“develop the “critical technologies of orbital assembly, automated docking, and propellant transfer and storage.” In fact, none of these technologies were needed to go to the moon in the 1960s, and none are needed to send astronauts to the moon or Mars today.”
And what’s more, all of these technologies *exist today*. There’s no reason not to incorporate them in your plans Mr. Zubrin.
“It is clear that a mission-driven space program should be more optimal for actually accomplishing missions, but why should it be so much better at technology development than one that allegedly purports to be technology-driven?”
The mission driven approach would indeed be better *if* it used fair, competitive and redundant procurement of propellant launch services as it easily could. All technology development could flow from that – safely off the critical path – and be prioritised by market forces. Why don’t you support that?
“The per-pound cost of space launch decreases as launch vehicle capacity increases, so by shunning heavy lift for orbital refueling, the depot approach will increase the cost of interplanetary ventures.”
Are you the same Bob Zubrin who was involved with Black Horse? And now you’ve been reduced to advocating minor cube/square advantages of large launchers when the real gains are to be had by using reusable instead of expendable launchers or at least cheap mass produced ones? What would the Bob Zubrin of fifteen years ago have had to say about such pathetic gains, which will in all probability be swamped by the inefficiency of a pork-driven government project? Do you realise why you never got funding for Black Horse? A large and fiercely competitive propellant launch market in support of a mission driven exploration program would be your best bet for both cheap lift and manned exploration of Mars.
“Worse yet, it will greatly increase the mission risk, since the more launches that are needed to mount a mission, the greater the chance that one will fail.”
Ah, the old 1-(1-p)^n myth. Unless you stupidly design your mission so that it cannot accommodate a single failure (a la Constellation in other words) a failed propellant launch is not a problem. It’s only propellant after all. Use of EELVs and propellant transfer would *decrease* risk, not increase it.
“Moreover, an orbital depot needs to be in an orbit at a particular inclination to the Earth’s equator. If a high inclination orbit, like that of the Space Station, is used, this will further reduce the payload that can be delivered to it by any booster. If a low inclination orbit is used instead, access to the depot will be restricted.”
This is only an issue for HLVs, which is begging the question. In addition, the trick is to use both LEO and Lagrange point depots, initially probably only Lagrange point depots (or better yet, refuelable spacecraft instead of dedicated depots), which would be enough for moon / Mars missions. That will solve all your issues and it is highly preferable for a large number of reasons anyway. You cannot plead ignorance on this point.
“In addition, propellant delivered to an orbital depot will have to be stored in heavy thickly-insulated tanks, which are a waste of launch capacity and so disadvantageous for use on an interplanetary mission that duplicate lightweight flight tanks will also have to be launched, thereby running up mission mass and costs still further.”
Then use hypergolics. You will even gain some dry weight advantage by launching the spacecraft dry. If you use LEO rendez-vous with a separately launched EELV upper stage for efficiency you can transport the individual pieces to L1/L2, from where use of hypergolics will be acceptable. Use of small SEP tugs operating between EML1/2 and SML1 (proven technology for storable propellant) to preposition return propellant (a la Huntress’ plans of which you cannot be ignorant) will keep IMLEO and therefore cost acceptable, even if RLVs don’t emerge soon as a result of the large market for propellant launch services.
