Saturday, July 21, 2012

Towards a reusable rocket system with a fast turnaround

Speculation alert

That's the idea fermenting in my mind as of the moment. I recall that the Saturn IVB of the Apollo Era almost had single stage to orbit (SSTO) capability on its own. Quite impressive.  Now, the idea is to mate that capability with a microwave thruster that Parkins described in his doctoral thesis.  The twist is not to make an SSTO as Parkins envisioned, but to make a staged rocket system that is reusable with a fast turnaround.

From the video of the Falcon 9 launch seen in the previous post, I obtained some trajectory information which shows that the first stage of a similar system would remain within the range of a proposed microwave beam system.  That means that it can stay in contact with the microwave beam on the way up and the way back down.  In other words, it could be made as a flyback first stage that powers a second stage to near space, separates, then returns back to the ground for reuse and fast turnaround.

The advantage of a microwave thruster is its high ISP.  It can reach an ISP of a nuclear thermal engine without having to carry all that weight, since the energy would be beamed toward the craft as opposed to generating it onboard.  This would give it more safety and structural margins while exceeding current conventional rocket designed performance, which could in turn allow the second stage to be configured into a fully reusable system in its own right.

By the way, the high ISP is like fuel efficiency.  The less fuel you have to carry, the less weight penalty there is to pay to get to orbit.  The weight penalty is what makes reusable rockets an elusive goal.  So much of the mass has to go for fuel that it forces very tight limits on what can be put into orbit- which also entails a very high cost per launch.

Parkins thesis proposes the use of truck batteries for power.  This may sound ridiculous, but it isn't really.  The amount of power required is enormous, but only for a short duration.  It would not make sense to use so much installed energy for only such a short term.  Hence, the use of batteries instead of the grid or a separate power station.

The batteries could be recharged for the next launch.  A LFTR may do the charging over a several hour period before the next launch.  Thus, the power system could be trucked in, being that it would be fairly portable.

Below is a jet power requirement for an orbital system.  The amount is in Gigawatts, but the time required is measured in minutes.  Power is measured over time, so 25 Gigawatts over a fraction of an hour translates into less than a Gigawatt hour of power, or a few Gigawatt-hours.  A 100 MW power plant can deliver 2.4 Gigawatt-hrs per day of power.  A LFTR that size can fit on a flat bed truck.

The circled numbers are for orbit, a suborbital jet power requirement should be much less.

The power system would generate the microwaves which would be beamed to the heat exchanger of the aeroshell of the spacecraft.  It would heat the reaction mass, which would be liquid hydrogen, and this would provide the thrust for liftoff.  The first stage would rise to about 120 km at 2g, then separate.  It would be mostly a vertical trajectory.  By the way, this acceleration appears to be less than the Falcon 9, which may entail some reworking of that system if it is to be mated with this one.

That reminds me of the Stratolaunch concept, which mates a Falcon 5 with a large plane that takes off from a runway.  The Parkins concept could work similarly to this concept, so maybe a modified Falcon 5 could be the payload for this proposition.  The added safety and structural margins could make this a good fit for a fully reusable system with the fast turnaround needed in order to bring down launch costs.


With a 2g acceleration to 120 km, the delta v for LEO would be about 4.275 km/sec.  Plugging that into the rocket equation, and assuming an ISP of 450, you could allow for 30k of payload plus dry mass of second stage rocket giving a propellant requirement of 50k.  Converting back to pounds from kilograms, the wet mass comes in at 176k pounds.  Not too bad. 

The Falcon 5 arrangement above weighs much more than that.  Dropping a rocket from a plane doesn't help much in getting to orbit.  A Falcon 5 weighs almost as much as a Falcon 9, assuming the payload is the same.  A Falcon 9 on the launch pad weighs in at 750k pounds.  You're cutting wet mass down by a factor of 4.

With all that extra margin, you could make that sucker fully reusable.  There's enough room for all kinds of bells and whistles.


Next in series, part 2.

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