[ExI] darpa's notion of using a retrofitted fighter jet to launch payloads
spike66 at att.net
Wed Feb 11 06:32:36 UTC 2015
From: extropy-chat [mailto:extropy-chat-bounces at lists.extropy.org] On Behalf Of Rafal Smigrodzki
Sent: Tuesday, February 10, 2015 9:40 PM
To: ExI chat list
Subject: Re: [ExI] darpa's notion of using a retrofitted fighter jet to launch payloads
On Mon, Feb 9, 2015 at 3:10 PM, BillK <pharos at gmail.com> wrote:
The rocket's design is also unusual, mounting four engines just below
the payload on the vehicle. The engines are used for the first and
second stages of the rocket, with propellant tanks below the engines
dropping away when exhausted. This approach avoids the expense and
complexity of separate sets of engines for the first two stages.
### Interesting. Imagine a bladder made of sturdy carbon-fiber foil, suspended from a wing-shaped bar with attached rocket engines and with payload on top. During launch the bladder is being pulled rather than squished, and we all know that tensile strength of long and thin objects is amazing compared to their compressive strength. So the bladder would have a tiny weight compared to a stiff fuel tank of the same capacity.
>…The vehicle would be suspended from a gantry for launch, its bladder almost touching the ground. The massive booster engines would run at full power all the way to LE orbit. Bladder would be jettisoned and promptly burn up on re-entry. Engines and their wing glide to an unpowered landing and the payload capsule flies wherever it's supposed to.
>…Spike, isn't this a neat idea? :) Rafał
Ja! I like the notion of an air-breathing first stage if there is any way to get that done. Reason: a lot of structural design weight is in handling the aerodynamic loads of supersonic flight while still fairly low in the atmosphere. If we could work out a means of climbing out at a leisurely couple hundred knots, then accelerating on up to just below Mach 1 when you get up around 0.2-ish atmospheres, you get to save weight in structure, in the nose radome, and so forth.
With any rocket, you need to accelerate hard; otherwise the gravity losses are too great. As you pointed out, structurally there are big advantages to tensile stresses rather than compressive, but you need to consider bending moment rigidity in either case. I don’t know what happens to your control model if the rocket flexes very much. I have seen that any nozzle-forward rocket design loves to go into oscillating instability.
Somewhere I saw a video of Robert Goddard’s first liquid fueled rocket, which was nozzle-forward. It went unstable.
Like everything in control theory, it seems like systems do the opposite of what you would think. Nozzle forward seems stable to me: center of thrust way forward of the center of mass and center of pressure. That looks to me like it would fly straight, but it doesn’t. Nearly everyone here as seen and played with toy rockets. For some odd reason those things do fly straight, with the center of thrust well aft of the center of gravity.
In any case, I am eager to see what DARPA can do with this idea.
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