Ok... a list of the various objections:
1. Orcs? In space? Really?
2. That ship looks like a porcupine.
3. You're telling me a person get's knocked off of a ship, in space, and he takes an envelope of oxygen with him? He doesn't, immediately suffer from explosive decompression?
4. They think the point and go mechanic is too simple. They think the modifications I made to make it more challenging are too hard. I added two additional "technologies" to the vessels: Radian drives, and Metta Sails.
Radian crystals - Specially prepared gems of various size, value and type can be alchemically treated to make them store kinetic energy. A Radian Pilot can operatoe up to two Radian Arrays on a vessel (usually comprising 70-100 gems per array, usually artfully imbedded in various patterns on the hull, keel and especially the rudder of a vessel. For every 5 tons of a vessel a minimum of 1 array is needed to effectively boost speed in wildspace, and most importantly boost maneuverability. Only of use in an atmosphere or in wildspace.
Metta Sails - Specially prepared sailcloth that catches metamagical particles that move in, with, and in some cases significantly faster than the flow. Used exclusively to increase SR in the flow.
5. they see DnD as being crawling in dungeons and fighting dragons. Their view of the game tends to be simpler and doesn't allow for a whole lot of out of the box thinking
6. It's dumb
7. It doesn't make sense
8. I thought we were playing DnD
Okay gotcha. I share some of their concerns, although not as strongly, I am guessing.
What I would do is ignore the science aspect...the how of it all. In a world where magic exists, I would say just use that as the default means. Have spelljammers simply be magical ships. I realize you probably put a lot of work into the Mettasails and Radian Crystals....but I would leave that stuff alone for now. Bring it up down the road if it becomes a point of interest.
I would also let space work how we know it works, for the most part. Ships have a magical atmosphere, but if you fall of the ship, you're in big trouble. Unless you've got an airsuit or something like that. Keep gravity working how they expect it. Make it all less jarring...don't mess with their expectations too much. Similar to how the actual world of your game shares expectations with our real world. I think this makes the buy in easier on their part.
Don't do orcs in space. Do a new or unused race, one specifically designed for these space adventures. This way the players have to learn about the race instead of altering what they already know.
I would ease them in, as many others have said. Don't just throw them into space and then explain that there are galactic empires of Illithida and Giff and so on. Instead, have them transported elsewhere somehow...keep it mysterious, and slowly reveal the situation. This prepares then a little at a time so that by the time you reveal that they're in space or on another world (a magical space station would probably be best, since it's essentially a dungeon), they'll have been broken in to the idea. Have the folks they encounter be a bit different than expected...unfamiliar with their home world and so on. Leave little clues. This may be tricky if your players know about your enthusiasm and anticipate it, but this is what I would try.
Finally, have an out. Let them return to their world after the adventure concludes. Now they'll know their world is part of a bigger one, but they can ignore that bigger world if that's what they want, or they can get more involved. You'll know which it will be. No need to end a campaign and the careers of characters they may like just because they don't like the Spelljammer aspect.
All in all, I say focus on it just being magic and treat it accordingly, and keep it subtle at first.
