>even if you managed to launch at the speed of 25000km/h from the ground, the payload would slow down before it left the atmosphere. Project HARP was shooting

>even if you managed to launch at the speed of 25000km/h from the ground, the payload would slow down before it left the atmosphere. Project HARP was shooting 16 inch artillery shells at 2100m/s, and they reached 180km altitude, i.e. the atmosphere was slowing it down only for 15%. While higher speeds cause higher losses, the larger payloads would benefit from decreasing surface-to-volume ratio, plus if one launches from say a plateau in Andes where atmosphere is aleady thinner, I think the losses will be similar for large payloads at speeds close to orbital.