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16 July 2012
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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T Williams & P Collins, 1999, "Orbital Considerations in Kankoh-Maru Return Flight Operations", Proceedings of 8th ISCOPS, Xian.
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Orbital Considerations in Kankoh-Maru Return Flight Operations
Trevor Williams * and Patrick Collins **

In the first phase of the Space Tourism Study Program of the Japanese Rocket Society, the VTOL reference-vehicle " Kankoh-maru" was designed to carry 50 passengers to low Earth orbit. While the initial service was specified to be a flight comprising two orbital revolutions, market research showed that flights to and from accommodation in orbit will be needed in order for the market for space tourism services to reach its full potential.

In an earlier paper presented at the 7th ISCOPS the authors discussed the orbital considerations influencing Kankoh-maru operations from take-off through rendezvous and docking with an orbiting hotel. Factors considered included the orbit plane constraints, the effects of launch site latitude, orbital hotel orbit selection, and orbit phasing constraints. A typical rendezvous sequence was described, and potential consequences for the design of Kankoh-maru discussed.

The need to reach a selected landing-site on Earth from a given orbital position has implications for Kankoh-maru return flight operations. The present paper introduces the main factors involved in returning to Earth from an orbiting hotel, and considers the various issues in some detail, making some preliminary estimates for representative cases. Orbit plane, landing-site latitude, orbit phasing, orbital traffic control issues, pre-re-entry manoeuvring, re-entry, atmospheric braking and cross-range issues are introduced and discussed. An example sequence of Kankoh-maru departing from an orbital hotel, manoeuvring to reach a selected 'window' for re-entry, re-entry and landing is described.

The viewpoint of operating commercial travel services to and from low Earth orbit is very different from US and Russian government operations: for example, commercial operators of Kankoh-maru will need to obtain high utilisation from their vehicles, as airline companies do. Thus they may choose to make a longer return flight with an earlier landing time, rather than stay longer in orbit followed by a shorter return-flight and a later arrival. However they must also consider customers' preferences and the cost of consumables.

At present, much of the detailed information that companies need in order to plan orbital flight schedules, such as detailed passenger vehicle specifications, actual orbital hotel orbits, levels of demand on different routes, passenger preferences for various service features, and others is not yet available. Consequently the discussion in this paper is simplified and schematic. Nevertheless it is hoped that it will be useful as an introduction to the subject for airline planners who are starting to consider the possibility of offering space flight services, but for whom existing technical publications are not very helpful, being written from a very different point of view from that of commercial operators.


The passenger vehicle " Kankoh-maru" was designed to carry 50 passengers to a 200 km altitude orbit with inclination of some 40 degrees, as part of the Space Tourism Study Program of the Japanese Rocket Society started in 1993 [1, 2]. Initial flights are to comprise two orbital revolutions, and the service is expected to grow progressively. Kankoh-maru operators will have considerable freedom to select orbital routes which give passengers interesting views of the Earth, and attractive flight paths have begun to be analyzed for this purpose [3]. According to economic analyses performed to date, the cost per passenger could fall to some $25,000 if 52 vehicles are produced, and demand grows to some 700,000 passengers per year [4].

Figure 1: Kankoh-maru passenger launch vehicle

However, in market research performed in several countries to date, most potential customers express a preference for staying in orbit for several days. In order to provide this service, there will be a need for orbital accommodation, growing eventually to a capacity of several thousands of guests: this will involve design, manufacture, launch, assembly, operation and maintenance of these facilities. It will also require Kankoh-maru operators to operate round-trip flights to and from orbital hotels, in addition to simple orbital flights. Outward-bound flights to an orbiting hotel were analysed in an earlier paper [5]; this paper considers return flights.

Commercial operators planning return flight services will have to consider a wide range of factors, including physical constraints imposed by orbital mechanics; vehicle performance factors such as Kankoh-maru's propellant and payload capacity; operations factors such as the landing-sites being used; commercial factors such as flight schedules and price-elasticity of demand for passenger flights to different destinations; and possible future developments such as the availability of obtaining propellant supplies in orbit.

Of these factors, orbital considerations are the most fundamental since they determine the physical frequency of landing opportunities from a given orbiting hotel to a given landing site, the time taken from departure to landing for a given orbital hotel-landing site pair on each occasion, and other matters. The concepts underlying these orbital factors are not particularly complex, although there are many subtle details. However, they are unfamiliar to the airline companies and hotel chains which are now beginning to consider the feasibility of offering orbital passenger flight services (6, 7). This paper is an attempt to make these matters more approachable.

The viewpoint of a commercial operator planning orbital passenger flights is different from that of a government space agency performing a research mission; in particular they will generally be trying to maintain or increase their profitability, which leads to a continuous interest in both increasing revenues and reducing costs. At the operational level this will lead to other objectives, of which three will be particularly important:

  1. Minimizing the duration of round-trip flights to and from orbit, thereby increasing the number of revenue-earning flights that each vehicle can perform (up to Kankoh-maru's design target of 300 flights per year).

  2. Minimizing propellant usage (and so propellant cost), thereby also maximizing each round-trip flight's revenue-earning payload capacity (whether passengers or cargo).

  3. Ensuring that the length of any flight, either outward or return, is not too long for the comfort and convenience of passengers.

Consequently, in drawing up plans for passenger space flight services, companies will have these objectives in mind when selecting between possible alternatives.


A major difference between outward and return flights to and from Earth orbit is that while the former involve using large quantities of rocket propellants to accelerate to orbital speed of some 8 km/sec, the latter require deceleration from orbital velocity to zero at landing. If this deceleration was achieved using thrust from rocket engines, it would require a very large quantity of propellants, all of which would have had to be carried to orbit on the outward flight, making the activity prohibitively expensive. Fortunately there is a way to decelerate that requires almost no propellant, namely using aerodynamic friction from the atmosphere. "Re-entry" is the term used to describe the process of entering the Earth's atmosphere from above at hypersonic velocity - some 8 km/second or approximately Mach 26 - and slowing to sub-sonic speed through the braking effect of air resistance.

During re-entry, a returning space vehicle's external surface facing the air-flow absorbs a large quantity of heat, and reaches very high temperatures. Designing the external shape to minimise heating rates and selecting appropriate materials to permit frequent re-flight without requiring maintenance, are key parts of the design of a vehicle such as Kankoh-maru. (Note that in traveling to orbit, Kankoh-maru leaves the effective atmosphere before reaching such high speed, and so the frictional heating on Kankoh-maru's leading surfaces on outward flights is much less than the heating of the base during return flights.)

Reentry actually begins when the "vacuum perigee" - that is the altitude to which the perigee (the lowest point in the orbit) would fall if there was no atmosphere to resist its motion - of a spacecraft's orbit falls within the effective atmosphere, typically to an altitude of less than 100 km. However, the Earth's atmosphere does not start abruptly at this height: 100 km is the approximate altitude below which air-resistance and heating on an orbiting vehicle typically become substantial. And the vacuum perigee is reduced by the spacecraft performing a re-entry engine-burn: that is, using its engines to decelerate sufficiently that it's path falls toward the Earth along a selected trajectory.

"Cross-range" refers to a vehicle's ability to manoeuvre away from a purely ballistic trajectory - that is, steering to left or right of its flight path in the plane of its orbit, and flying above or below a purely ballistic path. The extent of a vehicle's cross-range capability depends on its lift-to-drag ratio (L/D ratio), which depends on its shape and its lifting surfaces. The larger and more complex these are, the greater the cross-range capability, but the more severe the heating from air-resistance during re-entry manoeuvring. For blunt re-entry bodies the L/D ratio is around 0.2. The Gemini spacecraft had an L/D of some 0.19, while the more aerodynamic Apollo vehicle was some 0.25. Kankoh-maru is designed to have cross-range of some 200 km [1]. By comparison the winged US space shuttle has a nominal cross-range of more than 1000 km.

The duration of the re-entry process depends on the vehicle design, on the orbital altitude from which the vehicle is returning, and on the planned altitude of the vehicle's post-burn vacuum perigee. Vehicles used to date, which have typically returned from orbital altitudes of a few hundred kilometres and aimed at vacuum perigees of some 75 km, have taken some 25-30 minutes from retro-fire burn to the start of aerodynamic deceleration. The time from re-entry to actually landing also depends on the vehicle's aerodynamic design, as well as on its flight plan, including whether it lands using wings, parachutes or rockets, but typically lasts a further 15-20 minutes. The time between Kankoh-maru's re-entry engine burn and landing will be more or less fixed for a given altitude at re-entry engine burn. As discussed below, Kankoh-maru may frequently perform its re-entry engine burns at relatively low altitude, around 150 km; thus its re-entry engine burn will generally be between 30-45 minutes before landing.


Orbital mechanics imposes severe constraints on vehicles traveling between a particular orbit and points on the Earth's surface. The most fundamental of these constraints is determined by the orbit plane, that is the plane containing the vehicle and passing through the centre of the Earth. It is a fundamental property of orbital mechanics that the plane of a hotel's orbit is fixed relative to the distant stars (that is, it is "inertially" fixed) - though only to a first approximation [8, 9]. For a polar orbit (an orbit that passes over the Earth's poles) since the Earth rotates once every 24 hours any point on the Earth's surface passes through the orbital plane twice/day. For inclined orbits (ie orbits inclined at less than 90 degrees to equator) all points at latitudes below the angle of inclination also pass through the orbital plane twice/day. There are two special cases: equatorial orbits pass repeatedly over the equator several times/day - typically 15-16 times for low orbits of a few hundred km; and points at the same latitude as the orbital inclination lie within the orbit plane just once/day.

These conditions are important because in cases where Kankoh-maru is planning to return to a selected site on Earth from an orbiting hotel to which it is docked, it is not economic for the hotel to change its orbital plane, due to the very large quantities of propellants that would be needed. Furthermore, for Kankoh-maru to use its rocket engines to alter its orbital plane would also be prohibitively expensive in propellant. (For example, if the space shuttle used all its on-board propellant it could alter its orbital plane by only about 1.8 degrees - representing a distance of less than about 200 kilometers on the ground.)

Thus Kankoh-maru can land only at times when its intended landing site passes through its orbital plane. (In practice, since it will take Kankoh-maru some 30-45 minutes to de-orbit, re-enter and land, it must in fact de-orbit some 30-45 minutes before the landing site is due to pass through its orbital plane.) Consequently opportunities to land at a selected site typically occur once or twice per day, at specific times.

The two landing opportunities/day are spaced 12 hours apart for landing sites near the Equator, but for non-equatorial landing sites, the higher the latitude the closer together are the two successive "planar landing windows". The limiting case is when the orbital inclination is the same as the landing site latitude, there is only one landing opportunity/day, but it is extended in duration (as discussed below).

If orbit planes were truly inertially fixed, these "planar landing windows" would occur precisely twice per day, as the Earth rotates once every 24 hours inside the hotel's fixed orbital plane. In reality, orbital planes rotate slowly relative to the stars, as a consequence of the Earth's "oblateness" (ie the slight flattening at the poles due to the spinning of the planet which effectively creates a "bulge" around the Equator: this bulge of additional matter will exert a slight force on an orbiting hotel, so causing its orbit plane to rotate, or "precess", slowly).

The rate of precession depends strongly on an orbit's inclination i, being proportional to Cos(i), as well as on the orbit's altitude and eccentricity. Orbits at 28.5 degrees inclination, typical of space shuttle flights from Cape Canaveral, precess at around 7.5 deg/day towards the west. For higher inclinations such as the 50 - 60 degree orbits likely to be used for many space hotels, the resulting precession rate will be around 6 degrees/day towards the west - that is about 180 degrees/month or a full 360 degrees roughly every 2 months. As a result of this slow rate of precession, Kankoh-maru's landing opportunities or "planar landing windows" will actually repeat with a period of around 23.6 hours; that is, they will drift approximately 24 minutes earlier every day, thereby returning 'full circle' by drifting 24 hours roughly every two months.

Further points that need to be made are first, the planar landing window repeat interval is not exactly 23.6 hrs, but varies slightly with the orbit's inclination.and also its eccentricity. For simplicity this paper considers only the special case of circular orbits. Second, if a vehicle has a longer cross-range it will have additional landing opportunities, since during re-entry it will be able to move further outside its orbit plane. Third, the length of time between Kankoh-maru depart from an orbiting hotel and making its de-orbit engine-burn will vary greatly in different cases, as discussed below.


The latitude of Kankoh-maru's intended landing site not only determines the spacing during the day of the two planar landing windows, but it is also important in orbital analysis for another reason. As already described, landing opportunities occur when the hotel's orbit plane passes through the intended landing site. The hotel orbit must therefore reach at least as far North as the landing site; that is, it must have an inclination, or angle to the Equator, equal to or greater than the landing site latitude.

Most of the large centers of population in the currently economically more developed countries from which most passengers will derive in the early stages of space tourism services, tend to be at mid-to-high latitudes. Consequently, the orbits of most space hotels will probably have fairly high inclination: for instance, a value of 50 degrees would allow Kankoh-maru to land at sites in most of the United States, Japan and Europe. An inclination as great as this has the additional advantage that a great deal of the Earth's surface is overflown, so providing a wide range of views for passengers. A disadvantage is that it requires more propellant to launch to this inclination than to a lower one. Balancing these contrary factors will be one of the tasks of commercial hotel operators.

If Kankoh-maru's landing sites are typically at relatively high latitudes, its two landing opportunities per 23.6 hr 'day' will typically be close together. In principle, these landing 'windows' are instantaneous, occurring as the hotel's orbit plane passes through the landing site. In practice, since Kankoh-maru can make a significant cross-range maneuver during re-entry and subsequent atmospheric flight, its planar landing-windows will have a finite duration, typically of the order of one minute or so. Thus the sequence of manoeuvres in a typical return flight will need to be timed precisely.

The exception to this occurs when the hotel orbit inclination is within around 0.5 degrees of the landing-site latitude: in this case, the portion of the hotel orbit farthest from the equator remains close to the landing-site for an extended period and the two daily landing opportunities merge, giving a single landing-window during which the hotel and landing site are moving almost parallel, which may last tens of minutes. Note that these figures are highly dependent on the details of each different case, and particularly on the parameters of the hotel's orbit. Consequently more detailed analyses of particular cases is essential.


The planar landing windows described above specify the times at which Kankoh-maru can land at a selected landing site. However, in order to do so the hotel and the landing site must not only be in the same plane, but Kankoh-maru itself must also of course reach the same point as the landing-site at the same time. In order to do this Kankoh-maru must re-enter at the right time from the right position in its orbit.

A re-entry window of, say, 2 minutes is only a small fraction of the typical orbital period of some 90 minutes - just a few percent. Thus the chance that Kankoh-maru's position in its orbit would be at the right 'phase' to reach a particular landing site would occur with a probability of only some 2/90 - or only about once per 45 planar landing windows, or about once every 3 weeks. Thus, in general, unless Kankoh-maru alters its position relative to the hotel before re-entering it will not be able to reach its intended landing-site. The need for Kankoh-maru to manoeuvre within its orbital plane in order to reach the re-entry window appropriate for a particular landing site is known as the "Landing Phasing Problem".

The fundamental orbital property that is used to achieve correct phasing for landing at a selected site is that lower orbits have shorter periods: For instance, an orbit at 350 km altitude has a period of 91.5 minutes, whereas one at 150 km has a period some 4 minutes shorter. Consequently, if the hotel is in the higher of these orbits, and Kankoh-maru descends to the lower one, it will complete one revolution of the Earth approximately 4 minutes faster than the hotel. At the orbital speeds involved, this implies that Kankoh-maru will move forward relative to the hotel by nearly 2000 km each revolution. This leads to what is referred to as the "Ten-to-One Rule": the distance by which the lower of two spacecraft in low Earth orbits (in the same plane) will pull ahead of the higher during each orbital revolution is approximately ten times their average vertical separation. This provides Kankoh-maru with a systematic means of altering its position along its orbit.

In principle Kankoh-maru could also manoeuvre into a higher orbit than that of the hotel from which it is departing, whereby it would fall behind the hotel. However, this would use more propellant than departing downwards, which would be unattractive to commercial operators, though it could enable shorter return flights, which could be attractive on occasions. In this paper, for simplicity, we consider only cases in which Kankoh-maru departs downwards from an orbiting hotel - and so, from the Ten-to-One Rule, moves ahead of it. A more detailed analysis to determine the conditions under which upwards departures could be advantageous would be interesting.

From the above discussion we can see that, in order to achieve the correct orbital phasing to permit landing at a selected site during a particular planar landing window, Kankoh-maru's departure from the hotel will have to be timed to occur not during the re-entry window some 30-45 minutes before the hotel's orbit plane is due to pass through the landing site, but at a time such that after Kankoh-maru has completed its pre-re-entry manoeuvring - which may last several orbits, and hence several hours - it will reach the re-entry window. This pre-re-entry manoeuvring is constrained by the fact that there will be a limit to how many hours operators will wish to keep passengers on board Kankoh-maru.

A question of interest to Kankoh-maru operators planning scheduled services between sites on Earth and orbiting hotels is the following: suppose that on a particular day Kankoh-maru can depart from a particular hotel, manoeuvre for an acceptable length of time and reach a given landing site. How long will it be before the same relation between the hotel and landing site is next satisfied? The answer to this question will determine the maximum frequency of round-trips between the hotel and the landing site.

The solution is that the hotel and landing-site return to the same alignment when 23.6 hours or a multiple of it is equal to an integral number of hotel orbit periods. As already noted, the period of an orbit depends on its altitude; consequently, if we are to achieve the desired "resonance" between the planar and phasing landing-windows (ie whole numbers of each coincide periodically), only certain precise orbital altitudes will be acceptable for the hotel. The orbital altitudes that permit landing-windows repeating at intervals of one, two, three or four days are shown in Figure 2. (It is interesting to note that the Salyut and Mir space stations made extensive use of two-day and three-day repeat-cycle orbit altitudes, in order to provide conveniently frequent launch opportunities for Soyuz and Progress rendezvous missions [10].)

Figure 2: Hotel altitudes convenient for return flights

As described in an earlier paper orbital hotels are likely to use orbits with altitudes around either 350 km or 400 km, which typically give phasing landing windows every 2 days [5]. Another important point is that atmospheric drag will reduce the altitude of a hotel in a nominally 350 km orbit by approximately 1 km every 2 days (though the exact figure will depend on the size, shape and orientation of the hotel). This will lead to the hotel moving forward relative to its nominal orbital position, and hence to a cumulative phasing error of about 160 km or 20 seconds after 2 days, or about 1 minute per week. In 1 minute the hotel moves some 500 km along its orbit: to cancel such a discrepancy Kankoh-maru would need to travel for one additional orbit at 50 km lower altitude, half an orbit at 100 km lower altitude, quarter of an orbit at 200 km below, or etc.

This suggests that small (up to a few m/s) reboost maneuvers should be performed on roughly a weekly basis in order to preserve the hotel's accessibility. Obviously such manoeuvres will be scheduled in advance to facilitate operators' planning. Periodic re-boost manoeuvres will give hotels' altitude a "saw-tooth" pattern falling a few kilometres over a week or two before being re-boosted over a few hours. Maintaining a particular orbital position will involve the hotel following a cycle of drifting below and then being boosted above its nominal altitude. It is clearly important for the hotel's orbital position to be convenient for landing by operators of passenger vehicles such as Kankoh-maru.


For commercial operators there will be constraints on how long a returning flight can be allowed to take. However, Kankoh-maru operators will need to achieve high utilization rates, and so will be keen to operate even long return-flights if they permit faster round-trips. Most of the flights will be in weightlessness, and passengers will be able to move around the spacious cabin, and so will be more comfortable than on a long airliner flight.

The main consequence of requiring a short landing procedure is a reduction in the maximum angular distance (or "phase difference") between the hotel and the landing site that is acceptable. This reduces the number of landing-windows at a particular site. For example, if a return journey lasting more than, say, 9 hours was judged to be unacceptable, then this would limit the number of orbits that Kankoh-maru could travel in a lower orbit to 6 orbits. In the case of a hotel orbiting at 350 km altitude, traveling in a 150 km altitude allows Kanjoh-maru to advance by some 2,000 km/revolution relative to the hotel. 6 orbits would enable it to move 12,000 km ahead, or some 120 degrees around the orbit.

2,000 km is covered by Kankoh-maru in its orbit in about 5 minutes. Consequently, each revolution spent by Kankoh-maru at 150 km allows it to correct for an initial phasing error of 5 minutes. If the hotel were actually halfway around its orbit (ie 45 minutes) from the landing site during the planar landing window, this phasing error would require about 11 low orbits by Kankoh-maru in order to compensate - or about 16 hours. A 9-hour constraint would therefore reduce the number of usable landing-windows and increase the importance of having the hotel in a repeating orbit as described above, in which case the planar and phasing windows overlap more frequently than by chance.

It should also be noted that in practice it is efficient to use elliptical orbits, in which the apogee (the maximum altitude) and the perigee (the minimum altitude) are significantly different (in addition to circular orbits) during return flights. This adds considerably to the range of possibilities and so to the complexity of the analysis. For simplicity we do not discuss this topic in detail here, but airlines will certainly make use of all such possibilities, since even such detailed orbital analysis is possible with off-the-shelf computer software today.


The overall flight path used by Kankoh-maru in returning to Earth and landing at an airport will be managed by the traffic control authorities of the target airport. Details of future orbital traffic control systems have not yet been finalised, but a first study of the feasibility of extending controlled airspace upwards to include low Earth orbital space is currently under way in the FAA and an interim report has been published (11).

Perhaps the most significant proposal in this report is the conceptualisation of "Space Transition Corridors" (STC) - zones linking an area on the ground to an area in orbit reserved for either a vehicle returning from orbit or a launching vehicle, into which other aircraft are not permitted for the duration. This proposal resolves the potential problem that a returning vehicle such as Kankoh-maru will not be able to carry sufficient fuel to be able to manoeuvre significantly within the atmosphere before landing. For example it will not be able to hover for several minutes, nor reroute to another landing site as scheduled airliners can. Kankoh-maru's pilot will therefore need to receive irrevocable permission to land at its planned destination airport before departing from the orbiting hotel to which it is docked, as discussed in (12).

The concept of STC solves this problem elegantly: it is not a permanent fixed route like an air-lane, but a temporary zone defined in space and time within a computerised air traffic control network. As such it enables efficient and economical use of airspace. Details of such a system remain to be decided, and will require international support to become an international standard. The FAA has also proposed the formation of the International Space Flight Organisation (ISFO) to play the same role for space travel as the ICAO does for aviation. Other countries' governments have yet to comment on this, as they are behind the USA in making plans for the advent of reusable launch vehicles capable of round-trips between the Earth and space.

Local traffic control in orbit

Vehicle movements in the vicinity of an orbital hotel will be controlled through traffic rules, and real-time control. Such systems have already been conceptualized, involving series of nested zones in the same orbital plane as the hotel, as outlined in [13]. These rules will cover the routes which Kankoh-maru may take in departing from the hotel, as well as co-orbiting and approaching vehicles. In addition, a staff member on board the hotel is likely to have the role of Traffic Controller, giving instructions directly to the pilots of vehicles in the vicinity. At a time when several orbital hotels are in operation, it is considered likely that some will use the same orbits due to the operational benefits they will obtain, although this will require a number of technical and legal complexities to be resolved [14, 15].


Kankoh-maru's initial departure from a hotel may be either horizontal or vertical. During the Gemini and Apollo programs, the initial departure from a docked configuration was made tangentially along the orbital velocity vector, or "VBAR". An advantage of using this type of departure is that it can be halted at any point if problems arise, and the two vehicles continue to co-orbit in the same relative positions without the need for additional thruster firings. An alternative is to depart from a hotel vertically, that is along the orbital radius vector, or "RBAR". In such a departure, orbital mechanics automatically moves the vehicle away ahead of the hotel as its altitude decreases. The RBAR departure has been used for return flights by the Space Shuttle from the Mir space station.

For neither type of departure will Kankoh-maru initially fire its thrusters: the initial impulse will be provided by a spring-loaded or compressed-air device. This avoids using thrusters that face towards the hotel, from which exhaust gases might cause "pluming", impingement on vulnerable parts of the hotel, such as windows, solar arrays, or other equipment.

Space shuttle return flights from Mir have included a "fly-around" of Mir after separation, allowing inspection of the station for external damage, etc. A similar fly-around of a hotel could be performed by Kankoh-maru with very little propellant use. The velocity-change ("delta-v") needed to perform a fly-around of a space hotel depends on the speed and radius of the track to be followed. However, if Kankoh-maru's return-flight schedule allows a full orbital revolution, approximately 90 minutes, only a very small radial delta-v is required, after which orbital mechanics "propels" the vehicle.

For example a single initial impulse giving a radial delta-v of just 0.03m/second will initiate an elliptical path around the hotel within the orbital plane, with +/-25 metre radial movement and +/-50 metres movement along the orbit. One half of such a fly-around might be convenient. Alternatively, if the pilot makes many more small thruster burns, a faster, more nearly circular fly-around can be achieved over half an orbital revolution, or some 45 minutes.

A fly-around of the hotel would be likely to be popular with hotel guests, since it would give them a range of interesting views of the hotel against the backgrounds of both the Earth and the stars. Consequently, when the return-flight schedule permits, an appropriate fly-around of the hotel may be performed on initial departure.


The preceding discussion is used as the basis of a possible return flight sequence by Kankoh-Maru from an orbiting hotel to a selected landing site. For definiteness, a hotel orbit inclination of 60 degrees is assumed:  such an orbit would provide views of most of the Earth's surface and permit landings in USA, Canada, Japan and most of Europe. The hotel is assumed to be operating in a repeating orbit with an altitude of 355 km, giving a repeat cycle of 2 days, as described in [5].

  1. Departure of Kankoh-Maru from the hotel at a time calculated to enable it to land at a selected airport when the orbit plane next passes through it. Having made a partial fly-around of the hotel, Kankoh-maru drifts below the hotel where the pilot uses thrusters to move further down away from the hotel.

  2. Kankoh-maru fires two of its booster engines to alter its circular orbit to become elliptical with perigee at 150 km altitude (still coplanar with the hotel).

  3. Half an orbit (i.e. some 45 minutes) after the first firing, Kankoh-maru again fires two booster-engines (a "retrograde" or "slow-down" manoeuvre) so as to lower the apogee of its orbit, while keeping the perigee at 150 km. The new apogee altitude will also be 150 km, that is 205 km below the hotel.

    If Kankoh-maru's departure is slightly delayed, it will need to achieve a greater than nominal phase-angle catch-up to reach the re-entry window, which could be achieved in the same time by lowering its orbital altitude to less than 150 km.

  4. Over 2.3 revolutions, that is over some 3.5 hours, Kankoh-maru moves ahead of the hotel by some 4700 km, putting it "in phase" to reach its planned re-entry window, and hence its target landing site.

  5. At the optimal moment, some 30-45 minutes before Kankoh-maru is due to land, the pilot fires two booster engines to lower Kankoh-maru's "vacuum perigee" (the perigee to which it would fall if the vacuum of space continued down to the Earth's surface) to 75 km, well within the effective atmosphere. As Kankoh-maru's altitude falls, aerodynamic friction creates drag which slows the vehicle, thereby lowering its trajectory further and causing further deceleration, building up to 3 g.

  6. Any significant cross-range manoeuvring that is needed to reach the landing site is performed during this phase, during which aerodynamic forces, and so Kankoh-maru's potential for aerodynamic manoeuvring (which requires minimal propellant use) is at a maximum. Body flaps allow manoeuvring to raise or lower Kankoh-maru's trajectory, or to move to left or right of its ballistic path.

  7. Some 20 minutes later, Kankoh-maru clears re-entry and its velocity has fallen to sub-sonic. It is now some 340 km below the orbit path of the hotel, and falling towards the Earth at its terminal velocity of some 70 m/sec. The pilot restarts all 4 booster engines to provide redundancy for propulsion for final deceleration and landing.

    The Global Positioning System (GPS) is currently being studied by the FAA for use for navigation by scheduled airlines. It is expected that within a few years, leading airports' ground equipment will be upgraded to make use of GPS data transmissions. Kankoh-maru crews will use GPS for navigation in orbit as well in the atmosphere.

  8. Once Kankoh-maru is within a few thousand metres of the ground it deploys its landing legs; two booster engines are throttled up to slow Kankoh-maru and put it onto a trajectory that will reach the landing-site. Landing is cushioned by thrust from the engines. The vehicle's mass at landing is about 10% of its mass on take-off, and so the total thrust required for hovering is only about 8% as much as at take-off. As in an airliner, the pilots will monitor the wind speed and direction to direct the vehicle to achieve a soft landing.

    Unlike an airliner, Kankoh-maru will not carry sufficient propellant to be able to abort its landing and "go round again". However, there will be no need for this, since Kankoh-maru does not use a runway; in emergency it can land almost anywhere. This makes the planning, definition and management of STCs, as proposed by the FAA [11], relatively simple.


    In the preceding sections we have described some of the basic features of a return-flight by Kankoh-maru from a hotel in low Earth orbit. From the point of view of an airline planning such services, the return-flight including orbital manoeuvring, re-entry and landing is as important as launch and rendezvous, and it is necessary to extend this discussion to combine both activities. Combining both outward and return-flight analyses will give useful insights concerning the performance required of Kankoh-maru, attractive take-off and landing sites, convenient passenger flight schedules, propellant requirements for different services, and other matters. Even from the limited analysis performed so far there are some useful insights.

    1. The timing and positioning of Kankoh-maru's re-entry engine burn will need to be very accurate. Thus the selection of a convenient trajectory from the hotel that will provide opportunities for fuel-efficient correction of any errors in timing or navigation will be an important part of airline route-planning.

    2. The example describes a return from an orbit that repeats every 2 days. However, operators of Kankoh-maru will need to fly the vehicle near its design maximum of 300 flights/year, and so taking 2 days to make a round-trip would generally be unsatisfactory. This could be resolved by Kankoh-maru landing in the above example at a different site from the site from which it had earlier taken off to reach the orbiting hotel; it could then take off again within 24 hours to dock with a different orbital hotel than on the previous flight; and then depart within another 24-hours to return to the first take-off site. Such use of paired take-off and landing sites and hotels could permit more efficient flight schedules, providing regular 2-day round-trips to each destination. Another alternative would be to link 3 landing/take-off sites and orbital hotels in this way, providing 3-day round-trips. Such arrangements obviously depend on using landing-sites with the appropriate longitude separation, and hotels in the appropriate orbital positions.

    3. Kankoh-maru is nominally designed to return from a circular orbit of 200 km altitude at 40 degrees inclination [1]. Consequently to return from higher orbits and/or higher inclinations will require more propellant and/or a reduction in payload on those flights. This has implications for the economics of these services.

    4. The ability to refill propellant tanks in orbit would reduce the quantity of propellants that Kankoh-maru must carry to its orbital destination to use for its return-flight - thereby increasing its payload capacity and so its revenue-earning capacity on outward flights. Kankoh-maru uses liquid hydrogen ( LH2) as fuel and liquid oxygen ( LOX) as oxidizer, and these can be generated from water by solar-powered electrolysis - a technology particularly suitable for use in orbit where sunlight is readily available. This therefore leads to the question: could LOX and/or water delivery costs to LEO be reduced sufficiently through using dedicated "tanker" supply launches that, after paying the cost of storing on orbit (&/or the cost of electrolysis) propellant could be supplied to Kankoh-maru in orbit at a price less than the cost of carrying it itself? If so, this will be an attractive direction for business investment when hotels start operating in orbit. It has been suggested that this activity could stimulate commercial lunar development by creating demand in orbit for exports of lunar water to hotels in Earth orbit [16].

    These and many other such possibilities need detailed analysis, combining sophisticated orbital mechanics, advanced spacecraft design, market data and business analysis.


    This paper has discussed the main aspects of Kankoh-maru flights returning to Earth from an orbital hotel. In particular, the hotel's orbit has major implications for the relative convenience of different landing-sites. The present discussion has covered the fundamentals for one or two simple cases, and there is great scope for more detailed analysis of more complex cases.

    Although detailed orbital calculations can be performed easily using standard software on personal computers, the particular viewpoint of commercial travel service operators is new, and adds a range of novel constraints and complicating factors to the analysis. There is therefore a wide range of new issues for consideration which require more detailed analysis.

    This paper is intended to encourage work in this direction by presenting the analysis in a style that we hope is approachable by non-specialists. The ultimate objective of this work is that concepts such as vacuum perigee, orbital phasing error, RBAR, re-entry, the ten-to-one rule and STCs should become as familiar to airline flight dispatchers as great circle, jet-stream and go-around are today. Likewise, commercially important terms such as passenger load factor, block time, vehicle utilization and price elasticity should become equally familiar to spacecraft designers and operators.

    1. K Isozaki et al, 1994, " Considerations on Vehicle Design Criteria for Space Tourism", IAF-94-V.3.535, presented at 45th Congress of the International Astronautical Federation
    2. M Nagatomo et al, 1995, "Study on Airport Services for Space Tourism", AAS Vol. 91, pp 563-582; also downloadable from
    3. H Yoshida, 1996, " Engineering Analysis of Space Tourism as a Crewed Space Flight Mission", MSc Project Report, University of Tokyo, Department of Engineering, (in Japanese).
    4. P Collins (ed), 1998, " Report of Japanese Rocket Society Space Tourism Business Research Committee", (in Japanese).
    5. T Williams and P Collins, 1997, "Orbital Considerations in Kankoh-maru Rendezvous Operations", Proceedings of 7th ISCOPS, AAS Vol 96, pp 693-707; also downloadable from
    6. B Berger, 1999, " Billionaire Shops for Space Tourism Vehicle", Space News, V 10, N 18, p 6
    7. B Berger, 1999, " U.S. Developer Sets Sights on Space Tourism", Space News, Vol 10, No 20, pp 4, 20
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    10. P Clark, 1988, ' The Soviet Manned Space Program', Orion, New York
    11. P Smith, 1999, "Concept of Operations for the National Airspace 2005", FAA; also downloadable from
    12. E Anderson and P Collins, 1997, "Pilot Procedures for Kankoh-Maru Operations", Proceedings of 7th ISCOPS, AAS Vol 96, pp 647-692; also downloadable from
    13. M Nagatomo et al, 1984, " Orbital Operation of Co-orbiting Spacecraft with Space Station", IAF paper no IAF-84-42
    14. P Collins and T Williams, 1986, "Towards Traffic Systems for Near-Earth Space", Proc. 29th Colloquium on the Law of Outer Space, IISL, pp 161-70; also downloadable from
    15. P Collins, "Legal Considerations for Traffic Systems in Near-Earth Space", Proc. 31st Colloquium on the Law of Outer Space, IISL, pp. 296-303; also downloadable from www.
    16. P Collins, 1998, "Tourism in Low Earth Orbit: The Trigger for Commercial Lunar Development?", Proceedings of Space 98, ASCE, pp 752-756, 1998; also downloadable from
T Williams & P Collins, 1999, "Orbital Considerations in Kankoh-Maru Return Flight Operations", Proceedings of 8th ISCOPS, Xian.
Also downloadable from considerations in kankoh maru return flight operations.shtml

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