Author Topic: Falcon 9 booster for suborbital PTP (as tourism, not transport)  (Read 2531 times)

Offline mikelepage

I posted a variant of this in the "upper stage" thread yesterday, but had some further thoughts so I decided to edit and make it a new topic.

Elsewhere it's been established that the Falcon 9 booster could easily replicate what the New Shepherd rocket did earlier this month: The main idea of this post is that SpaceX could now make the existing contenders in the "suborbital space tourism" market segment obsolete, by providing an objectively better suborbital experience, and if it is done using reusable boosters, it may come out at a similar cost to what Blue Origin/Virgin Galactic/XCOR are asking.

Suborbital point to point (PTP) has been discussed before, usually in the context of the transport sector, and detractors have rightly pointed out that this is a very very expensive way to move people.  By the time you start having significant downrange distances, you're achieving a relatively high fraction of orbital speed.

But here's the thing: Elon stated in his OG2 prelaunch article that in return-to-landing-site mode, the Falcon 9 FT booster can accelerate a 125 ton upper stage/payload to ~5000km/h.  Looking at the CRS-3 (Falcon 9 v1.1) landing zone maps it seems that a ballistic trajectory for the first stage had it coming down roughly 880km (550 miles) downrange.  We know that the Full Thrust version is supposed to be 33% stronger, and from the OG2 launch telemetry we saw the booster momentarily achieved 6000 km/h at stage separation, reached apogee at roughly 200km and arrived back after 10 minutes.  That would suggest that a ballistic trajectory from the Falcon 9 FT would put the booster at around 1200km downrange.

Given that a "suborbital tourism" payload  is <10% of the current upper stage mass (for the sake of argument, let's say we put a Dream Chaser or Dragon 2 placed directly on top of the booster - both being largely developed, crew-rated craft with in-launch abort capability), and that for sub-orbital trajectories you would probably flatten out the path of the payload substantially, increasing downrange and lowering apogee, I'm thinking you might be able to increase that downrange distance by as much as 2x or more (2400km+) without putting any propulsive capability on the spaceplane/capsule itself, and still be able to return the booster to landing site.

Here's where the experience is objectively better than what the other suborbital companies are offering: Not only do you get to see the curvature of the Earth and feel zero gravity, but you have that for 15-20 minutes, and you also get to see the Earth turn under you.  This also means you can fly over cities/famous landmarks at orbital altitude.  Imagine launching from Cape Canaveral, flying North North East, following the coast all the way up, over New York and landing somewhere in Massachusetts .  Alternatively, maybe you can launch from Brownsville and land somewhere in Ohio.  Make it a 100% reusable system using components already developed, and for 6 people+pilot you're probably not paying much more than $250k per person either.

Summing up, I think suborbital PTP doesn't make sense as a transport solution, but as a suborbital tourism experience it would be amazing.  Also a great way to make money out of this excess of first stage boosters/Dragons which SpaceX looks likely to start accumulating.

Offline Jim

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There is a thread on this already.  Short answer, it is not a viable idea

Offline MattMason

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Might want to look at this thread and other similar threads.

SpaceX isn't in the tourism business. They're leveraging their development in LEO and BEO to get to Mars, period. Musk isn't interested in flying mere mortals yet until you can buy a ticket to live on Mars.

Look to Blue Origin for space tourism. The New Shepard is a suborbital joyride, and future launch vehicles are planned primarily for this purchase. Jeff Bezos is entirely interested in human spaceflight from a commercial point of view.
"Why is the logo on the side of a rocket so important?"
"So you can find the pieces." -Jim, the Steely Eyed

Offline mikelepage

Couldn't find any discussion on this variant of the idea, nor any actual show-stoppers in threads I did find, but the best rebuttal of the idea I've found is this:

That'd be a seriously expensive proposition! 

While both the 1st stage and Dragon2 are optimized for multiple launches, they aren't optimized for this type of flight where the full power of the booster isn't needed, nor the robustness of the Dragon capsule, nor the ability to launch dozens and dozens of times for an overpriced thrill ride.

Think VG but at ten times the price for a fraction of the useful life... :( 

Now a new system optimized for sub-orbital flight would be interesting!

It's a fair point about not being worth using the Dragon for this purpose: it's probably more likely that another company would build their own optimised craft (more seats/windows) and buy boosters from SpaceX, than SpaceX doing it themselves given their focus, but there's a lot of money sitting on the table for such "overpriced thrill rides" (which by definition makes them not overpriced), and someone is going to take it. 

The implicit assumption seems to be that the full power of the booster is not needed - which is true if you're only attempting to do what all the other providers are doing by barely touching the Karman line.  The difference is that I'm saying, using the full capability of the booster to get some downrange will give a better experience.  I'm curious if anyone has discussed this?  Like I said, the F9 booster as is, is clearly capable of launching a payload on a ballistic trajectory a significant distance downrange (obviously remaining a small fraction of orbital velocity) but still return to landing site.

I guess the real crux of the issue is around the actual numbers of times of reuse, and whether the VG/BO craft are really so much better optimised to this use case than the Falcon 9 boosters? I'm not sure why that should be so?  Launching "dozens and dozens of times" was one of the design guidelines of F9R, wasn't it?

Offline deltaV

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What is the approximate reentry speed and angle of your proposed trajectory? At speeds much less than orbital you can probably get good enough results assuming a flat Earth and an acceleration due to gravity that's a constant 9.8 m/s/s, i.e. elementary high school physics. I fear the vertical component of velocity at reentry would be high enough to make reentry acceleration exceed the levels customers would volunteer for (or possibly even survive). This is related to the concept of "black zones" for launch vehicles carrying humans, which has been discussed numerous times on these forums.

Offline Nilof

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What about using it to accelerate scramjet prototypes?
For a variable Isp spacecraft running at constant power and constant acceleration, the mass ratio is linear in delta-v.   Δv = ve0(MR-1). Or equivalently: Δv = vef PMF. Also, this is energy-optimal for a fixed delta-v and mass ratio.

Offline mikelepage

What is the approximate reentry speed and angle of your proposed trajectory? At speeds much less than orbital you can probably get good enough results assuming a flat Earth and an acceleration due to gravity that's a constant 9.8 m/s/s, i.e. elementary high school physics. I fear the vertical component of velocity at reentry would be high enough to make reentry acceleration exceed the levels customers would volunteer for (or possibly even survive). This is related to the concept of "black zones" for launch vehicles carrying humans, which has been discussed numerous times on these forums.

Ugh, that's what I missed. Yes fair point, the reentry angle is too steep, and to compensate by flattening out the booster path to that extent would either leave the booster too far down range to attempt RTLS, or would leave the tourist craft going too slow to achieve even 1200km down range.  So, that leaves us with keeping the booster trajectory more or less as is, and requiring the tourist craft to have some propulsion to flatten out the trajectory.  Some variant of Dream Chaser is probably the best craft currently suited to such a task, but nowhere can I find the specs of the "full-size" Dream chaser engines (hybrid or liquid), so I'm not sure if they're big enough to do what I'm talking about.

I'll be curious to follow what designs emerge, but this isn't really a Spacex thread anymore, so I won't continue it. I did want to make the point however that (for the meanwhile anyway) suborbital PTP actually makes more sense as a space-tourism activity rather than a practical form of transport.

What about using it to accelerate scramjet prototypes?

Still need to solve the flight angle issues I think , as the scramjets only operate efficiently in a narrow altitude range.

Offline Paul451

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What is the approximate reentry speed and angle of your proposed trajectory? [...] I fear the vertical component of velocity at reentry would be high enough to make reentry acceleration exceed the levels customers would volunteer for (or possibly even survive).

That's a weird objection when you're discussing the F9R. You know, the rocket that does retro-burns before reentry.

You are launching very light, you'd fully fuel the F9R as much for ballast as anything else, and still be running the engines throttled right down. You'd have more fuel left over than you know what to do with. So after the 3-400km peak and fall back towards the atmosphere, you'd do a pre-entry burn to zero your vertical before the capsule and booster separate for actual (low speed) reentry.

Online macpacheco

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What about using it to accelerate scramjet prototypes?
Scramjets are not a good idea.
Its not by chance that only the USA is pursuing them.
You can't take off on scramjet power.
Accelerating from zero to Mach 4+ is a daunting job (unless you use rockets, which are too expensive for civilian use).
« Last Edit: 01/06/2016 01:10 PM by macpacheco »
Looking for companies doing great things for much more than money

Offline JamesH

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What about using it to accelerate scramjet prototypes?
Scramjets are not a good idea.
Its not by chance that only the USA is pursuing them.
You can't take off on scramjet power.
Accelerating from zero to Mach 4+ is a daunting job.

Australia as well IIRC.

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