As is often said here and in XKCD, an orbit isn't merely high up... it's also going fast. The first two American astronauts -- Alan Shepard and Gus Grissom -- did indeed get "high up" enough to qualify as being in space (well above the Karman line). But their flights were "sub-orbital", and they fell back down before making it all the way around the Earth. Let's see if they were "going fast."

  1. What was the maximum speed each achieved by Shepard and Grissom? (You may use whatever units are convenient.)

  2. What speed would they have needed to acheive orbit?

  3. In percent, how did their maximum speeds compare to orbital velocity?

XKCD not fast enough for space

For further reading:


1 Answer 1


The Post Flight Mission Report for Grissom's flight compares key parameters for the two manned suborbital missions (Shepard's MR-3 and Grissom's MR-4):

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The corresponding document for John Glenn's orbital flight gives its orbital velocity.

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Assuming the Glenn number is inertial velocity, the percentages are 28.7% and 29.5%.

In metric figures, MR-3 reached 2252 m/s, MR-4 2310 m/s, and Glenn's MA-6 7842 m/s.

  • 3
    $\begingroup$ I wanted to add the metric figures (2252, 2310, and 7842 m/s) but couldn't think of a non-intrusive way to do it. $\endgroup$ May 2, 2019 at 1:04
  • 1
    $\begingroup$ Nice edit. Add a paragraph at the end w/ the other numbers if you like. $\endgroup$ May 2, 2019 at 1:08
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    $\begingroup$ I find it interesting that what we'd call "ascent" they called "exit". $\endgroup$ May 2, 2019 at 1:16
  • 3
    $\begingroup$ So if this were KSP, both Shepard and Grissom would've reached orbit... $\endgroup$
    – Vikki
    May 2, 2019 at 3:08
  • 2
    $\begingroup$ Velocity in which direction? perpendicular to the Earth? in line with the centre of the rocket? $\endgroup$
    – Baldrickk
    May 2, 2019 at 9:26

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