Why Retrofit is limited to +50% per order?

No reason?

Design two ships:
Empty hull, that fills the requirements (crew, bridge etc.).

Design a proper ship with same hull type.

Build the empty hulls. They are cheap, so you likely will build few every turn even at low space yard rates.

Upgrade the empty hulls into real ships. You must repair the components, but with few repair ships you will be able to build half a dozen ships per planet per turn.

I'd say that is why the limit is there.

If the retrofit cost is not capped, then you can have a situation where a player can build a basic hull and then upgrade it to a very expensive type of ship, like a Star Destroyer in basically two turns assuming they have the resources and repair capability. This would be an extreme case of "retroseries" building, which in this case is imbalanced. This is also goes to cover your second query, where you cite the construction speed advantage. If the retrofit cost cap is a balanced value, then retroseries building is a legitimate tactic in "emergency" type situations, but it doesn't have to be the standard building practice.

Space Empires Depot | SE:V Balance Mod

I find retro-series building to be a very good standard practice inmo. Although it can become a pain. I find having to copy an obsolete ship with outdated engines a pain to upgrade weapons. Sometimes forget Engines are old and just click upgrade and then stuck with a ship having a ton of components to repair rather than just the couple of guns/sensors/armor/shields I wanted.

~Slimbo

Everything that has transpired has done so according to my design...

I was unaware you could do that.

~Slimbo

Everything that has transpired has done so according to my design...

The main drawback to retroseries building as standard practice is the micromanagement and costs involved. It's usually easier to have more ships constructed at a slower rate (ie use more base space yards) to build up forces.

Space Empires Depot | SE:V Balance Mod

You can't issue multiple retrofit orders in the same turn in simultaneous move games (since retrofit order clears the queue), aka multiplayer.

SpaceEmpires.net | Space Empires Wiki

If you don't like 50%, it is a very simple thing to change the cost limit for retrofits in settings.txt.

You can issue multiple retrofit orders in the same turn in simultaneous move games, I do that to build star bases with building ships at warp points in a few turns over taking 15 turns to build it.

That is how it works in 1.66, but I am not sure about 1.71 in the unmodded game

The retrofit command does not clear the orders list. The only things that could stop someone from retrofitting empty hulls to whatever final design they want are cost, repair rate, and avoidance of micromanagement.

Sigh... why must Aaron forget every balance fix he added in previous iterations of the series every time he makes a new one...

SpaceEmpires.net | Space Empires Wiki

Heh... I still don't get this one. Repair speed is the entire reason for this? I can see how this would let you totally redo your ships each turn. Massproduction would get a boost too. and... is that it?

It took until SEV v1.39 to get back the abiliity for a game host to reset a player's password, which wasn't added until SEIV v1.47 the last time.

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"Good morning, Pooh Bear," said Eeyore gloomily. "If it /is/ a good morning," he said. "Which I doubt," said he.

SpaceEmpires.net | Space Empires Wiki

I know. I regularly do stuff like that in single to side step the 50% limitation. Seems kinda odd that it's THAT big of an issue though.

It is a big issue to be able to build expensive SM ships in 2 turns, instead of 20.

SpaceEmpires.net | Space Empires Wiki

Ah, that is something that it does prevent.