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#960 by Proponent on 22 May, 2017 13:20
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Ya know we could start a new thread.
mdot = thrust / (isp * g)
isp = thrust / g / mdot
As such, isp is: for an ideal rocket, how long can we maintain that mass-force?
The reason isp is popular in the US (whereas exhaust velocity is more popular in Europe/Russia) is because non-metric people prefer to talk in terms of mass-force.
No such quantity as mass-force exists, but I think I get your drift. The standard measure of the amount of oomph that a propellant provides is the impulse delivered, i.e., the product of thrust and time. If, as is common in English-language textbooks written in the 1960s or earlier, propellant quantity is expressed in terms of weight, then the natural figure of merit of for propellants is impulse divided by weight. Since both thrust and weight are forces, impulse over weight is expressed as a time. That's what we usually call specific impulse, though it would be more precise to call it weight-specific impulse (impulse per unit weight of propellant).
If we express propellant quantity in terms of mass, then the natural figure of merit is impulse divided by mass, which has units of velocity. We usually call that effective exhaust velocity, but it can also be thought of as mass-specific impulse.
Weight-specific impulse is, of course, the time that a given quantity of propellant can produce a thrust equal to its own Earth weight. Since thrust-to-weight ratios at stage ignition are typically nearish to unity and since, at stage ignition, the mass of a space vehicle is usually dominated by the stage's propellants, that time does provide a bit of insight into how long a stage should burn for optimal efficiency.
More often, however, effective exhaust velocity provides the greater insight: if the delta-V needed from a stage greatly exceeds its effective exhaust velocity, the rocket equation tells you you're really going to have to tighten your belt. If it were up to me, we'd do away with weight-specific impulse and use effective exhaust velocity.
EDIT: Deleted two fragmentary sentences accidentally attached at the end of the final paragraph. -
#961 by Steven Pietrobon on 22 May, 2017 22:49
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I thought this overview of Electron was pretty good. Has great graphics of Electron and includes a Kerbal Electron simulation!
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#962 by Steven Pietrobon on 22 May, 2017 22:53
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There's a comment in this New Zealand article saying the launch is being privately livestreamed to their Auckland and US HQ.
https://www.nbr.co.nz/article/rocket-lab-counts-down-first-test-launch-ck-202970 -
#963 by QuantumG on 22 May, 2017 23:02
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There's a comment in this New Zealand article saying the launch is being privately livestreamed to their Auckland and US HQ.
https://www.nbr.co.nz/article/rocket-lab-counts-down-first-test-launch-ck-202970
I think you mean there's a comment on the article, by some random internet commenter. I'd take that with a dump truck of salt.
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#964 by Lars-J on 22 May, 2017 23:25
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There's a comment in this New Zealand article saying the launch is being privately livestreamed to their Auckland and US HQ.
https://www.nbr.co.nz/article/rocket-lab-counts-down-first-test-launch-ck-202970
I can believe that.
It's not going to be livestreamed for the rest of it. Accept it and move on. -
#965 by Steven Pietrobon on 23 May, 2017 00:00
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You can download the Payload Users Guide from this website.
https://drive.google.com/file/d/0B-NvtQjqI6SeQ2J4Sk5tS3gxVWc/view -
#966 by Steven Pietrobon on 23 May, 2017 00:17
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The Rocket Lab live news updates link has died.
https://www.rocketlabusa.com/latest/live-news-updates/ -
#967 by MATTBLAK on 23 May, 2017 00:27
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#968 by CameronD on 23 May, 2017 02:02
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The Rocket Lab live news updates link has died.
https://www.rocketlabusa.com/latest/live-news-updates/
Well.. like everything RocketLab have done to date, "it's a test".
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#969 by Scylla on 23 May, 2017 02:19
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The Rocket Lab live news updates link has died.
https://www.rocketlabusa.com/latest/live-news-updates/
Try this.
https://www.rocketlabusa.com/latest/its-a-test-live-news-updates-2/ -
#970 by Steven Pietrobon on 23 May, 2017 22:33
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#971 by vaporcobra on 23 May, 2017 22:41
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As of three hours ago, it appears that Rocket Lab is in the process of preparing for another attempt today.
Slick timelapse can be found here https://twitter.com/RocketLabUSA/status/867106745390292992. -
#972 by Steven Pietrobon on 24 May, 2017 02:53
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I've sent the following message to Rocket Lab asking to make their official Youtube livestream public. If you want to also see the launch live, please also send a message to Rocket Lab. It won't hurt to ask and who knows, we may get Rocket Lab to change their mind.
https://www.rocketlabusa.com/contact/
Dear Rocket Lab,
We understand that there is an official Youtube livestream of the first Electron launch at youtube.com/watch?v=CBEQWSqoeFc , but it is private only. Could you please make this livesteam public? Rocket Lab has said it won't be providing a live stream because of the variability of the launch time. As a keen space enthusiast we understand this and are quite happy to wait all day and all week for the launch to happen. We would like to experience this historic moment for New Zealand that is being followed all over the world. You guys are about to send a payload into space! Please let us share the experience.
Yours sincerely,
Steven S. Pietrobon, B.Eng., M.Eng., Ph.D. -
#973 by meekGee on 24 May, 2017 04:17
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They are trading extra weight of batteries and motors for easier development cycle. Engine is more efficient as all fuel is used for thrust, instead of some being burnt for pumping fuel. That extract fuel saving helps to off set extra weight of batteries. Every year or two vehicle reduces weight by changing to latest battery technology for very little R&D cost.
For RLV the batteries shouldn't change, they are sized for fuel capacity of LV.
Also optimized for manufacturing. Probably cheaper to build and assemble many electric motors then many turbines and valving systems. -
#974 by high road on 24 May, 2017 06:55
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I've sent the following message to Rocket Lab asking to make their official Youtube livestream public. If you want to also see the launch live, please also send a message to Rocket Lab. It won't hurt to ask and who knows, we may get Rocket Lab to change their mind.
https://www.rocketlabusa.com/contact/
Dear Rocket Lab,
We understand that there is an official Youtube livestream of the first Electron launch at youtube.com/watch?v=CBEQWSqoeFc , but it is private only. Could you please make this livesteam public? Rocket Lab has said it won't be providing a live stream because of the variability of the launch time. As a keen space enthusiast we understand this and are quite happy to wait all day and all week for the launch to happen. We would like to experience this historic moment for New Zealand that is being followed all over the world. You guys are about to send a payload into space! Please let us share the experience.
Yours sincerely,
Steven S. Pietrobon, B.Eng., M.Eng., Ph.D.
Doesn't hurt to ask. However, given that there are even people on this forum already questioning their choice of launch site and calling the triboelectricity problems incompetence in thinly veiled terms, the prospect of reaching out/exposing themselves to a much larger, far more visible and considerably less informed public is probably not quite appealing to them. -
#975 by speedevil on 24 May, 2017 13:47
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As a rather silly comment, I note that this rocket would easily fit on a F9 launch.
You'd need to pull off the fairing, and arrange an arm to stack the payload on top, unless you can split the payload into two panniers.
(other minor problems may exist). -
#976 by ArbitraryConstant on 24 May, 2017 16:05
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As a rather silly comment, I note that this rocket would easily fit on a F9 launch.
I was thinking the Electron upper stage might make an interesting kick stage. The default American kick stage seems to be a Star 48 or similar, which is solid and hence suffers from <300 ISP and doesn't restart.
You'd need to pull off the fairing, and arrange an arm to stack the payload on top, unless you can split the payload into two panniers. -
#977 by envy887 on 24 May, 2017 16:07
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Fueling a cryo kick stage inside a fairing seems like a pain. And the ISP not much different than hypergols.As a rather silly comment, I note that this rocket would easily fit on a F9 launch.
I was thinking the Electron upper stage might make an interesting kick stage. The default American kick stage seems to be a Star 48 or similar, which is solid and hence suffers from <300 ISP and doesn't restart.
You'd need to pull off the fairing, and arrange an arm to stack the payload on top, unless you can split the payload into two panniers. -
#978 by ArbitraryConstant on 24 May, 2017 16:46
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Fueling a cryo kick stage inside a fairing seems like a pain.
Atlas V 5xx and Delta IV have the cryo upper stage inside the fairing.And the ISP not much different than hypergols.
ISP not that much different than Russian staged combustion hypergols. Beats pressure fed hypergols. -
#979 by envy887 on 24 May, 2017 17:44
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Fueling a cryo kick stage inside a fairing seems like a pain.
Atlas V 5xx and Delta IV have the cryo upper stage inside the fairing.And the ISP not much different than hypergols.
ISP not that much different than Russian staged combustion hypergols. Beats pressure fed hypergols.
Delta IV does not, only the 5 meter Atlas - and they aren't noted for easy handling. I'm not saying it's impossible, it's just a lot of effort compared to enclosing a hypergol or solid kick motor. You need to have a fairing and GSE designed specifically to support it.
AJ-10 gets 319 sec Isp. Not that far from the Ruthurford vac engine.
