Sunday, June 30, 2013

Even MIT students can make misteaks.

Finding Delta V between various places in the solar system has been one of my hobbies. I like to learn as well as share what I know about this topic. So I occasionally Google search strings that include the term "Delta V".

Googling: Delta V Mars. The first hit is a Wikipedia article, the second hit is How Much Delta V do you need to get to Mars - Yahoo! Answers.

A fellow who calls himself ronwizfr confidently asserts that the delta V from earth orbit to Mars transfer is 6.6 km/s. Ronwizfr cites an MIT student project for a class called Solving Complex Problems.

The MIT page cited is a well done presentation until the students get to constants:

Then the students made an arithmetic error. The distance from the earth to the sun is 1.49X1011 meters, not 1.49X1010 meters. Their radius for Mars' orbit is also off by a factor of 10.

The MIT students correctly plug in these wrong quantities to get speeds that are off by sqrt(10):

The velocity of the earth is not 94,384 m/s but closer to 29,846 m/s. Likewise velocity of Mars isn't 74,467 m/s but closer to 23,548 m/s. Their quantities would have been correct if the earth and Mars were .1 A.U. and .152 A.U. from the sun.

Even the very accomplished can make errors. So we should examine every assertion, no matter the source. My dad used to tell me we all put our pants on one leg at a time. His way of saying we're all human and capable of making mistakes.


Tage said...

Hi Mr. David,

It seems you didn't do any arithmetic errors in this blog post either, so I'll just mention that when you wrote "if the earth and moon were .1 A.U. and .152 A.U. from the sun", you probably meant "Mars" and not "moon". Excellent post either way, though. :)

The last days I have spent several hours going through the calculations that you did in your Hohmann Excel spreadsheet, planning to use them for my astrophysics project on Mars colonization. When going through your spreadsheet I assumed (by the quality of the spreadsheet) that you were working for NASA or something, but you are refering to delta V calculations as a hobby? Are you completely self-taught? No academic background in natural sciences? I think you've done a very impressive piece of work either way.

Best regards,
Tage Augustson
MSc of Mechanical Engineering

Hollister David said...

Tage, thanks for the heads up. Made the correction.

I'm strictly amateur. My credentials: I graduated in the top 60% of my high school class.

I am passionate about space exploration. Maybe we are on the cusp of opening a new frontier. If so, that'd be a major turning point in human history.