Space Travel in “Waldo”

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?

• Assuming you have sufficient Delta-V, you could reach orbit at 1mph, it would just take you a while. – Valorum Jan 11 '15 at 19:21
• 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 Jan 11 '15 at 19:25
• Orbital height, yes, orbital speed, no; what-if.xkcd.com/58 – Valorum Jan 11 '15 at 19:32
• 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 Jan 12 '15 at 4:18
• @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 Jan 12 '15 at 15:42