable and still
quite a good exhaust velocity.

 

[enigma]

Originally posted by marcus

where u is the exhaust velocity and M_0/M is the ratio
of the initial to the final mass

Just FYI, you won't find terribly much information giving the exhaust velocities of engines. It is tied up into the specific impulse, Isp.

Isp * sea level acceleration = exhaust velocity.

You will also sometimes see it listed as a characteristic exhaust velocity, 'c'. 'c' is the velocity which the propellant would have if you expand it out the nozzle to an infinite area, which of course, you can't do. It's just the theoretical limit for a specific fuel. If that's all you find, it's usually a decent first cut unless the nozzle is really crappy.

Isp changes with ambient pressure because of the pressure difference between the front of the rocket and the engine outlet. That accounts for 'sea-level' and 'vacuum' Isp.

 

[enigma]

Originally posted by marcus
does anyone have different figures.

Check the astronautix website. They have statistics on practically every rocket, fuel combination, and mission ever built.

 

[marcus]

Originally posted by enigma
Check the astronautix website.

thanks for the link. I went to it earlier, when you gave the link, but was confused by the menu---couldnt see any menu item for propulsion data (may have been staring me in the face, it happens)

the Dutch page http://dutlsisa.lr.tudelft.nl/Propulsion/Data/Rocket_motor_data.htm
seems pretty good though
how does it compare with astronautix?

BTW I got curious about the TU-Delft site and looked further up
in the directory
http://dutlsisa.lr.tudelft.nl/Propulsion/Rocketpropulsion.htm

"In this lecture series the basics of space propulsion are delt with to a level sufficient for selecting the best propulsion system for a given space mission and to perform a preliminary dimensioning and sizing of the system. The lecture series consists of 8 lecture hours of 45 minutes each and forms part of a compulsory lecture series on the basics of space engineering given in the first three years (undergraduate program) of the study for aerospace engineer at TU-Delft, faculty of aerospace engineering."

I guess TU-Delft is the technical university at Delft in the Netherlands. The site has a lot of pages and so far I've only checked out a couple.

 

[enigma]

Originally posted by marcus
the Dutch page http://dutlsisa.lr.tudelft.nl/Propulsion/Data/Rocket_motor_data.htm
seems pretty good though
how does it compare with astronautix?

If you look at the bottom, it cites the astronautix website.

when you get to the main page, look at the links on the right. It has performance characteristics based on rocket/fuel/etc.