Friday, January 6, 2017

'Carrier' spacecraft in the news

Here's an IEEE article describing two planned multi-satellite missions.

One is the Sherpa vehicle from Spaceflight Industries, set to fly on a Falcon 9 as a secondary payload on the Formosat 5 mission. This is expected to launch some time this year, so keep an eye out. The vehicle is described as a space tug by SI, and is capable of carrying mixed configurations of smallsats, cubesat launchers and other custom hardware. This first flight offers an 87-satellite capacity. The vehicle itself seems to be very flexible, with options for additional propulsion and add-on services.

 Another is the Indian Space Research Organization's C37 mission on PSLV, expected to launch near the end of this month. This is a medium-lift vehicle reaching polar orbits, with multiple upper stage restarts available. This launch system has already deployed multiple satellites to different orbits in one launch. Their next launch is expected to deploy over 100 cubesats and two rideshare satellites as secondary payloads to the Cartosat-2d mission.

These craft are significantly smaller than my proposal. That's not surprising; even though the smallsat market is rapidly growing, there just isn't enough demand to justify launching a thousand or more of them a year. It will also take time to develop technologies on this scale that can be used beyond LEO; cubesats face unique and difficult challenges when they can't easily be reached by ground-based communication. This is one of the reasons I focused on the carrier craft providing local comms and possibly beamed power. Sherpa in particular has the volume, mass and technical ability to add a comm relay in the future. One problem with that idea is the payloads drift after deployment; for anything more than short-term experiments, a network of LEO commsats providing data relay services would be a big help.

Edit:
The upcoming Iridium NEXT constellation (first launch expected this month) will offer satellite crosslink services. Spacecraft in polar orbit with appropriate receivers would have access to high-quality redundant communication paths, while their operators would receive data from ground stations over the internet or via an Iridium terminal. We might soon be able to say 'There's an app for that' to people wanting to control cubesats from their smartphone.

Exciting times.

Thursday, October 20, 2016

Reddit is distracting

Low post rate here lately has been because I've gotten sucked into reddit's r/spacex forum.

Partly for my own reference, here are links to some comments and submissions I've made:

Single-window round trip of the ITS ship
This was a trial run using Trajectory Optimization Tool to see if it was possible to send an ITS ship to Mars and return it within the same window. Provided you can refuel the ship in about a week and can handle a long (>200 days) return trip, it's definitely possible.

ITS system performance tables for near-Earth space
(public spreadsheet supporting tables)
A table of payload performance values for missions to various locations around the Earth and the Moon, with estimated costs.

ISRU system scaled to fuel one ITS ship per window, pt1
(pt2)
An extrapolation from an ISRU study that outlines the equipment needed to refuel an ITS ship (1950 tons of propellant) in one synod (~780 days). Includes mass estimates and is based on fairly good data. TL;DR is about 101 tons of gear and 20 tons of spares, with another 10-20 tons of spares each trip. The ISRU advantage is 16:1 for the first trip and about 98:1 for the next four trips. If the replacement cost is amortized over ten years then the advantage is 48.5 tons of propellant per ton of equipment.

Discussion of radiation shielding
This is an extension of my thoughts on shielding for large, permanent habitats that require Earthlike radiation levels. The conclusions do not apply for spacecraft in general because most spacecraft proposals cannot support several tons of shielding per square meter of surface area.

Wednesday, September 28, 2016

SpaceX ITS projections

 Now that Elon Musk has released engineering targets for the proposed interplanetary transport system (formerly BFR), there is some meat to work with when looking at possible applications. I'm going to extrapolate, extend and abuse those numbers as thoroughly as I can after the jump.


MCT predictions revisited

 I made a messy post with predictions for the MCT (SpaceX Mars Colonial Transporter, now known as the Interplanetary Transport Ship/System). There is a follow-on series of posts, but this one has the relevant numbers.

 In terms of architecture I did very poorly. The actual ITS is rigid-hulled, passengers travel in  microgravity, life support is ISS-style, and both Mars and Earth arrival is direct aerocapture and propulsive landing. There are a lot of windows. There are no propellant depots anywhere. Oh well... I like parts of my approach better but certainly Musk's approach is less risky up front and less expensive to develop.

 In terms of vehicle performance, I posted numbers for a 12m version and a 15m version. The vehicle will be 12m so I'll use those numbers. I'll compare to the reusable ship profile from Musk's talk.
In short, I didn't do very well. Details after the jump.

Thursday, September 22, 2016

A review of nuclear electric power

 This is a subject that's been stewing for a while now. I often see debates in comment sections over whether or not nuclear electric power is feasible in space. Only rarely do those arguing hold the same assumptions about what nuclear power actually means. As a result, these debates rarely convince anyone of anything beyond the stubborn natures of their opponents.

 The goal of this post is to briefly cover the range of commercial, military and scientific nuclear power systems ranging from a few kilowatts to over a gigawatt. I will follow up the (hopefully) useful background information in a later post with some fanciful projections and my usual call for unlikely investments in space.

 Read on after the break so you can be armed with facts for your next debate thread.


Monday, August 29, 2016

Tools - Tank estimator sheet updated

I discovered and corrected a mistake in my tank estimator spreadsheet. Corrected version is live.
{edit 2016-09-06:}Also added a fill factor and a structural factor for better estimates.{edit}

I also can't find my notes on how the sheet was put together, so I thought I should document the parts that were updated now so I'll know what I am looking at a few years from now.

Before I go into that, I'd like to reference a NASA document (pdf) describing helium ullage tests for the Centaur upper stage. Read it, it's full of retro cool. Their numbers are a tiny fraction (1%) of mine for several reasons: they use ambient temperature helium, assume a gravity-supported temperature gradient (while I assume the helium is pre-chilled to 20 K for hydrogen or 90 K for everything else), rely on vaporization of the liquid hydrogen to maintain pressurization during engine chilldown and assume warm hydrogen from the engine provides pressure during burns. As a result I'd suggest not taking the ullage masses and volumes from my sheet too seriously as my numbers appear to be extremely pessimistic. I've also used some constants that I didn't properly document, something that must have been tricky to calculate with thermal expansion of gases; that whole area of the sheet is probably due for a revisit.

I'll also warn you that my results can be up to 10% heavier when fueled than published masses; there is something I'm not properly considering in the model. Still, if you want a quick and dirty estimate that's probably within 10% and lets you enter most of your own parameters then here you go.

Read on after the break for more details. I even made a diagram.

Thursday, August 25, 2016

Understanding the biological effects of gravity - NASA PubSpace article

 The very first paper I dove into from the new NASA public server covered topics near and dear, primarily the fact that microgravity is not survivable over the long term. There was, shall we say, a very enthusiastic embrace of the term 'omics' but otherwise some very interesting points.

 In a nutshell, we know almost nothing about the effects of any level of pseudogravity (referred to as AG from here on) between microgravity and 1 g. This is important. NASA is considering human missions beyond LEO in less than a decade. We can and should do some kind of testing between now and then, and the only way I see to do that affordably is to launch a dedicated orbiting laboratory.

Read on for more. All costs are in current US dollars and are assumptions based on very little data.