Flight Question Mars trip and encounter speed

[Jason_25]

I am about to launch a fast manned mission to mars and am still coming up short of fuel in my calculations. I want to do a fairly straight trip using about 10km/s dv. This will be from lunar orbit so it will be pretty fast. I am leaving around December 11 this year so the planets are reasonably close.

This page: http://www-istp.gsfc.nasa.gov/stargaze/StarFAQ10.htm#q169 says on a non-hohmann trip to Mars from Earth the spacecraft will need to cancel 30km/s speed on arrival. When I launched an unmanned probe at about 7km/s dv from lunar orbit I found that to be seemingly true before I even read the article.

There are a couple of questions raised then. Is there any way to avoid having to slow down by 30km/s? Why is the 30km/s speed present at all when the average orbital speed difference between the two planets is less than 6km/s?

I want to clarify that with transx I can approach Mars very close at these speeds, but I am just moving way too fast. Aerobraking I have thought of but that is a last resort.

This topic is very confusing and is one of my last problems before I cast off. Thanks for any help.

I might add that the nature of the topic of this forum is such that each post must be well though out before posting. The 3 minute log out or whatever it is has surely left many people thinking they have lost a long winded post so that default needs to be changed.

 

[Mister Mxyzptlk]

Why are you leaving from lunar orbit?

 

[Jason_25]

I have 2 orions docked together along with alot of extra landers, fuel, oxygen and supplies. I have been building it all there for a long time now. As to why, it seems like radiation and EMP would be less of a concern all the way out there and I though it would be more fun too.

Maybe I will launch a probe from earth orbit instead around the same time and see what my encounter speed is like.

 

[Grover]

You can slow down, if you have the fuel, just turn almost retrograde (about 160 on the orbit HUD, but.it.depends on orbital parameters) and experiment with yaw until you can burn without affecting PeA

 

[Jason_25]

Thanks for that tip.

I tried a probe using 7.5km/s dv from Earth instead and got an encounter speed of around 20km/s. That mostly makes sense if you add the orbital speed difference between planets, the speed to get there, and gravity from mars speeding you up towards the end of the trip. I am hoping that the last probe speed was just a fluke.

At the beginning, I have about 70km/s dv to work with so I will start out with a 10km/s dv burn and have 2.5km/s dv for course corrections. That should make the trip take 2.5 to 3 months. Longer than the one month I originally wanted to get there but any faster and I would be cutting it too close on fuel. Very sobering.

A worst case scenario slow down should have me using 25km/s dv. I am unsure about exactly how much fuel I will use on the return trip but over 30km/s dv seems like enough. Especially given that among other docked vehicles I have an XR-5 with 15+ km/s dv on board. Though, I really want to avoid having to escape in the XR-5 leaving all that equipment in a solar orbit. I plan on staying for a month or less.

If anyone has any last-minute suggestions or advice that would allow me to get my crew home safely do let me know.

EDIT: the answer to the riddle in your signature is probably the horizon.

 

[Tommy]

I might add that the nature of the topic of this forum is such that each post must be well though out before posting. The 3 minute log out or whatever it is has surely left many people thinking they have lost a long winded post so that default needs to be changed.

The answer to this is to make sure that the "remember me" box is checked - this eliminates the time-out and you can stay logged in for days. It does NOT make your browser remember your password, and if you use the log-out link at the bottom of any page the forum removes ALL cookies it placed - making it safe for a public terminal.

Why is the 30km/s speed present at all when the average orbital speed difference between the two planets is less than 6km/s?

The simple, not qute complete, answer is that on a fast trajectory you are crossing the target's orbital path at a very high angle - close to perpendicular. This means that the target's orbital velocity is also relative velocity, as is your own velocity.