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Space Future has been on something of a hiatus of late. With the concept of Space Tourism steadily increasing in acceptance, and the advances of commercial space, much of our purpose could be said to be achieved. But this industry is still nascent, and there's much to do. this space.
9 December 2010
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K Aerospace & W Kistler, March 20-22 1997, "Design of a Transportation System for Space Tourism", Presented to International Symposium on Space Tourism, Bremen, Germany, March 20-22 1997.
Also downloadable from of a transportation system for space tourism.shtml

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Design of a Transportation System for Space Tourism
Walter P Kistler
Note: STV Co. is not connected to Kistler Aerospace Corporation. They are two independent entities and the design concepts presented here are different from those used in the design of the two-stage reusable launch vehicle being developed by KAC.

Before dealing with any space rocket design, it is important to revisit the basic rocket equation shown in Figure 1. It shows an inverse exponential relationship between the weight that reaches space and the required velocity. The curve in Figure 2 displays this relationship quite drastically, showing how little is left to reach orbit even using the most powerful propellants. The curve indicates the weight in orbit for two different velocity equivalents or delta V and for two of the most common propellants. The figures relating to the end velocity of 9 km/sec applies to a large, low drag design starting from ground while a delta V of 8 km/sec only applies when a rocket vehicle can start its run in space, without having to fight air drag and the pull of Earth gravity.

Figure 1: Basic Rocket Equation
Figure 2

In the case of VentureStar it is assumed that the weight of the vehicle after reaching orbit will amount to approximately 12% of its launch weight. This does not leave much to work with for the structural engineers. Figure 3 shows the main components that constitute the net weight of a reusable space rocket and very rough, approximate values for their maximum weight allowance.

Figure 3: Reusable Space Vehicle Using H2/O2 Propellants

Figures 4 and 5 show the one realistic concept of a single-stage-to-orbit plane which will hopefully be built someday, the VentureStar. Considering the numbers on Figure 3 the task will certainly not be easy, if it is possible at all. But even assuming this space vehicle will be flying someday, how well will it suit the task of bringing tourists for a trip in space at an affordable price? Not very well, I fear!

Figure 4
Figure 5

The designers of VentureStar figured the cost per flight to be 1/10th of the cost of a flight of the space Shuttle. Since the Shuttle trip costs $250,000,000, a trip on VentureStar may cost about $25,000,000. Assuming you are able to stack 50 people into the payload bay, which would not make the trip very pleasant for the customers, the price per person would still amount to $500,000. This is 10 times more than even the most enthusiastic passengers would be willing to pay. A flight in the tight payload bay with no view to the outside would hardly constitute a memorable experience.

Figure 6 shows a space vehicle designed by the Japanese Rocket Society, specially designed to bring tourists on a space voyage. The arrangement with all around windows and a space for gravity-free flotation will certainly be a lot more enticing to future space tourists. The vehicle takes off from the ground, is powered by H2/02, returns to Earth in base-first attitude, and lands on telescoping legs after firing its retro rockets.

Figure 6

This all sounds nice but can this vehicle really be built and will it fly? Taking another look at Figures 2 and 3, I would say this is rather doubtful. Because of the stout shape of the vehicle, ascent through the atmosphere will be slow, making for large losses due to drag and gravity. The required delta V would be around 10 km/sec as the designers correctly state. This however leaves only about 10% for the weight of the vehicle itself and such a low net weight would appear nearly impossible to achieve. Liquid hydrogen is very hard to handle and needs large, well-insulated tanks. The complex rocket engines required for this vehicle must have variable nozzles to adjust to the outside air pressure from sea level to vacuum. Furthermore, a considerable load of fuel is required to slow the vehicle down for landing.

What can be done to provide a technically feasible vehicle for space tourism? I myself do not believe that a pure SSTO vehicle will be the solution. The pure SSTO may be possible to build, but it might be technically so demanding in development costs, in maintenance and upkeep that it may remain an expensive and marginal approach. On the other hand, a two-stage vehicle is hardly an acceptable solution, it is just too big, too complex. However, there may be a solution in between.

When analyzing the entire trajectory of a SSTO we find that there are two operations that significantly add to fuel consumption and to the weight, these are the launch and the landing. During launch and ascent through the Earth's atmosphere the vehicle has to fight air drag and gravity adding more than 1000 in/sec to the required delta V. For landing either wings and retractable landing gear (like in the space Shuttle) are required, or a combination of parachutes, retro rockets and shock absorbing under carriage. This adds considerably to the empty weight. Obviously the SSTO needs some outside help to be able to do its job successfully.

A few years ago I proposed several devices that would help the SSTO overcome the two road blocks to some extent. The first one is a launch assist platform (LAP) which lifts the space vehicle above the bulk of the atmosphere (see Figure 7). It does not impart any velocity (except for a small vertical component) to the space vehicle, it just moves up and down vertically landing in the same area from where it was launched. Figure 8 shows one implementation of the idea. It is a low profile structure carrying four pods with rocket engines and their required fuel. The LAP helps the space rocket in several ways: losses through air drag are greatly reduced, the rocket can assume a flat trajectory right away, thus reducing g losses, and, thirdly, the rocket engines don't have to operate at sea level and can be designed for best efficiency in the vacuum of space.

Figure 7: STV-1/LAP-1 Trajectory
Figure 8

In a preferred implementation that would be used for the space tourism vehicle the pods would contain a cluster of two or three powerful, light-weight jet engines instead of the rocket engines, as shown in Figure 7.

The Shuttle as well as the VentureStar vehicle make use of wings and retractable landing gear to allow the space plane to land on a normal runway. This makes the task of keeping the structural weight within the extremely tight allowable limits for a SSTO even more difficult, maybe impossible.

According to the second suggestion I would like to present here, the heavy landing provisions would be left on the ground rather than bringing them up to space and back. Large flexibly mounted nets would be installed at the anticipated landing points providing for soft, shock-free landing of the fragile structure and its fragile load of tourists (Figures 9-10).

Figure 9: STV-1 Rocketship Descent & Landing Sequences
Figure 10: STV-1 Rocketship Landing

Part of the weight savings, however, will be balanced by the need for retro rockets required to slow down the descent velocity before the vehicle touches the net. If standard solid rockets are used their weight will be approximately 5% of the landing vehicle which would be considerably less than the weight of wings and retractable landing gear (See Figure 11).

Figure 11: STV Landing On the Net

More long-range development work will certainly be needed before a vehicle as presented here could be built. New special rocket engines will have to be developed and the proper jet engines powering the launch platform would have to be selected and modified for the job.

But sooner or later space tourism is bound to arrive and a vehicle to handle it will certainly be developed.

K Aerospace & W Kistler, March 20-22 1997, "Design of a Transportation System for Space Tourism", Presented to International Symposium on Space Tourism, Bremen, Germany, March 20-22 1997.
Also downloadable from of a transportation system for space tourism.shtml

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