There was considerable concern about the overall cost of such a program, but someone pointed out that there need not be a commitment to total cost, only to annual budgetary allocations. In other words, this is the type of exploration program that can be developed on a pay-as-you-go basis.

It starts with the existing human space flight infrastructure and then builds on that in a modular fashion. Thus, in any given budget year, progress would be proportional to the amount of funds available. This does not require retiring the shuttle and suffering a human space gap of at least five years and possibly 10 or more years.

There is no artificial deadline of 2020 to get to the moon. There is no new set of expensive and marginal launch vehicles to develop and test.

Let's take a look at some of the detail comments and try to respond:

Even though the Space Shuttle is a reusable, available and proven human transporter, several people appear concerned that it is unsafe, too expensive and 30 years old. The shuttle has had two fatal incidents in 127 flights. Yes, this is much worse than the record for airlines, but all of the factors that caused the shuttle incidents have been addressed and there appears to be no systemic weaknesses in the current system. This cannot be said of the Ares I.

As to expensive, development and test investments in the shuttle have been made. Even though it is expensive to operate, much of the expense is the result of low usage. With increased flight frequency and the addition of modernized turn-around processes, the costs can be significantly reduced.

Most airframes, such as that of the shuttle have lifetimes measured in flight hours. Many airliners fly for tens-of-thousands of hour while enduring extreme stresses and vibrations, and thousands of landings and takeoffs. The shuttle fleet has experienced a total of less than 130 takeoffs and landings. The total high stress flight time of the fleet, during ascent and reentry, is less than 120 hours. Thus, by airframe standards the Space Shuttle flight is "brand new."

Several writers stated that the International Space Station (ISS) is in the wrong orbit for lunar and solar system exploration. The fact is that there is no ideal low Earth orbit for lunar and planetary launches. Nevertheless, the U.S. has launched many planetary probes and Apollo flights from a 28.5-degree latitude launch site and Russia has launched many planetary probes from its high-latitude spaceport.

Launching from the ISS may impose some minor penalties in terms of launch windows and energies, but it does offer a central platform for international involvement by Russia and other countries. More importantly, launching from ISS reduces the energy requirement for many missions by more than 50 percent when compared to ground launches.

In fact, the launch energy for a lunar mission is reduced by roughly 70 percent just by using the ISS as a launch port instead of the Earth's surface.

Some writers suggested the use of commercial launch services instead of the shuttle. The suggested architecture is flexible in terms of transportation services between the ground and ISS.

The Space Shuttle is already operating and there are no commercial launch services yet available. Once these services become available and are proven cost-effective, safe and reliable, the shuttle may be phased out. Otherwise, the shuttle can continue human space flight services without interruption.

A few writers pointed out that when HETS returns to Earth orbit from the moon or beyond, an onboard propulsion system must slow the vehicle for Earth capture. This is true, but there is an important trade off to be considered.

A reentering spacecraft returning from the moon or beyond requires that the vehicle be designed according to a complex set of reentry requirements. This adds a great deal of mass and cost to the vehicle. HETS avoids this by carrying a propulsion system that slows its speed enough to enter Earth orbit.

For those of you that did not see last week's commentary, let's briefly review the key elements of Launchspace's suggested solar system architecture:

Continue flying the Space Shuttle until it can be replaced with a "better" system. Use it as the basic transportation vehicle from Earth's surface to ISS. It would ferry astronauts and valuable cargo to the ISS, while expendable launch vehicles would transport consumables and low-value cargo to low orbit.

Take advantage of the ISS as an assembly, integration and test facility for a modular reusable solar system Human Exploration Transfer Stage (HETS).

HETS would be a dedicated vehicle for solar system exploration that need not carry huge weight penalties associated with atmospheric ascent and reentry. It can be optimized for the vacuum flight of space, leading to performance, mass and cost advantages that are significantly superior to those of Constellation's architecture.

HETS would be able to take advantage of a modular design approach in order to optimize its shape, size and design for short, medium and long flights to the moon, asteroids and Mars. Robotic flights of HETS can allow safe check out and testing of the vehicle and its systems during short flights, e.g., lunar circumnavigation trips from the ISS.

Reusable orbit transfer vehicles (OTVs) can be used to raise HETS to high orbit for launch into asteroid and planetary transfers. These OTVs can also lower the orbit after rendezvous with HETS upon its return and injection into high earth orbit.

Any solar system exploration architecture is very complex because there are many variables and many options to be considered. The concepts suggested here represent just the beginning steps toward such a complex architecture.

Nevertheless, it does offer the continued use of already existing and proven systems for achieving low orbit, and eliminates the requirement for an exploration vehicle that would have to carry a great deal of added mass to survive atmospheric reentry. The only required new systems would be those elements needed to explore space above low Earth orbit.

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