Question about The Mercury Spacecraft

I just had a silly question about our first manned space missions. In the suborbital launches of the first two Mercury spacecraft using the Redstone could igniting the retro rocket pack after booster separation have propelled it into orbit? I know we have a lot of space geeks who are experts on space subjects.

As a general rule of thumb, if something isn’t designed to go into orbit, it can’t. Going into orbit is really, really hard.

The Redstone used for the suborbital launches had 78,000 pounds of thrust. The Atlas-D used for the orbital flights delivered 300,000 lbs.

Here’s a photo of technicians installing the retropack (for Godron Cooper’s launch, the final one.) As you can see, it’s pretty small, probably no more than 24" high. I don’t know much about rocket science, but I can’t believe the retropack could possibly produce anywhere near enough oomph to work as a second stage.

One way to consider this is in terms of how much change in velocity (or “delta V”) is necessary to reach orbit and then deorbit. The Mercury retrorocket pack produced a delta V of 170 meters/second. In comparison, the orbital velocity of the Mercury-Atlas missions was around 7800 meters/second, while the Mercury-Redstone missions had a peak velocity around 2300 meters/second. The retrorockets were nowhere near capable of putting the Mercury-Redstone capsule into orbit.

Deorbiting is simply a lot easier since you only need to descend into the atmosphere, which does the rest of the work.

(numbers from Wiki pages on various Mercury-Redstone and Mercury-Atlas missions)

The Mercury-Redstone missions were planned with trajectories that were specifically suborbital; that is, that they would fly in an ellipse with a ballistic track after burnout that reintersects the Earth downrange, with a total range of around 260 nmi and an apogee of just more 100 nmi. While extra thrust could be used to circularize the trajectory into a stable orbit, the return retrorocket packages had nowhere near enough thrust make up the impluse from the Redstone to an orbital trajectory. As others have noted, the impulse needed to return from low Earth orbit is a small fraction of that required to achieve orbit; basically, negating enough of the tangential (orbital) velocity such that the spacecraft plows through the atmosphere and is subject to controlled drag that further reduces its speed until its trajectory reintersects the Earth. The Mercury-Atlas flights achieved much higher velocities and apogees in addition to a circular orbit (25.6 kfps vs 7.5 kfps). So no, firing the retropacks after seperation would not allow a Mercury-Redstone vehicle achieve a stable orbit.


Not to mention that if you expended your retro rockets to get into orbit, even if you could make it, how would you slow down to de-orbit?

Assuming for the sake of argument that the retropack provided enough delta-V to reach orbit, remember that the Redstone reached an altitude of only forty or so miles by the time its fuel was spent. So the perigee of any orbit reached wouldn’t have been much higher than that, and as a result the orbit would have quickly decayed anyway. With a perigee of 40 miles and an apogee of 100 or so, I don’t think any spacecraft could complete more than one orbit before returning to earth.

For anyone interested in this subject I can’t recommend the Orbiter freeware package highly enough.

Orbiter space flight sim main page. The base package comes with the Space Shuttle and a hypothetical high-powered delta glider that can take off from a runway and reach orbit. Except for unlimited fuel in the case of fictional spacecraft like the delta glider, spacecraft handling and flight characteristics are based on RL physics. You can dig into the physics if you wish but it isn’t absolutely necessary.

There’s also a vast range of add-ons that have been built by Orbiter users, including several historical spacecraft at the “Meadville Space Center”. Those interested in the MR missions will find the the Mercury Redstone configuration here. I’ve always been fascinated by the Shepard and Grissom Mercury missions myself. It’s remarkable to think that America’s first two manned space missions could be covered in their entirety by network news programs, between one commercial break and another. (For some reason, I have found that the altitudes reached in the simulation are much higher than those reached in the historical missions, and I don’t know why that is.)

You could also do worse than check out the Gemini Joe add-on by yours truly. In reference to the Mercury Little Joe flights, I cobbled together two launch configurations using the Gemini spacecraft and either stage of its historical Titan II launch vehicle, but not both. In general, altitudes between 550 and 800km are reached in the simulations, but I am not sure how accurate this is.

A Gemini spacecraft blasts off with the second-stage configuration:
Gemini Joe 2nd Stage Takeoff

In a first-stage configuration, the Gemini spacecraft has just jettisoned the spent booster.
Gemini Joe Illustration 2

A quick look at the Earth’s surface through the open hatch
Gemini Hatch Opening

Because I adapted this from existing components, and I’m nowhere near a good enough C++ programmer to modify this at the code level, I should warn you that there are astronauts in the scenario, who will unaccountably insist on EVA during these short suborbital flights. If you do download the sim and my add-on, see if you can find a way to prevent that. Their virtual lives depend upon you!

Yes, we watched both MR missions from launch to recovery in the school auditorium. There were huge 19" black and white televisions on tall stands in the front tuned to the same station.