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Before the "Other World" business starts up Waldo is a hard sf story. I enjoyed reading the description of Waldo's free-fall habitat; given the lack of the computerization, I think it still holds up.

Heinlein was noted for doing his homework in the settings of his stories. For example, the orbital parameters of Waldo's home are consistent with physics. But what are we to make of this discussion about which spacecraft to take up to "Wheelchair"?

I'll have you there in three shakes. That tub of yours probably won't do over five hundred, and Wheelchair must be all of twenty-five thousand miles up.

What can the 500 refer to? The discussion is in English units; surely Heinlein didn't think a craft capable of 500 mph could make it into orbit?

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    Assuming you have sufficient Delta-V, you could reach orbit at 1mph, it would just take you a while.
    – Valorum
    Commented Jan 11, 2015 at 19:21
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    I disagree with you. You could attain any altitude, but to be in a particular orbit requires reaching a certain velocity. For a near-geosynchronous orbit like Wheelchair's it is about 3 km/sec.
    – Organic Marble
    Commented Jan 11, 2015 at 19:25
  • Orbital height, yes, orbital speed, no; what-if.xkcd.com/58
    – Valorum
    Commented Jan 11, 2015 at 19:32
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    It could well be 500MPH. Don't confuse acceleration needed to escape Earth's gravity with sustained or initial speed. About 25,000 mph is needed if you want to shoot into space from ground, with all the acceleration at the beginning, like a bullet being shot from a gun. But if you can maintain constant lift against 9.8 m/s^2, you will get to space.. Eventually, even at 1mph, as Richard points out. True, you would need an insane amount of fuel, but that's another issue.
    – K-H-W
    Commented Jan 12, 2015 at 4:18
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    @OrganicMarble - Well, that makes sense, but again, we are getting into max velocity v.s. acceleration; once out of the atmosphere, the max velocity no longer applies; eventually, you can accelerate up to any speed you like, assuming your fuel holds out. (Relatavistic issues aside.)
    – K-H-W
    Commented Jan 12, 2015 at 15:42

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The 500 is likely meant to be 500 MPH. The "space craft" are just regular flying cars in Waldo and are primarily used to travel across the Earth's Surface. On the ground and in atmosphere a vehicle's ~speed~ is typically measured in Maximum Velocity attainable. But once a vehicle gets outside of the atmosphere velocity is relative and Max Velocity is not a term that makes much sense (particularly for vehicles with unlimited fuel/energy such as those in Waldo). So, once the "Tub" left the atmosphere, it could presumably accelerate up to any necessary velocity.

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  • I don't think the text supports your interpretation. Why would they use a speed that, according to your interpretation, does not apply, when discussing how long it takes to get somewhere?
    – Organic Marble
    Commented Jul 27, 2016 at 3:47
  • Because that is what they are used to. When they went to buy their cars, the big important number given is maximum velocity, when they plan a trip the number that matters is maximum velocity. It is implied that Stevens (the person you are quoting) had never been to space. He is used to dealing with maximum velocities, and he is likely using that difference to point out the difference in acceleration as well.
    – Jonathon
    Commented Jul 27, 2016 at 4:00

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