Suborbital is Next to Useless for Point to Point Travel

Spacetransportnews has a link to another in the long list of nontechnical space dreams.

Earth’s radius is about 7000 km. The Van Allen belts start somewhat above 500 km from Earth’s surface. Hence, ballistic arcs have to be either very short or then very shallow. And shallow arcs mean high speed. Close to orbital. New York to Paris is about 6000 km. That’s roughly one seventh of the great circle. It is very clear that to travel such a distance ballistically and at low altitude, you need most of orbital velocity. ICBM:s have very high apogees because of this.

The corridor between the atmosphere’s top and the Van Allen belts is so narrow, just a couple hundred kilometers, compared to the horizontal distances, a couple thousand kilometers, that ballistic point to point travel for humans does not make sense.

Nobody seems to recognize this fact. We get vague dreamy projections by even the normally technically hard nosed engineer people in the alt space circles. Wake up. Orbital mechanics wins. Transatlantic point to point is harder than orbital.  Never mind transpacific.

Grumpy mode off…

Word coin: Narrow suborbital ballistic corridor.

This entry was posted in Architecture, Demotivation, Models, Motivation, Science Links, Spacecraft, Suborbital and tagged , , , , . Bookmark the permalink.

3 Responses to Suborbital is Next to Useless for Point to Point Travel

  1. GL,
    Did you actually read the paper? I didn’t read the whole thing, but they address the very point you make (that the height of the Van Allen belts is a key constraint for manned suborbital point-to-point). Their key point was that with a ballistic trajectory constrained to a less than 500km apogee, there’s a certain range you can get, but using a skip trajectory, you can extend that to antipodal ranges at less than orbital velocities.

    And with the exponential nature of the rocket equation, knocking off even a small amount of delta-V can have huge benefits cost-wise. So while it’s technically a very challenging problem, I don’t think it’s technically impossible at all.

    What I think the real challenge is, is that once such a system is close enough that governments will take it seriously, setting up some sort of regime so that Russia doesn’t think you’re starting WWIII when you’re really just flying some CEOs to Sydney. Ie, while the technical challenge is daunting, I think the regulatory/international issues are going to be pretty ugly as well.

    ~Jon

  2. gravityloss says:

    I skimmed thru the executive summary and their example uses big whole page graphics and no skips for New York – Paris. The craft pictured is an X-Prize contender if I remember correctly, could be wrong though.

    You need a high mass ratio for a balllistic New York to Paris and the vehicle will not look much like any X-Prize vehicle.
    Or then you need multiple skips and a weird hypersonic shape.

  3. Randy Campbell says:

    Gravityloss;

    Haven’t read the paper yet, just skimmed it but we’ve been down this road before. Yes you’d “skip” on just about any trajectory for Point-to-Point whether the graphics show it or not. The current trend of ‘sub-orbital’ tourism on simple ballistic arcs, (Class-1 Sub-Orbital which is basically straight up and down) is quickly going to reach a point where high-g forces incured during reentry will force either a change in configuration to a ‘fluffier’ more vertically enhanced design like the TGV VTVL vehicles or they will begin to move towards lifting reentry profiles which will mean they will require SOME horizontal delta-v.
    (As for the “Pictured” vehicle if you’re talking about the illustration on page 2 yes that is the Bristol Spaceplanes X-Prize entry but it is also supposed to be a proposal for a tourism vehicle and possible P-t-P short range vehicle)

    Skip-Glide trajectories are unlikely to be the “roller-coaster” rides most assume as the forces involved in each ‘skip’ are often found to be lighter than actual reentry forces. (Depending on the systems used and the flight profile the forces at the ‘bottom’ of the skip can be as high as 2-3gs for high angle entries while shallower angles can generate forces as low a 0.5g. A quick look at various hypersonic shapes shows that even a semi-X-prize vehicle like the Bristol Space plane could be used for skip glide flights with some modification but you don’t really NEED more than a Mach-6 able vehicle frame to handle skip-glide manuvers and even then with the proper TPS and cooling during the post skip phase of flight a “non-optimized” shape can be used. (Lenticualr or saucer shaped airframes were found to be particularly suited to such operations btw)

    Paris is listed on the breakdown of flight traffic, though it falls below London on traffic demand from New York, this may favor makeing the sub-orbital route NY-Paris for specifically that reason. Paris could probably handle the expansion and traffic easier than London coud despite London being the prefered destination.

    So there is no real problem with flying “sub-orbitally” other than the required delta-v and as long as you don’t insist that this be provided all at one during take off there are ways to add delta-v in small increments during the flight.
    (As a ‘note’ though ICBMs DO NOT have “high”apogees, they in fact are designed to use most of their delta-v to put the warhead bus into a atmosphere skiming suborbital trajectory to reduce warning time on target.)

    Randy

Leave a Reply

Your email address will not be published.