It is not possible to put a newline in an item name, though you should get the same results by adding it to the item's lore.

It is impossible to directly put a newline character in an item's name. Using the item's lore text as a second line will suffice though. How realistic it will look would depend on which version you are using.

1.12−

In 1.12 and below, neither the item name nor the item lore use JSON text formatting. Unfortunately, you can't make the item name and its lore look similar.

/give @s elytra 1 0 {display:{Name:"Rockets",Lore:["Rockets"]}}

1.13

In 1.13, the item name uses JSON text formatting, but the lore does not. This means you won't be able to make both lines look like an item name. If you are fine with one line white and one purple, use this command:

/give @s elytra{display:{Name:"{"text":"Rockets"}",Lore:["Rockets"]}}

1.14+

In 1.14 and above, both the item name and the lore use JSON text formatting. This means you can stylize the lore to look like a second line of the item name:

/give @s elytra{display:{Name:'{"text":"Rockets"}'},Lore:['{"text":"Rockets","color":"white","italic":false}']}